Metal chelates of arginine and related ligands

The interaction of Ca(II), Co(II), Cu(II), Hg(II), Mg(II), Mn(II), Ni(II) and Zn(II) with arginine, citrulline, and ornithine has been studied by potentiometric measurement of hydrogen ion concentration at 25° and ionic strengths of 0·10 and 1·0 M. Dissociation constants of the ligands at 25° and 0·10 M ionic strength are reported, and formation constants of the chelates containing 1:1 and 1:2 molar ratios of metal ion to monoprotonated ligand are reported at 25° and ionic strengths of 0·10 and 1·0 for arginine and ornithine. Stabilities under the same conditions are calculated for the chelates of citrulline containing 1:1 and 1:2 molar ratios of the metal ion to ligand. Dissociation constants of the 1:1 Co(II)-ornithine and Cu(II)-arginine chelates are used to calculate the formation constants of the corresponding chelates of the unprotonated ligands. Similar dissociation constants of the 1:1 and 1:2 Cu(II) and Ni(II) ornithine chelates are used to determine the stabilities of the corresponding chelates containing unprotonated ligands. The order of stabilities of the metal chelates was found to be Cu > Hg > Ni > Zn > Co > Mn > Ca > Mg. The greater stability of the chelates containing monoprotonated ornithine over the chelates containing monoprotonated arginine ligands is interpreted on the basis of the electronic interactions of the ligands with the metal ions. The number and arrangement of coordinate bonds in the metal chelates is deduced on the basis of their chemical constitution and relative solution stabilities.