Thermochemical studies of vitamin B12. Part I. Thermodynamic parameters for acid dissociation of aquocobalamin (vitamin B12a)

Abstract

The following thermodynamic data have been obtained for acid dissociation of aquocobalamin (Vitamin B_12a)→ hydroxocobalamin in aqueous solution at 25·0 °C and in an ionic background of 0·1M-NO₃^– ions: pK_a 7·62; ΔG⁰ 43·45 kJ mol^–1; ΔH⁰ 18·47 kJ mol^–1; and ΔS⁰–83·8 J K^–1 mol^–1. These results, obtained by potentiometric methods and direct calorimetry, are discussed. Attention is drawn to the invalidity of previous comparisons of ΔS for free and bound water molecules in this and similar systems.