The binding of Ni2+ to adenylyl-3',5'-adenosine and to poly(adenylic acid).

Studies of the binding of Ni2+ to adenylyl-3',5'-adenosine (ApA) at pH 6.0 by ultraviolet spectrophotometry indicate the formation of a 1:1 complex in the presence of a large excess of metal ion. At 25 degrees C, and ionic strength mu = 0.5 M, the stability constant of Ni(ApA) is evaluated to be K = 2.6 (+/- 0.6) M-1. The low stability is taken as evidence that the predominant complex species is one in which the ApA acts as a monodentate ligand, mainly through the adenine group. The rate constants for complex formation and dissociation, kf = 1430 M-1 s-1 and kb =665 s-1 (25 degrees C, mu=0.5 M), determined by the temperature-jump relaxation technique, are consistent with this interpretation. The binding strength of Ni2+ to poly(adenylic acid) [poly(A)] has been studied at pH 7.0 using murexide as an indicator of the concentration of free Ni2+. Within the concentration range [Ni2+]free = 1 x 10(-5) -1 x 10(-3) M the data can be represented in the form of a linear Scatchard plot, i.e., the process can be described as the binding of Ni2+ to one class of independent binding sites. The number of binding sites per monomer is 0.26, and the stability constant K=8.2 x 10(3) M-1 (25 degrees C, mu=0.1 M). In kinetic studies of the reaction of Ni2+ with poly(A), two relaxation effects due to complex formation were detected, one with a concentration-independent time constant of about 0.4 ms, the other with a concentration-dependent time constant in the millisecond range. The concentration dependence of the longer relaxation time can be accounted for by a three-step mechanism which consists of a fast second-order association reaction followed by two first-order steps. There is evidence, however, that the overall process is more complicated than expressed by the three-step mechanism.