Thermodynamics and Kinetics of Complex Formation between Cobalt (II), Nickel (II), and Copper (II) with Glycyl-L-leucine and L-Leucylglycine

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The thermodynamics of complex formation between cobalt(II), nickel(II), and copper(II) with glycyl-L-leucine and L-leucylglycine have been studied potentiometrically. Although the acidity constants of these two peptides and those of glycylglycine are nearly the same, the metal complexes of leucylglycine are considerably less stable than those of the glycyl dipeptides. The influence of the isobutyl group on the acidity constant for the deprotonation of the CuL+ complex is also very dependent on the place of attachment. The CuL+ complex of leucylglycine is far more acidic than those of the glycyl dipeptides. The kinetics of these reactions were studied using the temperature-jump technique. The forward rate constants for complexation show very little variation with the biggest effect appearing for the copper system. Even the rates of proton attack of the Cu(L–1H) species vary by less than a factor of 3 for the dipeptides. For the deprotonation effect, the rate constants correlate with the acidities of the CuL+ species leading to a variation of about 100. The thermodynamic and kinetic results are consistent with the Rabin model (coordination via the amino end group and the oxygen of the amide group) for bonding in metal–dipeptide complexes. © 1972, American Chemical Society. All rights reserved.

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Journal of the American Chemical Society

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