Abstract
The interactions of nickel(II) with 9-methylpurine, adenine, and hypoxanthine in aqueous solution at 25° and ionic strength ~0.1 M have been studied by the temperature-jump method. A kinetic analysis of the relaxation spectra of 9-methylpurine interacting with Ni2+ indicates formation of a 1:1 complex with the neutral form of this ligand. The complex formation rate constant, dissociation rate constant, and stability constant are k, = (6.7 ± 2.5) X 10s ilf-1 sec-1, k-, ~ 75 sec-1, and K, ~ 90 M~l, respectively. The mag- nitude of the formation rate constant indicates normal substitution kinetics for nickel(II) ion. The tem- perature-jump results of the Ni(II)-adenine system are interpreted in terms of a mechanism in which the neutral form of the ligand attacks the metal ion. However, the anionic form of the ligand is postulated to be attached to the metal ion in the complex. The rate and stability constants which fit the data best are ki — 3 X 102 Af-1 sec-1, &V
1 X 107 If-1 sec-1, and Kt~2X 105 ,M-1 for the formation, dissociation, and stability constants, respectively. Substitution for adenine with Ni2+ is slower than normal, an effect that may be due to rate-limiting ring closure if a bidentate chelate is formed. The relaxation spectra obtained for the Ni(II)-hypoxanthine system are not exponential, indicating the presence of multistep equilibria. Since these spectra were not resolvable into separate exponential functions, a quantitative kinetic analysis was not made.