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

The interactions of caffeine and theophylline with potassium(+), cobalt(2+) and nickel(2+) 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 Co2+. Ni2+ and K+ through O6 and N9 atoms (caffeine) and O6, N9 and N7 atoms (theophylline). The overall binding constants were. K(Co-caffeine) = 6.92 X 10(4) M-1, K(Ni-caffeine) = 2.22 X 10(4) M-1, K(K-caffeine) = 5.08 X 10(3) M-1, K(Co-theophylline) = 5.06 X 10(4) M-1, K(Ni-theophylline) = 4.84 X 10(4) M-1 and K(K-theophylline) = 2.13 X 10(3) M-1. The association constants showed weaker interaction for monovalent cation than divalent metal ions. (C) 2004 Elsevier B.V. All rights reserved.