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.