Metal complexation kinetics of serine

Abstract

Rate constants for the formation and dissociation of l-serine complexes of Ni(II), Co(II), Cu(II) and Zn(II) have been determined by the temperature-jump technique. For the overall reaction at 25° and I = 0.1 M, ML n−1 + L ⇌ k −n k n ML n (where M = Ni(II), Co(II), Cu(II) or Zn(II), L = anionic form of the ligand, n = 1, 2, or 3, and charges have been neglected), the results are: Ni(II), k 1 = 2.9·10 4 M −1·sec −1, k −1 = 0.11 sec −1, k 2 = 3.4·10 4 M −1·sec −1, k −2 = 1.6 sec −1, k 3 = 3.0·10 4 M −1·sec −1, k −3 = 28 sec −1; Co(II), k 1 = 2.0·10 6 M −1·sec −1, k −1 = 93 sec −1 k 2 = 2.0·10 6 M −1·sec −1, k −2 = 930 sec −1; Cu(II), k 1 = 2.5·10 9 M −1·sec −1, k 2 = 5.10 8 k −2 = 150 sec −1; Zn(II)_, k 1 approx. 1.10 8 M −1· sec t ̄ 1 approx. 2.10 3 sec − . These results are interpreted in terms of a mechanism in which the rate-determining step for complex formation is the loss of a water molecule from the metal ion's inner coordination sphere. (A possible exception is the reaction sifCuL + L ⇌ CuL 2, where changes in the metal ion's configuration may affect the forward rate.) All the dissociative rate constants appear to be influenced by the bond weakening effect of the β-hydroxyl group on the coordinate linkage.