Journal Article Structural analysis of crystalline R(+)-α-lipoic acid-α-cyclodextrin complex based on microscopic and spectroscopic studies

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

16 ( 10 )  , pp.24614 - 24628 , 2015-10-16 , MDPI
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. © 2015 by the authors; licensee MDPI, Basel, Switzerland.

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