Analytical Sciences

Structural Characterization of a Cyclodextrin/l-menthol Inclusion Complex in the Solid-state by Solid-state NMR and Vibrational Circular Dichroism

Takahiro SAKAI,* Yumiko AKAGI,* Hisashi SUZUKI,* Mitsuki IRIE,** Tetsuya NAKAMURA,* Hisako SATO,*** and Izuru KAWAMURA**

*R&D Center, T. Hasegawa Co., Ltd., 29-7 Kariyado, Nakahara, Kawasaki, Kanagawa 211-0022, Japan
**Graduate School of Engineering Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama, Kanagawa, 240-8501, Japan
***Graduate School of Science and Engineering, Ehime University, 2-5 Bunkyo, Matsuyama, Ehime 790-8577, Japan

Hydrophobic and volatile flavor molecules can be encapsulated inside cyclodextrins (CyDs). Inclusion complexes are frequently used in solid or dispersed states in preserved food and cosmetics. In this study, the solid-state structures of spray-dried inclusion complexes of l-menthol in α-CyD and β-CyD were analyzed using ¹³C solid-state NMR and vibrational circular dichroism (VCD). The NMR signals of l-menthol and CyDs were identified in the physical mixture and the l-menthol inclusion complex of α- and β-CyD. The NMR signal of the isopropyl–methyl group of menthol in the α-CyD inclusion complex exhibited a large low-field shift, which suggested a steric hindrance between menthol and α-CyD. VCD exhibited specific changes in the intensity of bands corresponding to C–C vibrations in α-CyD and O–C stretching vibrations in l-menthol. Our results indicated that l-menthol specifically fitted the narrow space within α-CyD. The combined solid-state NMR and VCD analysis provided structural insights into the flavor inclusion complex in the solid-state.

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