A fluorescence study of differently substituted 3-styrylindoles and their interaction with bovine serum albumin

Interaction of 3-styrylindoles 1-8 viz. 3-(2-phenylethenyl-E)-NH-indole (1), 3-[2-(4-nitrophenyl)ethenyl-E]-NH-indole (2), 5-bromo-3-[2-(4-nitrophenyl)ethenyl-E]-NH-indole (3), 5-methoxy-3-[2-(4-nitrophenyl)ethenyl-E]-NH-indole (4), 3-[2-(4-cyanophenyl)ethenyl-E]-NH-indole (5), 3-[2-(4-cyanophenyl)ethenyl-E]-N-ethylindole (6), 5-bromo-3-[2-(4-chlorophenyl)ethenyl-E]-NH-indole (7) and 5-methoxy-3-[2-(4-chlorophenyl)ethenyl-E]-NH-indole (8) with bovine serum albumin (BSA) was examined by UV-vis and steady-state fluorescence spectroscopy. The fluorescence intensity of 1-8 increases with the increasing BSA concentration. Upon binding with BSA, while 1 and 5-8 show a blue shift in their lambda(f max), 2-4 do not exhibit such behavior. Compounds 1-8 also quench the 345 nm fluorescence of BSA in phosphate buffer (lambda(ex), 280 nm). These compounds intercalate in the hydrophobic regions of BSA, as evidenced by the determination of BSA binding site micropolarity using compounds 2-8. As evidenced by the estimation of energy transfer efficiency and distance between the donor (BSA-Trp-212) and the acceptor (3-styrylindoles), the halo-substituted compounds 3 and 7 interact with BSA more effectively than the other 3-strylindoles. These compounds have potential for use as neutral and hydrophobic fluorescence probes for examining the microenvironments in proteins, polymers, micelles, etc.