Solvent effects on thiamine–enzyme model interactions. 2,3,4-Trimethylthiazolium iodide, a model for interaction with negative charges

Abstract

We have studied the solvent dielectric constant dependence of the ionic association process in 2,3,4-trimethylthiazolium iodide as a model for the interaction of the thiazolium ring of thiamine with a carboxylate or other negatively charged species in enzymic processes requiring thiamine pyrophosphate as a co-factor. The K_ass value for the ion pair (as measured by u.v. spectroscopy) increases as the solvent dielectric constant decreases from 3.4 in pure water to 175 in pure ethanol. The ion pair is subject to electrostatic stabilization as manifested by a linear log K_assvs. (dielectric constant)^–1 plot. This type of interaction would be stabilized in a hydrophobic environment, and may have a multiple role in enzymic processes requiring the thiamine pyrophosphate coenzyme.