Abstract
The ring opening-closing reaction of the thiazolium moiety of thiamine was studied by the pH-stat method at 20° and 30°. The reaction could be represented by the following equations.
thiazolium salt_??_pseudo-base_??_thiol form;
k1=k1'(OH-); k4 =k4'(H+)
Between pH 7 and pH 11, the rate constant k2 was nearly equal to k3 but it was considerably larger than k1, and k4 in magnitude. The negative dependency of k1 and k4 on the ionic strength was observed. The rate constants, k1' and k4', were also obtained in the analogous reactions of 3-benzyl-4-methylthiazolium chloride, 4-methyl-3-phenylethylthiazolium bromide and 3, 4-dimethylthiazolium iodide.
Reference was made to the mechanism of the thiol exchange reactions between thiamine derivatives of disulfide type (IV) and thiols, and it was confirmed that the rapid ring closing reaction to the thiazolium salt hindered the formation of IV from thiol form of thiamine below neutral pH.
