Differential pulse polarographic study of thiamine (vitamin B₁) in neutral and acidic aqueous solutions

Abstract

The differential pulse (dp) polarograms of thiamine in neutral aqueous solutions exhibited six peaks at low depolarizer concentration (⋦10⁻⁴ mol dm⁻³) and only three peaks at concentrations ≥10⁻³ mol dm⁻³. Only one of these was found to correspond to the diffusion-controlled reduction of this compound at the dme and this was shown to be an irreversible two-electron process. The kinetic parameters derived from the dp polarograms were found to be in good agreement with those calculated from classical polarograms and were:E _1/2=−1·261 Vvs SCE,an _a=0·54 andD≈3·5×10⁻⁶ cm² sec⁻¹ for 10⁻³ mol dm⁻³ thiamine in 0·1 mol dm⁻³ acetate buffer (pH 6·5). The reduction product has been identified as dihydrothiamine. The effect of pH on the dpp of thiamine was studied in the pH range 0–7. In the pH region 5·5 to 7·0 only one peak attributable to the B₁ ⁺ form of thiamine is present. In the pH region 3·5–5·5 another dpp peak attributable to the protonated form (B₁H²⁺) of thiamine was also observed. At pHs less than 3 only one peak was observed which could be attributed to the doubly protonated form (B₁ H₂ ³⁺) of thiamine. Surfactants like triton-X-100 and CTABr were found to inhibit the electroreduction of thiamine due to the strong adsorption of these compounds on the dme. Thiamine itself was found to have an inhibitory effect on its own electroreduction, although to a smaller extent.