Optical responses of BSA protein under re-entrant condensation in presence of trivalent ions

Abstract

Fluorescence behavior of globular protein like BSA is studied under the re-entrant condensation in the presence of trivalent salts like YCl3 and LaCl3. BSA shows a re-entrant behaviour in solution where a specific turbid phase is formed between the two critical concentrations of the trivalent salts. Optical behaviors of proteins are explored by UV–Vis, photoluminescence and time resolved photoluminescence (TRPL) spectroscopy, whereas hydrodynamic size is obtained from the dynamic light scattering (DLS) technique. BSA shows maximum emission intensity at ≈338 nm for an excitation at 278 nm, however, the value of the peak intensity of emission gradually increases with the increase in salt concentration and becomes maximum at the turbid phase but for higher salt concentration as the re-dissolution takes place, the emission intensity again decreases. TRPL study suggests that the decay time of BSA under re-entrant condensation is correlated with the salt concentration, whereas DLS study shows that the hydrodynamic size gradually increases up to a certain salt concentration where turbid phase is formed and then again decreases for higher salt concentrations. The variation of emission intensity of BSA nearly follows the modification of diffusion coefficient or hydrodynamic size and the phase transition behaviors.