Enhanced catalytic activity of carbonate buffer for isomerization of D-galactose into D-tagatose

D-tagatose (Tag) is a rare monosaccharide with health benefits. In this work, catalytic activity of CAPS (pH 10.4), carbonate (pH 10.4), triethylamine (pH 11.2), quinuclidine (pH 11.5), and L-arginine (pH 12.5) was examined for isomerization of D-galactose (Gal) to Tag. The maximum yields of Tag were 15.0 % with CAPS, 15.2 % with carbonate, 19.3 % with triethylamine, 19.6 % with quinuclidine, and 18.1 % with L-arginine. Despite identical pH, the Tag formation rate with carbonate buffer was 3-8 times higher than with CAPS. For carbonate buffer, the reaction orders for hydroxide and carbonate anions were ∼ 1 and ∼ 0, respectively. Operando NMR studies of Gal-1-¹³C isomerization in carbonate and CAPS buffers indicate similar tautomeric distributions of the substrate in both buffers. The deuterium kinetic isotope effect demonstrated that carbonate facilitates isomerization through a proton transfer mechanism, with hydroxide anions acting as the catalytically active species whereas carbonate anions stabilize the enediolate anion and/or the transition state.