Ternary complexes in solution. Comparison of the coordination tendency of some biologically important zwitterionic buffers toward the binary complexes of some transition metal ions and some amino acids

Potentiometric equilibrium measurements have been performed at (25.0 +/- 0. 1) degrees C and ionic strength I = 0.1 mol dm(-3) (KNO3) for the interacti on of glycine (aminoethanoic acid), serine (2-amino-3-hydroxypropanoic acid ), methionine (2-amino-4-(methylthio)butanoic acid), aspartic acid (aminobu tanedioic acid), glutamic acid (2-aminopropanedioic acid), and histidine (a lpha-amino-1H-imidazole-4-propanoic acid) and Cu(II), Co(II), Ni(II), Mn(II ), and Zn(II) with the biologically important secondary ligand zwitterionic buffers beta-hydroxy-4-morpholinepropanesulfonic acid (MOPSO), 4-morpholin epropanesulfonic acid (MOPS), 3-[bis(2-hydroxyethyl)amino]-2-hydroxy- acid (DIPSO), and 3-[N-tris(hydroxymethyl)methyl)amino]-2-hydroxypropanesulfonic acid (TAPSO) in 1:1:1 and 1:1:2 ratios, and the formation of various 1:1:1 ternary complexes and 1:1:2 quaternary complex species was inferred from t he potentiometric pH titration curves. Initial estimates of the formation c onstants of the resulting species and the acid dissociation constants of th e different amino acids and secondary ligands studied have been refined wit h the SUPERQUAD computer program. The order of stability of the different n ormal ternary complexes in the systems under investigation in terms of meta l ion follows generally the trend Cu(II) > Ni(II) > Co(II) > Zn(II)> Mn(II) .