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

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 wi...

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Bibliographic Details
Published inInternational journal of molecular sciences Vol. 16; no. 10; p. 24614
Main Authors Ikuta, Naoko, Endo, Takatsugu, Hosomi, Shota, Setou, Keita, Tanaka, Shiori, Ogawa, Noriko, Yamamoto, Hiromitsu, Mizukami, Tomoyuki, Arai, Shoji, Okuno, Masayuki, Takahashi, Kenji, Terao, Keiji, Matsugo, Seiichi
Format Journal Article
LanguageEnglish
Published Basel MDPI AG 01.10.2015
Subjects
Acids
Crystallization
Fourier transforms
Glucose
Spectrum analysis
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Summary: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.
ISSN:1661-6596
1422-0067
DOI:10.3390/ijms161024614