Fluorescence modulation and associative behavior of lumazine in hydrophobic domain of micelles and bovine serum albumin

The photophysical behavior of the deprotonated form of lumazine (Lum-anion) was studied in biologically relevant surfactant systems like sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB) and TritonX-100 (TX-100) and also model water soluble protein, bovine serum albumin (BSA), using steady-state and time-resolved fluorescence spectroscopy in buffer solution of pH 12.0. The association constant values were calculated from modulated fluorescence intensity of Lum-anion in different medium. The interaction of non-ionic surfactant TX-100 was found to be about 10 times greater than SDS and CTAB. However, while the driving force of binding in SDS and/or TX-100 is mainly hydrophobic in nature, electrostatic interaction with the oppositely charged micellar head group is the predominant factor in CTAB. The thermodynamic parameters like enthalpy (ΔH) and entropy (ΔS) change, etc., corresponding to the binding of Lum-anion with BSA were estimated by performing the fluorescence titration experiment at different temperatures. Thermodynamically favorable and strong binding of Lum-anion (K ∼ 104 M−1) into BSA is due to hydrophobic interaction in the ligand binding domain II. However, the binding mechanism is entirely different in presence of protein denaturing agent like urea and electrostatic interaction plays a major role under this condition.