The effects of mono- and divalent metal cations on the solution structure of caffeine and theophylline

The interactions of caffeine and theophylline with potassiumþ, cobalt2þ and nickel2þ ions were studied in aqueous solution at physiological pH with constant ligand concentration and various metal ion contents. Fourier Transform infrared spectroscopy and absorption spectra were used to determine the cation binding mode and association constants. Spectroscopic results showed direct and indirect cation interactions for Co2þ, Ni2þ and Kþ through O6 and N9 atoms (caffeine) and O6, N9 and N7 atoms (theophylline). The overall binding constants were, K(Co–caffeine) ¼ 6.92 £ 104 M21, K(Ni–caffeine) ¼ 2.22 £ 104 M21, K(K–caffeine) ¼ 5.08 £ 103 M21, K(Co– theophylline) ¼ 5.06 £ 104 M21, K(Ni–theophylline) ¼ 4.84 £ 104 M21 and K(K–theophylline) ¼ 2.13 £ 103 M21. The association constants showed weaker interaction for monovalent cation than divalent metal ions. q 2004 Elsevier B.V. All rights reserved.