Effect of mixed-ligand complex formation on the ionization of the pyrrole hydrogens of histamine and histidine

Abstract

Equilibrium studies have been carried out by pH titrimetry to determine the stability constants of the parent complexes of Cu^II Ni^II and Zn^II with histamine and histidine, and of the mixed-ligand complexes of these with, glycine, ethylenediamine, 4,5-dihydroxybenzene-1,3-disulphonate, and 2,2′-bipyridyl. From titrations in an alkaline medium it is concluded that nickel(II) and zinc(II) ions do not affect the ionization of the pyrrole hydrogen significantly compared to the free ligand, but Cu^II decreases the relevant pKvalues in the parent complexes by about three orders of magnitude. As a consequence of charge-transfer processes, the deprotonation can also be in-fluenced significantly by the formation of mixed-ligand complexes.