Complexation kinetics of leucine with nickel(II), cobalt(II), and copper(II)

Abstract

The kinetics of leucine complexation with nickel(II), cobalt(II) and copper(II) have been determined with the temperature-jump technique. The reactions are of the type ML + L ⇌ ML where M = Ni , Co , or Cu , L refers to the attacking form of the ligand which is the anion in this case, and n = 1 or 2. At an ionic strength of 0.1 M and 25°, the rate constants are: Ni k = 1.7 × 10 M sec , k 7.1 × 10 sec , k = 4.1 × 10 M sec , k = 1.5 sec ; Co k = 1.3 × 10 M sec , k = 67 sec , k = 1.7 × 10 M sec , k = 600 sec ; Cu k = 1.6 × 10 M sec , k = 12 sec , k = 8 × 10 M sec , k = 15 sec . The rate constants for the reactions of Co and Cu with the zwitterion form of the ligand were found to be zero within experimental error. These results are consistent with the dissociative mechanism suggested by Eigen. The nonpolar side chain of leucine appears to play no significant role in determining either the forward or reverse rates. Once again, as for other ligands, k < k unlike the situation found for other metal ions. n-1 k-n n 1 -1 2 -2 1 -1 2 -2 1 -1 2 -2 2 1 kn 2+ 2+ 2+ 2+ 4 -1 -1 -2 -1 4 -1 -1 -1 2+ 6 -1 -1 -1 6 -1 -1 -1 2+ 9 -1 -1 -1 8 -1 -1 -1 2+ 2+ Cu Cu