Comparative studies on the adsorption and association of 3-methylxanthine and 7-methylxanthine at a charged interface

A systematic comparative study of the adsorption and association of 3-methylxanthine (3MX an) and 7-methylxanthine (7MX an) at mercury-solution interfaces in acidic, neutral, and alkaline buffer solutions was undertaken by ac voltammetry. At bulk concentrations of 3MX an above a threshold value, the stacking interactions between vertically oriented molecules lead to a slow reorientation around E_ecm, and the molecules adopt a perpendicular orientation. The association of the adsorbed molecules of 7MX an is hindered and is not effective in promoting base-base stacking interactions. The enhanced surface activity of 3MX an compared to 7MX an results from the different position of the methyl group in the purine moiety. The effect of some divalent metal ions on adsorption stages and association of the investigated compounds has been studied. The results indicate that the complexation of methylated xanthine enhances the stacking interactions and hence would be expected to facilitate the formation of perpendicularly stacked layers of M(II)-MX an complexes on the electrode surface. The adsorption parameters of the investigated compounds have been computed in absence and presence of Cu(II) at different pH values. The results are compared with the behaviour of xanthine.