| Abstract: | Carbon 13 NMR spectra have been obtained for aqueous solutions of DL-2-(alpha-hydroxyethyl)thiamin, DL-2-(alpha-hydroxybenzyl)thiamin, DL-2-(alpha-hydroxybenzyl)oxythiamin, and related N-3 methyl and N-3 benzyl analogs. The unusually large downfield shift of the 13C resonance of C-2 of hydroxyethylthiamin suggests that this carbon bears a partial positive charge. This result stands in contrast to results of x-ray crystallographic studies of hydroxyethylthiamin, which place a partial negative charge on C-2 (Pletcher, J., and Sax, M. (1974) J. Am. Chem. Soc. 96, 155-165). A partial positive charge on C-2 helps to explain the facility of carbanion formation at the alpha carbon both enzymatically and in model systems. The rates of proton-deuteron exchange of (C-alpha)-H with solvent deuterium, and of release of aldehyde to regenerate thiamin have been measured for hydroxyethylthiamin and analogs. The differences in kinetic acidity of (C-alpha)-H and of rates of aldehyde release are rationalized in terms of differing electron-withdrawing abilities of the substituents attached to N-3, and appear not to be related to intramolecular basic catalysis of these processes by the C-4' amino group. |
