Abstract
Reactions of nickel(II) and cobalt(II) with lysine and ornithine have been studied over a wide range of pH by the temperature-jump relaxation technique at 25 °C and μ = 0.1 M. The attacking and binding form of the
ligand was found to be HL, where the proton is on the -nitrogen atom. Mono- and bis-protonated complexes are formed according to the reactions M2+ + HL ^ MHL2+ (kx, k-X), MHL2+ + HL ^ M(HL)22+ (k2, &_2), where
M = Ni or Co. Attempts to fit the data to other mechanisms, notably schemes with protonated complexes as minor species, failed. The rate constants (kx and k2 M"1 s-1, k-X and fc_2 sTM1) are the following: for nickel
andlysinekx = (4.2 ± 9.0%) X 103, k„x = 4.9 X 10“2, k2 = (4.7 ± 6.1%) X 103, fc_2 = 2.8 X 10-1; fornickeland ornithine kx = (2.2 ± 5.1%) X 10s, k.x = 4.1 X 10~2, k2 = (1.0 ± 9.3%) X 104, fc_2 = 9.8 X 10-1; for cobalt and lysine kx = (6.0 ± 18%) X 105, k.x = 8.8 X 101, k2 = (1.0 ± 5.0%) X 106, &_2 = 5.8 X 102; for cobalt and ornithine kx = (6.5 ± 5.1%) X 105, k_x = 1.5 X 102, k2 = (3.2 ± 2.2%) X 106, and fc_2 = 2.6 X 103. The rate-determining step is release of a coordinated water molecule. In each case k2 > kx, which indicates that the ligand labilizes the water molecules. For cobalt kx ~ k0Kos (the water exchange rate times the ion-pair formation equilibrium constant). That the nickel kx < k(¡K0S is due to screening of ligand charge, i.e., a lowering of . Lysine and ornithine are reactive zwitterions, but the proton is not located at the binding site, and ring closure is not rate determining. The unreactivity of glycine-like, -amino acid zwitterions does not appear to be due to rate-limiting ring closure.