A study by ultraviolet spectroscopy on self-association of purine, 6-methylpurine, benzimidazole, and imidazo [1,2-a]pyridine in aqueous solution

The self-association of four heteroaromatic compounds with fused five- and six-membered rings: purine, 6-methylpurine, benzimidazole, and imidazo[1,2- a]pyridine, was studied in aqueous solution at different pH values, by ultr aviolet spectroscopy. The variation in molar absorptivity with the concentr ation of these compounds was measured for the band systems in the mid-ultra violet region. Hypochromic deviations from the Beer-Lambert law with increa sing concentration were found for all compounds. These results were interpr eted in terms of self-association processes involving the formation of dime rs and, in certain cases, also the formation of polymers. From the fitting of the experimental curves of hypochromic effects, self-association constan ts for dimerization and polymerization were calculated. The results obtaine d are discussed in relation to the self-association properties of the build ing blocks of each compound, studied by us in previous works. The relevance of the position of the nitrogen atoms at the five- and six-membered rings was analyzed to elucidate the nature of the self-association processes, in particular the tendency to polymerization. The values of the calculated sel f-association constants are discussed in relation to the intermolecular int eraction energies of these compounds in aqueous solution. (C) 2000 Elsevier Science B.V. All rights reserved.