Abstract
The temperature-jump and NMR line-broadening techniques have been used to determine the stability constants, kinetics and binding sites of nickel(II)-purine complexes in aqueous solution. The temperature-jump relaxation spectrum consists of two processes, dependent on concentration and pH. Previous evidence has shown that the anionic form of the ligand binds to the metal ion, hence HL, Ni2+ and NiL+ alone are not sufficient to explain the kinetic data. The two most likely systems consistent with the data are: (1) complexes differing in their sites of metal attachment (linkage isomers) and (2) higher order complexes, i.e. and NiL2 complex in addition to the NiL+ complex. Both possibilities fit the temperature-jump data reasonably well, although neither is entirely satisfactory. The NMR experiments provide additional experimental evidence allowing a choice of mechanisms to be made. The preferred mechanism is The rate and equilibrium constants for this scheme are calculated. From the temperature-dependence of the NMR line width, the activation parameter for k−2 and the chemical shift ΔΩNi for H(8) on the bound purine are calculated. The results are consistent with a normal substitution mechanism for aqueous nickel(II) ion.