STABILITY OF THE ISOMERS OF DIHYDROTHIAMINE

Abstract

Stabilities of dilute solutions of n- and Ψ-TH2 in an alkaline solution and degradation by sulfite were investigated.
1. The stabilities of two isomers upon heating or standing at room temperature showed no difference; they were both unstable in a weakly acidic medium but relatively stable in a neutral medium. However, the n-form becomes labile in an alkaline medium, especially, it was readily degraded by heating, whereas the Ψ-form remained fairly stable upon heating.
2. Dilute solutions (final concentration, 17-33 μM) were quite unstable and the effect of sulfite was scarcely detectable, but at higher concentrations (final concentration, 3.7mM) both n- and Ψ-TH₂ were readily degraded (about 88% decomposed) by sulfite at pH 5.5-7.0 in the presence of 6 eqivalents of sulfite. In more acidic or alkaline solution, the degradation was decreased. Ψ-TH₂ was stable in an alkaline medium, being scarcely affected by sulfite, whereas n-TH₂ had a tendency to be degraded readily in the presence of sulfite.
3. These findings suggest that both n- and Ψ-TH₂ take the same, probably a quarternary form in a dilute aqueous solution, whereas in an alkaline solution, the Ψ-form takes a stable cyclic structure, and n-form is degraded gradually through a dihydro-form of thiamine and its labile thiol form.