2D NMR OF THE METABOLIC ANTIOXIDANT DIHYDROLIPOIC ACID AND ITS DERIVATIVES

Dihydroplipoate and lipoate are physiological thiols which in addition to their coenzyme functions exhibit antioxidant activity. For NMR inv estigations of their protective mechanism in biological and model syst ems it is very important to know the full assignment of proton and car bon spectra of these molecules in water (D2O). An unambiguous assignme nt of proton and carbon NMR spectra has been made for dihydrolipoate a nd its short chain derivatives bisnor-and tetranor-lipoic acid in D2O and CDCl3 solutions using 2D NMR methods. Oxidation of dihydrolipoic a cid produces substantial electron density deshielding of the carbons n earest to the SH groups with the largest shift found at the inner SH g roup (17.79 ppm in D2O, 16.93 in CDCl3) and almost no changes in the t ail portion of the molecule. However, bisnor-dihydrolipoic acid and es pecially 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 pos ition 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.