Alkaline degradation of monosaccharides V: Kinetics of the alkaline isomerization and degradation of monosaccharides

Article abstract of DOI:10.1002/recl.19871060201

A new kinetic model for the alkaline isomerization and degradation of monosaccharides is presented which includes the interconversion of D-glucose, D-fructose, D-mannose and D-psicose.Computer simulations used in this model fit the experimental data and allow the determination of all relevant rate constants.Additionally, it has been established that, for the alkaline degradation of either D-fructose, D-glucose or D-mannose, substantial amounts of acidic products, i.e. ca. 65percent and ca. 20percent, are formed via D-fructose and D-psicose, respectively. - The influence of some reaction parameters on the pseudo-first-order rate constants involved in the kinetic model has been investigated.The enolization of monosaccharides appears to be the rate-limiting step in both the isomerization and the degradation reactions.The enolization rate is markedly enhanced at higher HO^- concentration or by the addition of calcium(II).Direct retro-aldolization of D-fructose and D-psicose occurs at -> > 10^-2 M or in the presence of calcium(II), leading to an additional increase in the degradation rate towards lactic acid.The shift of the isomerization equilibrium by borate towards D-fructose and D-psicose is accompanied by a decrease of the degradation rate constants, due to stabilization of the monosaccharides as their borate esters.