Schepkin V, Kawabata T, Tritschler H J, Packer L
Department of Molecular & Cell Biology, University of California, Berkely 94720-3200, USA.
Free Radic Res. 1996 Sep;25(3):195-205. doi: 10.3109/10715769609149045.
Dihydroplipoate and lipoate are physiological thiols which in addition to their coenzyme functions exhibit antioxidant activity. For NMR investigations of their protective mechanism in biological and model systems it is very important to know the full assignment of proton and carbon spectra of these molecules in water (D2O). An unambiguous assignment of proton and carbon NMR spectra has been made for dihydrolipoate and its short chain derivatives bisnor- and tetranor-lipoic acid in D2O and CDCl3 solutions using 2D NMR methods. Oxidation of dihydrolipoic acid produces substantial electron density deshielding of the carbons nearest to the SH groups with the largest shift found at the inner SH group (17.79 ppm in D2O, 16.93 in CDCl3) and almost no changes in the tail portion of the molecule. However, bisnor-dihydrolipoic acid and especially tetranor-dihydrolipoic acid have more carbon deshielding near the outer SH group of the molecule which correlates with their known diminished ion chelating activity. Moreover, the proton triplet at position 2 of lipoic acid has strong pH dependence (pK = 4.58) due to the close proximity to the carboxylic group and this feature may be used for monitoring pH.
二氢硫辛酸和硫辛酸是生理性硫醇，除了具有辅酶功能外，还具有抗氧化活性。对于它们在生物和模型系统中的保护机制进行核磁共振研究时，了解这些分子在水（D2O）中的质子和碳谱的完整归属非常重要。使用二维核磁共振方法，已对二氢硫辛酸及其短链衍生物双降和四降硫辛酸在D2O和CDCl3溶液中的质子和碳核磁共振谱进行了明确的归属。二氢硫辛酸的氧化会使最靠近SH基团的碳产生大量电子密度去屏蔽，在内侧SH基团处位移最大（在D2O中为17.79 ppm，在CDCl3中为16.93 ppm），而分子尾部几乎没有变化。然而，双降二氢硫辛酸，尤其是四降二氢硫辛酸，在分子外侧SH基团附近有更多的碳去屏蔽，这与其已知的离子螯合活性降低相关。此外，硫辛酸2位上的质子三重峰由于靠近羧基而具有很强的pH依赖性（pK = 4.58），这一特性可用于监测pH。
