Pulse polarography VI. Mutarotation of D-glucose

Abstract

The strictly rate-controlled reduction of D-glucose at a dropping mercury electrode in aqueous phosphate buffer solutions was studied with better accuracy (using pulse polarography) than previously attained1,2. The rate constants in the reaction scheme α⇌k'1k1γ⇌k2k'2β, where γ is the reducible free-aldehyde form, were determined. Contrary to k′1 and k′2, the constants k1 and k2, which cannot be affected by the choice of an erroneous diffusion coefficient, change linearly with the buffer concentration, cb, within experimental error, as expected for an acid-base catalyzed process. The fact that this does not equally apply to k′1 and k′2, was attributed to the use of a diffusion coefficient, D, which is relevant to α- and β-, but not to γ-glucose. Assuming that y∞/c*=(49±4)×10−6, which was found for the relative concentration of γ-glucose in pure water at 25°, applies also to the buffered solutions, a set of diffusion coefficients, D*, related to the transport of γ-glucose, could be calculated. These are very different from D. Most likely, γ-glucose associates much more strongly with the phosphate ions of the buffer than α- or β-glucose. Corresponding with the D*-values, new and apparently much better values of k′1 and k′2, namely k′1* and k′2*, were obtained.