Thermodynamic considerations in co-ordination. Part VII. Solubility of the histidine–H+ system and stability constants, free energies, enthalpies, and entropies of protonation of histidine and tryptophan and of formation of their manganese(II), iron(II), cobalt(II), nickel(II), copper(II), and zinc(II) complexes

Abstract

Acidic or basic perchlorate solutions of histidine precipitate histidine hydroperchlorate (histidine HCIO₄,H₂O), solubility product –log K_s= 17·65 in 3M-perchlorate at 25·0°. Tryptophan has higher pK values than histidine in this medium. Near pH 7·4 (the pH of blood) the formation constants indicate that the complexes of Mn^II, Fe^II, Co^II, and Ni^II with histidine are tridentate and that those with tryptophan are bidentate. The zinc–histidine complex is less stable than that of cobalt, whereas for all other amino-acids the reverse is true. Calorimetry shows that the main driving force behind metal ion–amino-acid complex formation is an enthalpy effect, the bond strengths obeying the Irving–Williams order. It is suggested that the zinc ion is tetrahedral, the carboxy-group not being chelated.