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

Abstract

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 λ_fmax, 2-4 do not exhibit such behavior. Compounds 1-8 also quench the 345 nm fluorescence of BSA in phosphate buffer (λ_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.