Studies on the stability of thiamine and its modified compounds. IX. Stability of thiamine-O-esters in acid aqueous solutions

Abstract

Quantitative studies were made on the anaerobic degradation of eight thiamine O-esters (B1-O-CO-R ; R=CH3, C3H7, C6H5, C6H4-p-NO2, OC2H5, OC4H9, OCH2CH2C6H5, and N(CH3)2) in aqueous solution at 100° in a pH range of 1 to 5. Each of these esters was degraded by a pseudo-first-order reaction and all of their minimum rate constants were found within a range of pH 2 to 3. These esters were found also to give thiamine and 2-methyl-4-amino-5-hydroxymethyl-pyrimidine (IX), together with the corresponding thiazole moiety, as their degradation products. The relative amount of thiamine formed from the respective esters varied with the pH of the solution as well as with the ester moiety. On the basis of an assumption that IX was produced not only from thiamine but also from each of the thiamine O-esters directly, the rate constants of the hydrolysis of the esters to thiamine, and of IX formation directly from these esters were calculated by using an analog computer. Hydrolysis of the esters to thiamine had a dominant dependence on pH and all of their minimum rates were found at about pH 3. IX was produced more directly from each of these esters than from thiamine. All of the rates of direct formation of IX from each of the esters were less affected by the variation of the ester moiety, while the rates of hydrolysis of the respective esters to thiamine were more affected.