The binding of Ni2+ to adenylyl-3',5'-adenosine and to poly(adenylic acid) |
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Authors: | MJ Hynes H Diebler |
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Institution: | Max-Planck-lnstitut fur Biophysikalische Chemie, 34 Göttingen-NikolaushergF.R.G. |
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Abstract: | 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 °C. and ionic strength μ = 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°C. μ = 0.5M). 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+ = 1 × 10?5 × 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 = 8.2×103 M?1 (25°C μ = 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. |
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Keywords: | Poly(A) Thermodynamic stability Relaxation kinetics |
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