Excited state proton transfer of 2-(2'-hydroxyphenyl)benzimidazole and its nitrogen substituted analogues in bovine serum albumin

The interaction of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) and its nitrogen substituted analogues 2-(2'-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine (HPIP-b) and 2-(2'-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine (HPIP-c) with BSA was explored. Upon interaction with BSA both normal and tautomer emissions are significantly enhanced. However, the fluorescence ratios of the normal band to the tautomer band of HPBI and HPIP-b decrease, but that of HPIP-c increases. From the tautomer emission, the stoichiometry and association constants were determined. HPBI exists as cis- and trans-enolic and zwitterionic forms, whereas HPIP-b and HPIP-c are present as monoanions in addition to cis- and trans-enols. The study shows that different conformers of all three molecules bind at different binding sites of BSA.