Kinetics of stereoselective complexation reactions of histidine and leucyltyrosine with divalent metal ions in aqueous solution

Abstract

The optical activity of amino acids or peptides influences the chemical equilibrium. Bis histidine complex ML_2 and MLD differ in the stability in aqueous solution (M^2^+ is the metal ion and D^-, L^- are the L and D forms of histidine anion). Dipeptides, L-leucyl-L-tyrosine and D-leucyl-L-tyrosine, differ in the pK values and the stabilities of complexes with metal ions. The complexation reactions of these stereoselective systems were studied kinetically by the temperature jump method. On histidine complex systems, the rate constants at these types were obtained. ML^+ + L^- k_2 ⇌ k_-_2 ML_2 ML^+ + D^- k_3 ⇌ k_-_3 MLD. At 25℃ and ionic strength of 0.1, rate constants are: nickel (II), k_2 = 4.1 x 10^5M^-1sec^-^1, k_-_2= 0.059 sec^-^1, k_3 = 4.3 x 10^5M^-^1sec^-^1, 2k_-_3 = 0.027 ; cobalt (II), k_2 = 1.4 x 10^6M^-^1sec^-^1, k_-_2 = 5.7sec^-^1, k_3 = 1.2 x 10^5M^-^1sec^-^1, 2k_-_3 = 2.9sec^-^1 ; zinc (II), k_2 = 6.0 x 10^7M^-^1sec^-^1, k_-_2 = 190 sec^-^1. k_3 = 5.2 x 10^7M^-^1sec^-^1, 2k_2 = 96 sec^-^1 ; cadmium (II), k_2 ≈ 7 x 10^7M-^1sec^-^1, k_2 ≈ 4 x 10^3sec^-^1 (The coefficient 2 of k_3 is the statistical factor). The formation rate constants k_2 and k_3 are equal within experimental error for nickel (II), cobalt (II) and zinc (II), but 2k_3 is about half of k_2. On leucyltyrosine systems, the rate constants of the following types were obtained. Ni^2^+ + A^- k_1 ⇌k_-_1 NiA^+, NiA^+ + A^- k_2 ⇌ k_-_2 NiA_2, where A^- is the optical active dipeptide anion. The results at 25℃ and ionic strength of 0.15 are : L-leucyl-L-tyrosine k_1 = 2.2 x 10^3M^-^1, k_-_1 = 1.3 sec^-^1, k_2 = 1.7 x 10^3M^-^1sec^-^1, k_-_2 = 3.0sec^-^1 ; D-leucy1-L-tyrosine, k_1 = 2.4 x 10^3^-^1sec^-^1, k_-_1 = 0.45sec^-^1, k_2 = 2.0 x 10^3M^-^1sec^-^1 , k_-_2 = 2.4sec^-^1. The value of k_1 for L-leucyl-L-tyrosine is equal within experimental error to that for D-leucyl-L-tyrosine, but in dissociation reactions the value of k_-_1 for D-leucyl-L-tyrosine is about one third of the value for L-leucyl-L-tyrosine. The effect of the optical activity of ligands influences kinetically the dissociation reaction of the complex. The fact that there is no effect on the rate constant of the formation reaction indicates that the rate-determining step is the first unidentate complex formation step of the incoming ligand involving the water dissociation from the inner hydration shell of the metal ion via diffusion-controlled ion-pair formation.