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Int. J. Mol. Sci. 2015, 16(10), 24614-24628;

Structural Analysis of Crystalline R(+)-α-Lipoic Acid-α-cyclodextrin Complex Based on Microscopic and Spectroscopic Studies

Graduate School of Medicine, Kobe University, Kobe 650-0017, Japan
College of Science and Engineering, Kanazawa University, Kanazawa 920-1192, Japan
Department of Pharmaceutical Engineering, School of Pharmacy, Aichi Gakuin University, Nagoya 464-8650, Japan
CycloChem Bio Co., Ltd., Kobe 650-0047, Japan
Author to whom correspondence should be addressed.
Academic Editor: Bing Yan
Received: 4 September 2015 / Revised: 5 October 2015 / Accepted: 8 October 2015 / Published: 16 October 2015
(This article belongs to the Collection Bioactive Nanoparticles)
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R(+)-α-lipoic acid (RALA) is a naturally-occurring substance, and its protein-bound form plays significant role in the energy metabolism in the mitochondria. RALA is vulnerable to a variety of physical stimuli, including heat and UV light, which prompted us to study the stability of its complexes with cyclodextrins (CDs). In this study, we have prepared and purified a crystalline RALA-αCD complex and evaluated its properties in the solid state. The results of 1H NMR and PXRD analyses indicated that the crystalline RALA-αCD complex is a channel type complex with a molar ratio of 2:3 (RALA:α-CD). Attenuated total reflection/Fourier transform infrared analysis of the complex showed the shift of the C=O stretching vibration of RALA due to the formation of the RALA-αCD complex. Raman spectroscopic analysis revealed the significant weakness of the S–S and C–S stretching vibrations of RALA in the RALA-αCD complex implying that the dithiolane ring of RALA is almost enclosed in glucose ring of α-CD. Extent of this effect was dependent on the direction of the excitation laser to the hexagonal morphology of the crystal. Solid-state NMR analysis allowed for the chemical shift of the C=O peak to be precisely determined. These results suggested that RALA was positioned in the α-CD cavity with its 1,2-dithiolane ring orientated perpendicular to the plane of the α-CD ring. View Full-Text
Keywords: cyclodextrin; lipoic acid; ATR/FT-IR; microscopic Raman; solid-state NMR cyclodextrin; lipoic acid; ATR/FT-IR; microscopic Raman; solid-state NMR

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Ikuta, N.; Endo, T.; Hosomi, S.; Setou, K.; Tanaka, S.; Ogawa, N.; Yamamoto, H.; Mizukami, T.; Arai, S.; Okuno, M.; Takahashi, K.; Terao, K.; Matsugo, S. Structural Analysis of Crystalline R(+)-α-Lipoic Acid-α-cyclodextrin Complex Based on Microscopic and Spectroscopic Studies. Int. J. Mol. Sci. 2015, 16, 24614-24628.

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