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Formation and characterization of furfuryl mercaptan-β-cyclodextrin inclusion complex and its thermal release characteristics

Formation and characterization of furfuryl mercaptan-β-cyclodextrin inclusion complex and its... Polish Journal of Chemical Technology Pol. J. Chem. T , 23, 4,ech., V 35—40, 10.2478/pjct-2021-0035 ol. 23, No. 4, 2021 35 Formation and characterization of furfuryl mercaptan-β-cyclodextrin inclusion complex and its thermal release characteristics 1 *,1 *,1 2 Guangyong Zhu , Zuobing Xiao , Gengfa Yu , Guangxu Zhu , Yunwei Niu, Junhua Liu Shanghai Institute of Technology, No.100 Haiquan Road, Shanghai, 201418, PR China 3035 Sable Ridge Dr. Ottawa. ON K1T 3R9, Canada Corresponding authors: e-mail: zbingxiao@sina.com, genquan1@sina.com Furfuryl mercaptan has the aroma characteristics of coffee. However, it is unstable during storage of coffee brew and roasted coffee. In order to enhance the stability of furfuryl mercaptan, furfuryl mercaptan- β-cyclodextrin inclusion complex was synthesized using the precipitation method in this work. Fourier transform infrared spectroscopy, x-ray diffraction, and thermogravimetric analysis (TG) were used to characterize the resulting products. The interaction of furfuryl mercaptan with β-cyclodextrin was inve- stigated by the molecular mechanics (MM) method. These changes in FTIR and XRD gave supporting evidence for the successful formation of furfuryl mercaptan-β-cyclodextrin inclusion complex. The TG results showed that the formation of furfuryl mercaptan-β-cyclodextrin inclusion complex could improve the thermal stability of furfuryl mercaptan and provide a long-lasting effect. The structure of furfuryl mercaptan-β-cyclodextrin inclusion complex with the minimum energy was obtained by MM2 calculation, –1 –10 and the minimum binding energy was –77.0 kJ mol at –1.96 × 10 m. Keywords: Furfuryl mercaptan-β-cyclodextrin, Inclusion complex, XRD, FTIR, Thermal analysis, Molecular mechanics. INTRODUCTION of coffee and affect the quality and stability of coffee aroma, which has become a problem that plagues the Furfuryl mercaptan (see Fig. 1), an important aroma industry . How to reduce the loss of furfuryl mercaptan, constituent of fl avor, was found in coffee, sesame seed keep its aroma stable, and improve the aroma quality oil, popcorn, grilled pork, cooked beef, and roasted 1 of coffee, has become an urgent problem to be solved chicken . It has the aroma characteristics of coffee, in the industry. How to effectively enhance the stability sesame, onion, garlic, and meat. Furfuryl mercaptan has of furfuryl mercaptan, control its release, improve its been widely used in baked goods, gelatins, alcoholic and aroma retention, and enhance its water solubility is of nonalcoholic beverages, condiments, relishes, frozen dairy great signifi cance. gravies, hard and softy candy, and meat products. Furfuryl Core materials can be encapsulated in wall materials mercaptan, which is soluble in oils but insoluble in water, o 1 by formation of inclusion complex. It can realize the is a colorless oily liquid with boiling point 154–155 C . protection of core materials and improve their stabil- Furfuryl mercaptan exerts an important effect on coffee ity. Therefore, by encapsulation of furfuryl mercaptan fl avor profi les. It is a key ingredient of the characteristic to form inclusion complex, it is possible to realize coffee fl avor. In coffee fl avor fi eld, the identifi cation the protection of furfuryl mercaptan and enhance its of furfuryl mercaptan was considered as a milestone . stability. Among many wall materials, β-cyclodextrin, However, because of physical volatile loss and chemical as a safe and environmentally friendly wall material, reaction, its odor quality is unstable. Due to its interac- tion with the melanoidins (which were formed during has attracted much attention and becomes a research 8, 9 the Maillard reaction, e.g., coffee roasting), furfuryl hotspot . β-cyclodextrin molecule has a slightly tapered mercaptan is unstable during storage of coffee brew and hollow cylinder three-dimensional ring structure formed roasted coffee . Melanoidins can promote the degradation by glucopyranose units. Its outer surface has hydrophilic of furfuryl mercaptan . It was one of the major reasons characteristics, while the inside of the molecule has for the sulfury-roasty odor quality decrease. When fur- hydrophobic characteristics because of the C-H bond furyl mercaptan was added to water plus melanoidins, shielding effect. Various organic molecules can be en- approximately 50% of furfuryl mercaptan was lost after capsulated in its hydrophobic cavity to form inclusion 10 min of storage and it was nearly absent after 30 min complexes and change the physicochemical properties of storage . Through the reactive quinone converted of these entrapped molecules . In most cases, inclusion from hydroxyhydroquinone, furfuryl mercaptan can also complex formation with β-cyclodextrin can improve stabil- 3, 6 react with hydroxyhydroquinone . Furthermore, furfuryl ity of poorly stable substances . Pires et al. found that mercaptan is unstable and tends to polymerize when it the shelf-life increase achieved for thymol standard was 3, 6 was heated in the presence of mineral acids . 354% and Lippia origanoides essential oil had a stability The reactions of furfuryl mercaptan and physical 12 increase of about 45% . Ikeda et al. found that water volatile loss are the main reasons for loss of the aroma solubility of anionic nateglinide could be improved by complexation of β-cyclodextrin . Furfuryl mercaptan-β-cyclodextrin inclusion complex was synthesized using the precipitation method in this work to improve the stability of furfuryl mercaptan, Figure 1. Furfuryl mercaptan chemical structure formula control its release, improve its aroma retention, and 36 Pol. J. Chem. Tech., Vol. 23, No. 4, 2021 enhance its water solubility. Fourier transform infrared mA and 40 kV, Cu Kα radiation was adopted. The 2θ o o (FTIR) spectroscopy, thermogravimetric (TG) analysis, range was 5 to 70 . and x-ray diffraction (XRD) were used to characterize the Characterization of furfuryl mercaptan release from its resulting products. The formation of furfuryl mercaptan- β-cyclodextrin inclusion complex was investigated by the inclusion complex by thermogravimetric (TG) analysis molecular mechanics (MM) method at the molecular The rate of mass loss and mass loss of samples were per- level. Binding energy calculation and structure optimiza- formed with a TGA-Q5000IR thermogravimetric analyzer tion were performed with MM2. (TA Instruments, USA). About 5 mg of β-cyclodextrin or furfuryl mercaptan-β-cyclodextrin inclusion complex EXPERIMENTAL in a ceramic crucible was weighed. The heating rate o –1 adopted in the pyrolysis process was 10 C min . High Materials purity nitrogen gas was used to avoid oxidation during Furfuryl mercaptan (food grade, C H OS, molecular 5 6 –1 the pyrolysis process and the gas fl ow was 20 ml min . weight 114, colorless oily liquid) was purchased from Guangzhou Levon Flavor & Fragrance Technology Co., Binding energy calculation and structure optimization Ltd. Anhydrous ethanol was of analytical grade and was by molecular mechanics (MM) calculations provided by Shanghai Sinopharm Chemical Reagent Co., Ltd. Deionized water adopted throughout the experi- MM2 calculations were adopted to examine furfuryl ments was produced in our laboratory. β-cyclodextrin mercaptan-β-cyclodextrin inclusion complex formation (C H O , molecular weight 1134, white crystalline 42 70 35 at the molecular level. Binding energy calculation and powder) was of pharmaceutical grade and was purchased structure optimization were carried out by Chem3D Ultra from Shandong Binzhou Zhiyuan Bio-Technology Co., (CambridgeSoft Corporation, MA, USA). The process of Ltd. Without out further purifi cation, all the raw materi- encapsulation of furfuryl mercaptan in β-cyclodextrin was als were used directly in the experiment. simulated by successively changing the Z coordinate of Methods the furfuryl mercaptan atoms after properly orientating β-cyclodextrin and furfuryl mercaptan molecules. The Formation of furfuryl mercaptan-β-cyclodextrin inclu- docking strategy as described in references was used, sion complex i.e. push furfuryl mercaptan molecule stepwise through With some modifi cations, the precipitation method the β-cyclodextrin orifice minimizing the energy of as described in references was used to prepare furfuryl 15, 17 14, 15 the complex at each step . The position of furfuryl mercaptan-β-cyclodextrin inclusion complex . Firstly, 5 g of β-cyclodextrin was added to 93 g of deionized water mercaptan molecule relative to β-cyclodextrin molecule and was stirred to form a suspension. The temperature was referred to the Z coordinate of C2 atom of furfuryl of the suspension was kept at 35 C. Then, 2 g furfuryl mercaptan. Furfuryl mercaptan moving direction during mercaptan was slowly added to this suspension. Excess MM2 calculation is shown in Fig. 2. furfuryl mercaptan was used in the experiment. Such molar ratio of furfuryl mercaptan to β-cyclodextrin was used to ensure the cavities of β-cyclodextrin molecules containing furfuryl mercaptan molecules. After the ad- dition of furfuryl, the temperature of the suspension remained at 35 C and the mixture was continuously stirred for 3 h to form furfuryl mercaptan-β-cyclodextrin inclusion. The suspension was stored in a refrigerator at 5 C overnight. The precipitate was obtained with the vacuum fi ltration method. Anhydrous ethanol was used to wash the precipitate. After drying in a freeze drier (FD-1A-50), the product was kept in a desiccator for further analysis. Characterization by FTIR A Vertex 70 Fourier transform infrared spectrometer (Bruker, Germany) was adopted to determine the FTIR spectra of furfuryl mercaptan, β-cyclodextrin and furfuryl mercaptan-β-cyclodextrin inclusion complex. The frequ- –1 ency range was 400–4000 cm . Characterization by XRD A D/Max 2000X X-ray diffractometer (Rigaku Corpo- ration, Japan) was used to determine x-ray diffraction of the β-cyclodextrin and furfuryl mercaptan-β-cyclodextrin Figure 2. Furfuryl mercaptan moving direction during MM2 14, 16 inclusion complex as described in references . At 100 calculation Pol. J. Chem. Tech., Vol. 23, No. 4, 2021 37 RESULTS AND DISCUSSIONS stic strong peaks of furfuryl mercaptan such as at 1012 –1 and 735 cm disappear as shown in the FTIR spectra of furfuryl mercaptan-β-cyclodextrin inclusion complex. The results of FTIR of furfuryl mercaptan, β-cyclodextrin The changes and the blue-shifting hydrogen bond of and furfuryl mercaptan-β-cyclodextrin inclusion complex β-cyclodextrin after interaction with furfuryl mercaptan Fig. 3 shows the FTIR curves of furfuryl mercaptan, give evidence of successful encapsulation of furfuryl β-cyclodextrin and furfuryl mercaptan-β-cyclodextrin inc- mercaptan in β-cyclodextrin. lusion complex. The obvious FTIR peaks of β-cyclodextrin –1 –1 –1 –1 appear at 3356 cm , 2922 cm , 1643 cm , 1404 cm , The results of XRD of β-cyclodextrin and furfuryl –1 –1 –1 –1 –1 1244 cm , 1151 cm , 1030 cm , 937 cm and 848 cm . mercaptan-β-cyclodextrin inclusion complex As shown in Fig. 3, except for some minor changes in As an effective instrument, XRD can be used to deter- peak position, the FTIR curves of β-cyclodextrin and mine the formation of furfuryl mercaptan-β-cyclodextrin furfuryl mercaptan-β-cyclodextrin inclusion complex have 16 inclusion complex . The XRD peaks of β-cyclodextrin –1 a similar shape. The broad peaks appear at 3356 cm will change after the formation of the furfuryl mer- –1 and 3337 cm in the FTIR curves of β-cyclodextrin and captan inclusion complex. The XRD curves of the furfuryl mercaptan-β-cyclodextrin inclusion complex re- furfuryl mercaptan-β-cyclodextrin inclusion complex and spectively can be attributed to stretching (O-H) vibration β-cyclodextrin are shown in Fig. 4. of hydroxyl groups in β-cyclodextrin molecule . After the formation of furfuryl mercaptan-β-cyclodextrin inclusion complex, the peak caused by the stretching vibration of hydroxy groups moved towards the low band and blue- –1 -shift was observed. The strong sharp peaks at 1030 cm –1 and 1032 cm in the FTIR curves of β-cyclodextrin and furfuryl mercaptan-β-cyclodextrin inclusion complex re- spectively can be assigned to stretching (C-O) vibration. However, after the formation of the inclusion complex, the peak moved towards the high band and red-shift was –1 observed. The shoulder peak at 1151 cm did not change before and after the formation of furfuryl mercaptan-β- cyclodextrin inclusion complex. The weak peak appearing –1 at 2565 cm in the FTIR curve of furfuryl mercaptan is due to stretching (S-H) vibration . Owing to furan ring stretching (C-H) vibration, a weak peak occurs at –1 3123 cm . Because of the furan ring stretching (C=C) –1 vibration, three peaks occur at 1597, 1501 and 1420 cm Figure 4. The XRD curves of β-cyclodextrin and furfuryl respectively in the FTIR curve of furfuryl mercaptan. The mercaptan-β-cyclodextrin inclusion complex –1 peaks at 1254, 1151 and 1012 cm can be assigned to 19–21 furan ring asymmetrical stretching (C-O-C) vibration . In the XRD curve of β-cyclodextrin, a strong peak –1 o The strong sharp peak that occurs at 735 cm is due to appears at 12.7 as shown in Fig. 4. However, in the the out-of-plane bending (C-H) vibration of furan ring. XRD curve of the furfuryl mercaptan-β-cyclodextrin In addition to the small peaks of furfuryl mercaptan at inclusion complex, this peak shifts to 12.2 . The peak –1 3123, 2526, 1597, 1510 and 1420 cm , these characteri- intensity decrease can also be observed. Furthermore, the peaks at 10.8, 9.4, and 6.4 in the XRD curve of β-cyclodextrin also shift to lower 2θ angles of 10.4, 8.7, and 6.0 respectively in the XRD curve of furfuryl mercaptan-β-cyclodextrin inclusion complex. A similar phenomenon for peak shift was also found in the XRD patterns of mentha-8-thiol-3-one-β-cyclodextrin inclusion complex and menthyl acetate-β-cyclodextrin inclusion 14, 22 complex . The increase in the intensity of the peak at 10.4 in the XRD pattern of β-cyclodextrin can be observed compared to the peak at 10.8 . However, the peaks at 16.6 and 19.0 shift to a higher 2θ angle of 16.9 o o and 19.4 , respectively, and the peaks at 13.4 and 18.1 disappear after the interaction of β-cyclodextrin and furfuryl mercaptan. Compared with β-cyclodextrin, some new peaks appear at 14.5, 15.2, 15.9 and 17.5 in the XRD curve of furfuryl mercaptan-β-cyclodextrin inclusion complex. The encapsulation of furfuryl mercaptan mole- cule in the cavity of β-cyclodextrin molecule may cause the XRD peaks shifting and the new ones appearing in Figure 3. The FTIR spectra of β-cyclodextrin (a), furfuryl the complex. Like the results of FTIR, these changes in mercaptan-β-cyclodextrin inclusion complex (b), and furfuryl mercaptan (c) XRD further give another supporting evidence for the 38 Pol. J. Chem. Tech., Vol. 23, No. 4, 2021 successful formation of furfuryl mercaptan-β-cyclodextrin cules might be expelled from the hydrophobic cavities of inclusion complex. β-cyclodextrin molecules . So, the water that is included furfuryl mercaptan-β-cyclodextrin inclusion complex can Furfuryl mercaptan thermal release characteristics from be ignored. Therefore, in the mass loss curve of furfuryl furfuryl mercaptan-β-cyclodextrin inclusion complex mercaptan-β-cyclodextrin inclusion complex, the mass loss Thermal analysis is another effective method of in- occurring in the fi rst stage may be explained simply as vestigation of the interaction between host and guest caused by the release of furfuryl mercaptan. In the fi rst molecules. The inclusion compound stoichiometry stage, the mass loss of furfuryl mercaptan-β-cyclodextrin can also be evaluated by thermal analysis. Therefore, inclusion complex is approximately 8%. Therefore, the thermal analysis was adopted in the experiment to loading capacity of furfuryl mercaptan, which is defi - study the interaction between furfuryl mercaptan and ned as the mass ratio of furfuryl mercaptan to furfuryl β-cyclodextrin. The rate of mass loss and mass loss curves mercaptan-β-cyclodextrin inclusion complex, is about of β-cyclodextrin and furfuryl mercaptan-β-cyclodextrin 8%. The molecular weights of furfuryl mercaptan and inclusion complex obtained are shown in Fig. 5. There β-cyclodextrin are 114 and 1134 respectively. Therefore, are three main stages in the mass loss curves of both the molar ratio of furfuryl mercaptan to β-cyclodextrin β-cyclodextrin and furfuryl mercaptan-β-cyclodextrin obtained from the mass loss is approximately 0.9:1. inclusion complex as shown in Fig. 5. The fi rst stage If guest:host stoichiometry is considered as 1:1, the refers to the temperature range from room temperature theoretical loading capacity of furfuryl mercaptan is to 290.6 C. In the fi rst stage, a slightly mass loss can be 9%. Therefore, the furfuryl mercaptan:β-cyclodextrin observed. The second stage refers to the temperature stoichiometry is close to 1:1. o o range from 290.6 C to 350 C. In the second stage, the Because of the thermal decomposition of β-cyclodextrin, major mass loss occurred. The third stage refers to the major mass loss can be observed in the second stage. Two o o temperature range from 350 C to 500 C. In the third strong peaks are appearing in the rate mass loss curves of stage, a slightly mass loss occurred again. both β-cyclodextrin and furfuryl mercaptan-β-cyclodextrin inclusion complex. In the third stage, a slightly mass loss occurred again because of the continuous decomposition of solid residuals of β-cyclodextrin at a very slow rate with the increase of temperature. Because the boiling point of furfuryl mercaptan is 154–155 C, furfuryl mercaptan should evaporate away completely before it reaches the boiling point during the pyrolysis process. However, the mass loss still oc- curred from 155 C to the decomposition temperature of β-cyclodextrin. It can be observed obviously from the TG curve of furfuryl mercaptan-β-cyclodextrin inclusion complex in this stage. During the pyrolysis process, the encapsulated furfuryl mercaptan was gradually released from its inclusion complex. It further demonstrates the successful encapsulation of furfuryl mercaptan in β-cyclodextrin. By the formation of furfuryl mercaptan-β- cyclodextrin inclusion complex, long-lasting effect can be provided and the thermal stability of furfuryl mercaptan can be improved. The results of molecular mechanics calculations Binding energy is defi ned as the difference between the total energy of a furfuryl mercaptan-β-cyclodextrin inclusion complex molecule and the sum of the total ener- Figure 5. The TG and DTG curves of β-cyclodextrin and gy of furfuryl mercaptan and β-cyclodextrin molecules. furfuryl mercaptan-β-cyclodextrin inclusion complex In other words, binding energy can be used to measure In the TG curve of β-cyclodextrin, the slightly mass the energy required to break up a host-guest inclusion loss occurring in the fi rst stage can be mainly attributed complex molecule completely into its host molecule to desorption water; while in the mass loss curve of and guest molecule. To some extent, the combination furfuryl mercaptan-β-cyclodextrin inclusion complex, the and the interaction between β-cyclodextrin and furfuryl release of furfuryl mercaptan caused the main mass loss mercaptan can be refl ected by binding energy. Fig. 6 occurring in the fi rst stage. In the temperature range of shows the plot of binding energy vs. the Z coordinate 140 to 250 C, the TG curve of β-cyclodextrin is leveling of C2 of furfuryl mercaptan molecule. off while the mass loss curve of furfuryl mercaptan-β- During the development of the furfuryl mercaptan-β- cyclodextrin is downward sloping. The difference can cyclodextrin inclusion complex, energy is released, so the be attributed to the release of furfuryl mercaptan from value of binding energy is negative. When the Z coordinate –10 –10 furfuryl mercaptan-β-cyclodextrin inclusion complex. of C2 increases from –21.4 × 10 m to –10.9 × 10 m, Furfuryl mercaptan is soluble in oils and insoluble in the binding energies change slightly as shown in Fig. 6. –10 water. During the inclusion process, all the water mole- When Z coordinate increases from –10.9 × 10 m, a sharp Pol. J. Chem. Tech., Vol. 23, No. 4, 2021 39 disulfi de-β-CD, neral-momochlorotriaziny-β-cyclodextrin, geranial-monochlorotriazinyl-β-cyclodextrin and menthol- HP-β-cyclodextrin inclusion complex. In the process of MM2 calculation, the total energy consists of 1,4 van der Waals, bend, non-1,4 van der Waals, stretch, stretch-bend, dipole/dipole, and torsion energy. The minimum binding energy consists of the following com- –1 ponents: 1,4 van der Waals (4.2 kJ mol ), bend (–2.5 kJ –1 –1 mol ), non-1,4 van der Waals (–75.4 kJ mol ), stretch (–1.4 –1 –1 kJ mol ), stretch-bend (–0.6 kJ mol ), dipole/dipole (–0.4 –1 –1 kJ mol ), and torsion energy (–0.8 kJ mol ). Non-1,4 van der Waals energy contributes a lot to binding energy, and is the main reason for the stability of furfuryl mercaptan- β-cyclodextrin inclusion complex. Once furfuryl mercaptan molecule entered the cavity of β-cyclodextrin molecule, β-cyclodextrin changed its shape and furfuryl mercaptan molecule also made conformation adjustments to maximize Figure 6. Plot of binding energy vs. the Z coordinate of C2 the stabilization. The complexation geometries of furfuryl in furfuryl mercaptan molecule mercaptan molecule and β-cyclodextrin were constantly readjusted until the most stable inclusion complexes were fall in the value of binding energy can be observed. At –10 obtained. By MM2 calculation, the obtained structure of –10.6 × 10 m, the value of binding energy is –49.3 kJ –1 furfuryl mercaptan-β-cyclodextrin inclusion complex which mol . With the increase of Z coordinate of C2 to –1.96 –10 has the minimum energy is shown in Fig. 7. It is a relatively × 10 m, the minimum value of binding energy, –77.0 –1 stable structure after shape and conformation adjustments kJ mol , was obtained. When the Z coordinate of C2 –10 –10 of furfuryl mercaptan and β-cyclodextrin molecules during changes from –1.96 × 10 m to 9.7 × 10 m, binding the process of MM2 calculation. energy has an increasing trend. With a further increase of –10 –10 Z coordinate from 9.7 × 10 m to 14.6 × 10 m, the CONCLUSIONS binding energy rises sharply. When the Z coordinate of –10 –10 C2 changes from 14.6 × 10 m to 35.3 × 10 m, the Furfuryl mercaptan-β-cyclodextrin inclusion complex values of binding energy keep almost unchanged. was successfully synthesized in this work. Some cha- The more energy is released during the complexation racteristic peaks of furfuryl mercaptan disappeared in process, the smaller the binding energy is, and the more the FTIR spectra of furfuryl mercaptan-β-cyclodextrin stable the furfuryl mercaptan-β-cyclodextrin inclusion inclusion complex, and blue-shifting hydrogen bond of complex becomes. The binding energy for the most stable β-cyclodextrin occurred after interaction with furfuryl furfuryl mercaptan-β-cyclodextrin inclusion complex is mercaptan. The peaks at 12.7, 10.8, 9.4 and 6.4 in the –1 –10 –77.0 kJ mol at –1.96 × 10 m. By MM2 calculation, XRD curve of β-cyclodextrin shifted to lower 2θ angles geranial-monochlorotriazinyl-β-cyclodextrin, difurfuryl of 12.2, 10.4, 8.7, and 6.0 respectively in the XRD curve disulfi de-β- cyclodextrin, neral-momochlorotriaziny-β- of furfuryl mercaptan-β-cyclodextrin inclusion complex. cyclodextrin, and menthol-HP-β-cyclodextrin inclusion These changes in FTIR and XRD gave supporting evi- complex were previously investigated and the calculated dence for the successful formation of furfuryl mercaptan- binging energy values were –135.2, –162, –143, –127 kJ β-cyclodextrin inclusion complex. The boiling point of –1 15, 17, 23 mol respectively . Compared with these previous furfuryl mercaptan is 154–155 C, while the mass loss –1 values, –77.0 kJ mol is the largest binding energy. It still occurred from furfuryl mercaptan-β-cyclodextrin means that furfuryl mercaptan-β-cyclodextrin inclusion inclusion complex in the temperature range of 155 C complex is relatively unstable compared with difurfuryl to the decomposition temperature of β-cyclodextrin. Figure 7. The MM2-computed structure of furfuryl mercaptan-β-cyclodextrin with the minimum energy 40 Pol. J. Chem. Tech., Vol. 23, No. 4, 2021 It further demonstrated that the encapsulation of fur- 10. Yildiz, Z.I., Celebioglu, A., Kilic, M.E., Durgun, furyl mercaptan in β-cyclodextrin was successful and E. & Uyar, T. (2018). Fast-dissolving carvacrol/cyclode- that long-lasting effect and thermal stability of furfuryl xtrin inclusion complex electrospun fi bers with enhan- mercaptan were improved. Using MM2 calculation, the ced thermal stability, water solubility, and antioxidant structure of furfuryl mercaptan-β-cyclodextrin inclusion activity. J. Mater. Sci. 53, 15837–15849. DOI: 10.1007/ complex was optimized and the minimum binding energy s10853-018-2750-1. was calculated. This data is helpful to understand the 11. Saffarionpour, S. (2019). Nanoencapsulation of interaction of furfuryl mercaptan and β-cyclodextrin. hydrophobic food fl avor ingredients nanoencapsulation Encapsulation of furfuryl mercaptan by the formation of hydrophobic food fl avor ingredients. Food Bioprocess of inclusion complex is a possible way to enhance the Tech. 12, 1157–1173. DOI: 10.1007/s11947-019-02285-z. stability of furfuryl mercaptan, control its release, impro- 12. Pires, F.Q., Pinho, L.A, Freire, D.O., Silva, I.C.R., Sa- ve its aroma retention, and enhance its water solubility. Barreto, L.L., Cardozo-Filho, L., Gratieri, T., Gelfuso, G.M. Furfuryl mercaptan-β-cyclodextrin inclusion complex can & Cunha-Filho, M. (2019). Thermal analysis used to guide be widely used in fl avor, fragrance and food industries. the production of thymol and Lippia origanoides essential oil inclusion complexes with cyclodextrin. J. Therm. Anal. Calorim. 137, 543–553. DOI: 10.1007/s10973-018-7956-6. ACKNOWLEDGMENTS 13. Ikeda, H., Fukushige, Y., Matsubara, T., Inenaga, The authors thank the fi nancial supports provided by the M., Kawahara, M., Yukawa, M., Fujisawa, M., Yukawa, E. National Key R&D Program of China (2016YFA0200300), & Aki, H. (2016). Improving water solubility of nateglinde National Natural Science Found of China (31972196), Shan- by complexation of β-cyclodextrin. J. Therm. Anal. Calo- ghai Alliance Program (LM201844), and Gaofeng & Gaoyuan rim. 123, 1847–1850. DOI: 10.1007/s10973-015-4714-x. Project for University Academic Program Development. 14. Zhu, G., Xiao, Z. & Zhu, G. 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Formation and characterization of furfuryl mercaptan-β-cyclodextrin inclusion complex and its thermal release characteristics

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10.2478/pjct-2021-0035
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Abstract

Polish Journal of Chemical Technology Pol. J. Chem. T , 23, 4,ech., V 35—40, 10.2478/pjct-2021-0035 ol. 23, No. 4, 2021 35 Formation and characterization of furfuryl mercaptan-β-cyclodextrin inclusion complex and its thermal release characteristics 1 *,1 *,1 2 Guangyong Zhu , Zuobing Xiao , Gengfa Yu , Guangxu Zhu , Yunwei Niu, Junhua Liu Shanghai Institute of Technology, No.100 Haiquan Road, Shanghai, 201418, PR China 3035 Sable Ridge Dr. Ottawa. ON K1T 3R9, Canada Corresponding authors: e-mail: zbingxiao@sina.com, genquan1@sina.com Furfuryl mercaptan has the aroma characteristics of coffee. However, it is unstable during storage of coffee brew and roasted coffee. In order to enhance the stability of furfuryl mercaptan, furfuryl mercaptan- β-cyclodextrin inclusion complex was synthesized using the precipitation method in this work. Fourier transform infrared spectroscopy, x-ray diffraction, and thermogravimetric analysis (TG) were used to characterize the resulting products. The interaction of furfuryl mercaptan with β-cyclodextrin was inve- stigated by the molecular mechanics (MM) method. These changes in FTIR and XRD gave supporting evidence for the successful formation of furfuryl mercaptan-β-cyclodextrin inclusion complex. The TG results showed that the formation of furfuryl mercaptan-β-cyclodextrin inclusion complex could improve the thermal stability of furfuryl mercaptan and provide a long-lasting effect. The structure of furfuryl mercaptan-β-cyclodextrin inclusion complex with the minimum energy was obtained by MM2 calculation, –1 –10 and the minimum binding energy was –77.0 kJ mol at –1.96 × 10 m. Keywords: Furfuryl mercaptan-β-cyclodextrin, Inclusion complex, XRD, FTIR, Thermal analysis, Molecular mechanics. INTRODUCTION of coffee and affect the quality and stability of coffee aroma, which has become a problem that plagues the Furfuryl mercaptan (see Fig. 1), an important aroma industry . How to reduce the loss of furfuryl mercaptan, constituent of fl avor, was found in coffee, sesame seed keep its aroma stable, and improve the aroma quality oil, popcorn, grilled pork, cooked beef, and roasted 1 of coffee, has become an urgent problem to be solved chicken . It has the aroma characteristics of coffee, in the industry. How to effectively enhance the stability sesame, onion, garlic, and meat. Furfuryl mercaptan has of furfuryl mercaptan, control its release, improve its been widely used in baked goods, gelatins, alcoholic and aroma retention, and enhance its water solubility is of nonalcoholic beverages, condiments, relishes, frozen dairy great signifi cance. gravies, hard and softy candy, and meat products. Furfuryl Core materials can be encapsulated in wall materials mercaptan, which is soluble in oils but insoluble in water, o 1 by formation of inclusion complex. It can realize the is a colorless oily liquid with boiling point 154–155 C . protection of core materials and improve their stabil- Furfuryl mercaptan exerts an important effect on coffee ity. Therefore, by encapsulation of furfuryl mercaptan fl avor profi les. It is a key ingredient of the characteristic to form inclusion complex, it is possible to realize coffee fl avor. In coffee fl avor fi eld, the identifi cation the protection of furfuryl mercaptan and enhance its of furfuryl mercaptan was considered as a milestone . stability. Among many wall materials, β-cyclodextrin, However, because of physical volatile loss and chemical as a safe and environmentally friendly wall material, reaction, its odor quality is unstable. Due to its interac- tion with the melanoidins (which were formed during has attracted much attention and becomes a research 8, 9 the Maillard reaction, e.g., coffee roasting), furfuryl hotspot . β-cyclodextrin molecule has a slightly tapered mercaptan is unstable during storage of coffee brew and hollow cylinder three-dimensional ring structure formed roasted coffee . Melanoidins can promote the degradation by glucopyranose units. Its outer surface has hydrophilic of furfuryl mercaptan . It was one of the major reasons characteristics, while the inside of the molecule has for the sulfury-roasty odor quality decrease. When fur- hydrophobic characteristics because of the C-H bond furyl mercaptan was added to water plus melanoidins, shielding effect. Various organic molecules can be en- approximately 50% of furfuryl mercaptan was lost after capsulated in its hydrophobic cavity to form inclusion 10 min of storage and it was nearly absent after 30 min complexes and change the physicochemical properties of storage . Through the reactive quinone converted of these entrapped molecules . In most cases, inclusion from hydroxyhydroquinone, furfuryl mercaptan can also complex formation with β-cyclodextrin can improve stabil- 3, 6 react with hydroxyhydroquinone . Furthermore, furfuryl ity of poorly stable substances . Pires et al. found that mercaptan is unstable and tends to polymerize when it the shelf-life increase achieved for thymol standard was 3, 6 was heated in the presence of mineral acids . 354% and Lippia origanoides essential oil had a stability The reactions of furfuryl mercaptan and physical 12 increase of about 45% . Ikeda et al. found that water volatile loss are the main reasons for loss of the aroma solubility of anionic nateglinide could be improved by complexation of β-cyclodextrin . Furfuryl mercaptan-β-cyclodextrin inclusion complex was synthesized using the precipitation method in this work to improve the stability of furfuryl mercaptan, Figure 1. Furfuryl mercaptan chemical structure formula control its release, improve its aroma retention, and 36 Pol. J. Chem. Tech., Vol. 23, No. 4, 2021 enhance its water solubility. Fourier transform infrared mA and 40 kV, Cu Kα radiation was adopted. The 2θ o o (FTIR) spectroscopy, thermogravimetric (TG) analysis, range was 5 to 70 . and x-ray diffraction (XRD) were used to characterize the Characterization of furfuryl mercaptan release from its resulting products. The formation of furfuryl mercaptan- β-cyclodextrin inclusion complex was investigated by the inclusion complex by thermogravimetric (TG) analysis molecular mechanics (MM) method at the molecular The rate of mass loss and mass loss of samples were per- level. Binding energy calculation and structure optimiza- formed with a TGA-Q5000IR thermogravimetric analyzer tion were performed with MM2. (TA Instruments, USA). About 5 mg of β-cyclodextrin or furfuryl mercaptan-β-cyclodextrin inclusion complex EXPERIMENTAL in a ceramic crucible was weighed. The heating rate o –1 adopted in the pyrolysis process was 10 C min . High Materials purity nitrogen gas was used to avoid oxidation during Furfuryl mercaptan (food grade, C H OS, molecular 5 6 –1 the pyrolysis process and the gas fl ow was 20 ml min . weight 114, colorless oily liquid) was purchased from Guangzhou Levon Flavor & Fragrance Technology Co., Binding energy calculation and structure optimization Ltd. Anhydrous ethanol was of analytical grade and was by molecular mechanics (MM) calculations provided by Shanghai Sinopharm Chemical Reagent Co., Ltd. Deionized water adopted throughout the experi- MM2 calculations were adopted to examine furfuryl ments was produced in our laboratory. β-cyclodextrin mercaptan-β-cyclodextrin inclusion complex formation (C H O , molecular weight 1134, white crystalline 42 70 35 at the molecular level. Binding energy calculation and powder) was of pharmaceutical grade and was purchased structure optimization were carried out by Chem3D Ultra from Shandong Binzhou Zhiyuan Bio-Technology Co., (CambridgeSoft Corporation, MA, USA). The process of Ltd. Without out further purifi cation, all the raw materi- encapsulation of furfuryl mercaptan in β-cyclodextrin was als were used directly in the experiment. simulated by successively changing the Z coordinate of Methods the furfuryl mercaptan atoms after properly orientating β-cyclodextrin and furfuryl mercaptan molecules. The Formation of furfuryl mercaptan-β-cyclodextrin inclu- docking strategy as described in references was used, sion complex i.e. push furfuryl mercaptan molecule stepwise through With some modifi cations, the precipitation method the β-cyclodextrin orifice minimizing the energy of as described in references was used to prepare furfuryl 15, 17 14, 15 the complex at each step . The position of furfuryl mercaptan-β-cyclodextrin inclusion complex . Firstly, 5 g of β-cyclodextrin was added to 93 g of deionized water mercaptan molecule relative to β-cyclodextrin molecule and was stirred to form a suspension. The temperature was referred to the Z coordinate of C2 atom of furfuryl of the suspension was kept at 35 C. Then, 2 g furfuryl mercaptan. Furfuryl mercaptan moving direction during mercaptan was slowly added to this suspension. Excess MM2 calculation is shown in Fig. 2. furfuryl mercaptan was used in the experiment. Such molar ratio of furfuryl mercaptan to β-cyclodextrin was used to ensure the cavities of β-cyclodextrin molecules containing furfuryl mercaptan molecules. After the ad- dition of furfuryl, the temperature of the suspension remained at 35 C and the mixture was continuously stirred for 3 h to form furfuryl mercaptan-β-cyclodextrin inclusion. The suspension was stored in a refrigerator at 5 C overnight. The precipitate was obtained with the vacuum fi ltration method. Anhydrous ethanol was used to wash the precipitate. After drying in a freeze drier (FD-1A-50), the product was kept in a desiccator for further analysis. Characterization by FTIR A Vertex 70 Fourier transform infrared spectrometer (Bruker, Germany) was adopted to determine the FTIR spectra of furfuryl mercaptan, β-cyclodextrin and furfuryl mercaptan-β-cyclodextrin inclusion complex. The frequ- –1 ency range was 400–4000 cm . Characterization by XRD A D/Max 2000X X-ray diffractometer (Rigaku Corpo- ration, Japan) was used to determine x-ray diffraction of the β-cyclodextrin and furfuryl mercaptan-β-cyclodextrin Figure 2. Furfuryl mercaptan moving direction during MM2 14, 16 inclusion complex as described in references . At 100 calculation Pol. J. Chem. Tech., Vol. 23, No. 4, 2021 37 RESULTS AND DISCUSSIONS stic strong peaks of furfuryl mercaptan such as at 1012 –1 and 735 cm disappear as shown in the FTIR spectra of furfuryl mercaptan-β-cyclodextrin inclusion complex. The results of FTIR of furfuryl mercaptan, β-cyclodextrin The changes and the blue-shifting hydrogen bond of and furfuryl mercaptan-β-cyclodextrin inclusion complex β-cyclodextrin after interaction with furfuryl mercaptan Fig. 3 shows the FTIR curves of furfuryl mercaptan, give evidence of successful encapsulation of furfuryl β-cyclodextrin and furfuryl mercaptan-β-cyclodextrin inc- mercaptan in β-cyclodextrin. lusion complex. The obvious FTIR peaks of β-cyclodextrin –1 –1 –1 –1 appear at 3356 cm , 2922 cm , 1643 cm , 1404 cm , The results of XRD of β-cyclodextrin and furfuryl –1 –1 –1 –1 –1 1244 cm , 1151 cm , 1030 cm , 937 cm and 848 cm . mercaptan-β-cyclodextrin inclusion complex As shown in Fig. 3, except for some minor changes in As an effective instrument, XRD can be used to deter- peak position, the FTIR curves of β-cyclodextrin and mine the formation of furfuryl mercaptan-β-cyclodextrin furfuryl mercaptan-β-cyclodextrin inclusion complex have 16 inclusion complex . The XRD peaks of β-cyclodextrin –1 a similar shape. The broad peaks appear at 3356 cm will change after the formation of the furfuryl mer- –1 and 3337 cm in the FTIR curves of β-cyclodextrin and captan inclusion complex. The XRD curves of the furfuryl mercaptan-β-cyclodextrin inclusion complex re- furfuryl mercaptan-β-cyclodextrin inclusion complex and spectively can be attributed to stretching (O-H) vibration β-cyclodextrin are shown in Fig. 4. of hydroxyl groups in β-cyclodextrin molecule . After the formation of furfuryl mercaptan-β-cyclodextrin inclusion complex, the peak caused by the stretching vibration of hydroxy groups moved towards the low band and blue- –1 -shift was observed. The strong sharp peaks at 1030 cm –1 and 1032 cm in the FTIR curves of β-cyclodextrin and furfuryl mercaptan-β-cyclodextrin inclusion complex re- spectively can be assigned to stretching (C-O) vibration. However, after the formation of the inclusion complex, the peak moved towards the high band and red-shift was –1 observed. The shoulder peak at 1151 cm did not change before and after the formation of furfuryl mercaptan-β- cyclodextrin inclusion complex. The weak peak appearing –1 at 2565 cm in the FTIR curve of furfuryl mercaptan is due to stretching (S-H) vibration . Owing to furan ring stretching (C-H) vibration, a weak peak occurs at –1 3123 cm . Because of the furan ring stretching (C=C) –1 vibration, three peaks occur at 1597, 1501 and 1420 cm Figure 4. The XRD curves of β-cyclodextrin and furfuryl respectively in the FTIR curve of furfuryl mercaptan. The mercaptan-β-cyclodextrin inclusion complex –1 peaks at 1254, 1151 and 1012 cm can be assigned to 19–21 furan ring asymmetrical stretching (C-O-C) vibration . In the XRD curve of β-cyclodextrin, a strong peak –1 o The strong sharp peak that occurs at 735 cm is due to appears at 12.7 as shown in Fig. 4. However, in the the out-of-plane bending (C-H) vibration of furan ring. XRD curve of the furfuryl mercaptan-β-cyclodextrin In addition to the small peaks of furfuryl mercaptan at inclusion complex, this peak shifts to 12.2 . The peak –1 3123, 2526, 1597, 1510 and 1420 cm , these characteri- intensity decrease can also be observed. Furthermore, the peaks at 10.8, 9.4, and 6.4 in the XRD curve of β-cyclodextrin also shift to lower 2θ angles of 10.4, 8.7, and 6.0 respectively in the XRD curve of furfuryl mercaptan-β-cyclodextrin inclusion complex. A similar phenomenon for peak shift was also found in the XRD patterns of mentha-8-thiol-3-one-β-cyclodextrin inclusion complex and menthyl acetate-β-cyclodextrin inclusion 14, 22 complex . The increase in the intensity of the peak at 10.4 in the XRD pattern of β-cyclodextrin can be observed compared to the peak at 10.8 . However, the peaks at 16.6 and 19.0 shift to a higher 2θ angle of 16.9 o o and 19.4 , respectively, and the peaks at 13.4 and 18.1 disappear after the interaction of β-cyclodextrin and furfuryl mercaptan. Compared with β-cyclodextrin, some new peaks appear at 14.5, 15.2, 15.9 and 17.5 in the XRD curve of furfuryl mercaptan-β-cyclodextrin inclusion complex. The encapsulation of furfuryl mercaptan mole- cule in the cavity of β-cyclodextrin molecule may cause the XRD peaks shifting and the new ones appearing in Figure 3. The FTIR spectra of β-cyclodextrin (a), furfuryl the complex. Like the results of FTIR, these changes in mercaptan-β-cyclodextrin inclusion complex (b), and furfuryl mercaptan (c) XRD further give another supporting evidence for the 38 Pol. J. Chem. Tech., Vol. 23, No. 4, 2021 successful formation of furfuryl mercaptan-β-cyclodextrin cules might be expelled from the hydrophobic cavities of inclusion complex. β-cyclodextrin molecules . So, the water that is included furfuryl mercaptan-β-cyclodextrin inclusion complex can Furfuryl mercaptan thermal release characteristics from be ignored. Therefore, in the mass loss curve of furfuryl furfuryl mercaptan-β-cyclodextrin inclusion complex mercaptan-β-cyclodextrin inclusion complex, the mass loss Thermal analysis is another effective method of in- occurring in the fi rst stage may be explained simply as vestigation of the interaction between host and guest caused by the release of furfuryl mercaptan. In the fi rst molecules. The inclusion compound stoichiometry stage, the mass loss of furfuryl mercaptan-β-cyclodextrin can also be evaluated by thermal analysis. Therefore, inclusion complex is approximately 8%. Therefore, the thermal analysis was adopted in the experiment to loading capacity of furfuryl mercaptan, which is defi - study the interaction between furfuryl mercaptan and ned as the mass ratio of furfuryl mercaptan to furfuryl β-cyclodextrin. The rate of mass loss and mass loss curves mercaptan-β-cyclodextrin inclusion complex, is about of β-cyclodextrin and furfuryl mercaptan-β-cyclodextrin 8%. The molecular weights of furfuryl mercaptan and inclusion complex obtained are shown in Fig. 5. There β-cyclodextrin are 114 and 1134 respectively. Therefore, are three main stages in the mass loss curves of both the molar ratio of furfuryl mercaptan to β-cyclodextrin β-cyclodextrin and furfuryl mercaptan-β-cyclodextrin obtained from the mass loss is approximately 0.9:1. inclusion complex as shown in Fig. 5. The fi rst stage If guest:host stoichiometry is considered as 1:1, the refers to the temperature range from room temperature theoretical loading capacity of furfuryl mercaptan is to 290.6 C. In the fi rst stage, a slightly mass loss can be 9%. Therefore, the furfuryl mercaptan:β-cyclodextrin observed. The second stage refers to the temperature stoichiometry is close to 1:1. o o range from 290.6 C to 350 C. In the second stage, the Because of the thermal decomposition of β-cyclodextrin, major mass loss occurred. The third stage refers to the major mass loss can be observed in the second stage. Two o o temperature range from 350 C to 500 C. In the third strong peaks are appearing in the rate mass loss curves of stage, a slightly mass loss occurred again. both β-cyclodextrin and furfuryl mercaptan-β-cyclodextrin inclusion complex. In the third stage, a slightly mass loss occurred again because of the continuous decomposition of solid residuals of β-cyclodextrin at a very slow rate with the increase of temperature. Because the boiling point of furfuryl mercaptan is 154–155 C, furfuryl mercaptan should evaporate away completely before it reaches the boiling point during the pyrolysis process. However, the mass loss still oc- curred from 155 C to the decomposition temperature of β-cyclodextrin. It can be observed obviously from the TG curve of furfuryl mercaptan-β-cyclodextrin inclusion complex in this stage. During the pyrolysis process, the encapsulated furfuryl mercaptan was gradually released from its inclusion complex. It further demonstrates the successful encapsulation of furfuryl mercaptan in β-cyclodextrin. By the formation of furfuryl mercaptan-β- cyclodextrin inclusion complex, long-lasting effect can be provided and the thermal stability of furfuryl mercaptan can be improved. The results of molecular mechanics calculations Binding energy is defi ned as the difference between the total energy of a furfuryl mercaptan-β-cyclodextrin inclusion complex molecule and the sum of the total ener- Figure 5. The TG and DTG curves of β-cyclodextrin and gy of furfuryl mercaptan and β-cyclodextrin molecules. furfuryl mercaptan-β-cyclodextrin inclusion complex In other words, binding energy can be used to measure In the TG curve of β-cyclodextrin, the slightly mass the energy required to break up a host-guest inclusion loss occurring in the fi rst stage can be mainly attributed complex molecule completely into its host molecule to desorption water; while in the mass loss curve of and guest molecule. To some extent, the combination furfuryl mercaptan-β-cyclodextrin inclusion complex, the and the interaction between β-cyclodextrin and furfuryl release of furfuryl mercaptan caused the main mass loss mercaptan can be refl ected by binding energy. Fig. 6 occurring in the fi rst stage. In the temperature range of shows the plot of binding energy vs. the Z coordinate 140 to 250 C, the TG curve of β-cyclodextrin is leveling of C2 of furfuryl mercaptan molecule. off while the mass loss curve of furfuryl mercaptan-β- During the development of the furfuryl mercaptan-β- cyclodextrin is downward sloping. The difference can cyclodextrin inclusion complex, energy is released, so the be attributed to the release of furfuryl mercaptan from value of binding energy is negative. When the Z coordinate –10 –10 furfuryl mercaptan-β-cyclodextrin inclusion complex. of C2 increases from –21.4 × 10 m to –10.9 × 10 m, Furfuryl mercaptan is soluble in oils and insoluble in the binding energies change slightly as shown in Fig. 6. –10 water. During the inclusion process, all the water mole- When Z coordinate increases from –10.9 × 10 m, a sharp Pol. J. Chem. Tech., Vol. 23, No. 4, 2021 39 disulfi de-β-CD, neral-momochlorotriaziny-β-cyclodextrin, geranial-monochlorotriazinyl-β-cyclodextrin and menthol- HP-β-cyclodextrin inclusion complex. In the process of MM2 calculation, the total energy consists of 1,4 van der Waals, bend, non-1,4 van der Waals, stretch, stretch-bend, dipole/dipole, and torsion energy. The minimum binding energy consists of the following com- –1 ponents: 1,4 van der Waals (4.2 kJ mol ), bend (–2.5 kJ –1 –1 mol ), non-1,4 van der Waals (–75.4 kJ mol ), stretch (–1.4 –1 –1 kJ mol ), stretch-bend (–0.6 kJ mol ), dipole/dipole (–0.4 –1 –1 kJ mol ), and torsion energy (–0.8 kJ mol ). Non-1,4 van der Waals energy contributes a lot to binding energy, and is the main reason for the stability of furfuryl mercaptan- β-cyclodextrin inclusion complex. Once furfuryl mercaptan molecule entered the cavity of β-cyclodextrin molecule, β-cyclodextrin changed its shape and furfuryl mercaptan molecule also made conformation adjustments to maximize Figure 6. Plot of binding energy vs. the Z coordinate of C2 the stabilization. The complexation geometries of furfuryl in furfuryl mercaptan molecule mercaptan molecule and β-cyclodextrin were constantly readjusted until the most stable inclusion complexes were fall in the value of binding energy can be observed. At –10 obtained. By MM2 calculation, the obtained structure of –10.6 × 10 m, the value of binding energy is –49.3 kJ –1 furfuryl mercaptan-β-cyclodextrin inclusion complex which mol . With the increase of Z coordinate of C2 to –1.96 –10 has the minimum energy is shown in Fig. 7. It is a relatively × 10 m, the minimum value of binding energy, –77.0 –1 stable structure after shape and conformation adjustments kJ mol , was obtained. When the Z coordinate of C2 –10 –10 of furfuryl mercaptan and β-cyclodextrin molecules during changes from –1.96 × 10 m to 9.7 × 10 m, binding the process of MM2 calculation. energy has an increasing trend. With a further increase of –10 –10 Z coordinate from 9.7 × 10 m to 14.6 × 10 m, the CONCLUSIONS binding energy rises sharply. When the Z coordinate of –10 –10 C2 changes from 14.6 × 10 m to 35.3 × 10 m, the Furfuryl mercaptan-β-cyclodextrin inclusion complex values of binding energy keep almost unchanged. was successfully synthesized in this work. Some cha- The more energy is released during the complexation racteristic peaks of furfuryl mercaptan disappeared in process, the smaller the binding energy is, and the more the FTIR spectra of furfuryl mercaptan-β-cyclodextrin stable the furfuryl mercaptan-β-cyclodextrin inclusion inclusion complex, and blue-shifting hydrogen bond of complex becomes. The binding energy for the most stable β-cyclodextrin occurred after interaction with furfuryl furfuryl mercaptan-β-cyclodextrin inclusion complex is mercaptan. The peaks at 12.7, 10.8, 9.4 and 6.4 in the –1 –10 –77.0 kJ mol at –1.96 × 10 m. By MM2 calculation, XRD curve of β-cyclodextrin shifted to lower 2θ angles geranial-monochlorotriazinyl-β-cyclodextrin, difurfuryl of 12.2, 10.4, 8.7, and 6.0 respectively in the XRD curve disulfi de-β- cyclodextrin, neral-momochlorotriaziny-β- of furfuryl mercaptan-β-cyclodextrin inclusion complex. cyclodextrin, and menthol-HP-β-cyclodextrin inclusion These changes in FTIR and XRD gave supporting evi- complex were previously investigated and the calculated dence for the successful formation of furfuryl mercaptan- binging energy values were –135.2, –162, –143, –127 kJ β-cyclodextrin inclusion complex. The boiling point of –1 15, 17, 23 mol respectively . Compared with these previous furfuryl mercaptan is 154–155 C, while the mass loss –1 values, –77.0 kJ mol is the largest binding energy. It still occurred from furfuryl mercaptan-β-cyclodextrin means that furfuryl mercaptan-β-cyclodextrin inclusion inclusion complex in the temperature range of 155 C complex is relatively unstable compared with difurfuryl to the decomposition temperature of β-cyclodextrin. Figure 7. The MM2-computed structure of furfuryl mercaptan-β-cyclodextrin with the minimum energy 40 Pol. J. Chem. Tech., Vol. 23, No. 4, 2021 It further demonstrated that the encapsulation of fur- 10. Yildiz, Z.I., Celebioglu, A., Kilic, M.E., Durgun, furyl mercaptan in β-cyclodextrin was successful and E. & Uyar, T. (2018). Fast-dissolving carvacrol/cyclode- that long-lasting effect and thermal stability of furfuryl xtrin inclusion complex electrospun fi bers with enhan- mercaptan were improved. Using MM2 calculation, the ced thermal stability, water solubility, and antioxidant structure of furfuryl mercaptan-β-cyclodextrin inclusion activity. J. Mater. Sci. 53, 15837–15849. DOI: 10.1007/ complex was optimized and the minimum binding energy s10853-018-2750-1. was calculated. This data is helpful to understand the 11. Saffarionpour, S. (2019). Nanoencapsulation of interaction of furfuryl mercaptan and β-cyclodextrin. hydrophobic food fl avor ingredients nanoencapsulation Encapsulation of furfuryl mercaptan by the formation of hydrophobic food fl avor ingredients. Food Bioprocess of inclusion complex is a possible way to enhance the Tech. 12, 1157–1173. DOI: 10.1007/s11947-019-02285-z. stability of furfuryl mercaptan, control its release, impro- 12. Pires, F.Q., Pinho, L.A, Freire, D.O., Silva, I.C.R., Sa- ve its aroma retention, and enhance its water solubility. Barreto, L.L., Cardozo-Filho, L., Gratieri, T., Gelfuso, G.M. 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Journal

Polish Journal of Chemical Technology – de Gruyter

Published: Dec 1, 2021

Keywords: Furfuryl mercaptan-β-cyclodextrin; Inclusion complex; XRD; FTIR; Thermal analysis; Molecular mechanics

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