The effect of co-ordination on ionisation. Part VI. The imidazole and histamine complexes of cobalamin Factor B

Abstract

Thermodynamic data have been obtained at 25° for the formation of cobalamin Factor B–imidazole and for the acid ionisation of the imino NH group in this complex. For the formation reaction ΔG°=–5·58 ± 0·03 kcal. mole^–1, ΔH°=–6·3 ± 0·5 kcal. mole^–1, and ΔS°=–2·4 ± 1·7 e.u. For the acid ionisation of the free imino-group of the bonded imidazole, pK₃°= 11·39 ± 0·03, ΔH₃°= 12·1 ± 1·0 kcal. mole^–1, and ΔS₃°=–12 ± 4 e.u. The acid ionisations of the non-bonded amino –NH₃⁺ and imino NH in factor B–histamine have also been investigated, and yield the following thermodynamic data: for the amino –NH₃⁺, pK₅°= 4·49 ± 0·02, ΔH₅°= 4·7 ± 0·9 kcal. mol^–1, and ΔS₅°=–5 ± 3 e.u.; for the imino NH, pK₆°= 11·00 ± 0·02, ΔH₆°= 11·7 ± 1·0 kcal. mol^–1, and ΔS₆°=–11 ± 3 e.u. The significance of these results is discussed.