Complexes of the nickel(II) ion with purine bases: relaxation spectra with theophylline

Abstract

Temperature-jump experiments with theophylline (HL) and Ni²⁺ yield spectra characterized by a single relaxation process. Similar experiments with caffeine and Ni²⁺ in the concentration range 10^–3–10^–2M, pH 3·85–6·10, do not yield detectable effects. Potentiometric determination of the number and stability of the theophylline complexes formed at 25 ± 0·1 °C and variable ionic strength in the range 0·010–0018M gave K₁=[NiL⁺]/[Ni²⁺][L^–]=(1·8 ± 0·3)× 10³ l mol^–1 and K₂=[NiL₂]/[NiL⁺][L^–]=(1·0 ± 0·4)× 10³ l mol^–1. Evaluation of the relaxation data, based on the formation of two theophylline complexes, results in the values k₁≈ 1 × 10² l mol^–1s^–1 and k_–1≈ 10⁷ l mol^–1s^–1for the reaction Ni²⁺+ HL[graphic omitted]NiL⁺+ H⁺, and k₂≈ l ×10² l mol^–1s^–1and k_–2≈ 2 × 10⁷ l mol^–1 s^–1 for the reaction NiL⁺+ HL[graphic omitted]NiL₂+ H⁺. It is concluded that the formation rate constant is smaller than that which would be expected for a neutral ligand. A possible explanation of this retardation may be hindrance to the loss of the dissociable proton, due to internal H-bond formation in theophylline.