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Int. J. Mol. Sci. 2015, 16(10), 24614-24628; doi:10.3390/ijms161024614

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

1
Graduate School of Medicine, Kobe University, Kobe 650-0017, Japan
2
College of Science and Engineering, Kanazawa University, Kanazawa 920-1192, Japan
3
Department of Pharmaceutical Engineering, School of Pharmacy, Aichi Gakuin University, Nagoya 464-8650, Japan
4
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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Abstract

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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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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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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