Fig. 2

Characterization of HSA/BPBBT complexes. a Fluorescence emission spectra of BPBBT (200 µM) in THF:water (5%, v/v) with different HSA-to-BPBBT ratio (w/w) excited at 831 nm. b Temperature-time curves of BPBBT (200 µM) in THF:water (5%, v/v) with different HSA-to-BPBBT ratio (w/w) under 808 nm laser irradiation (0.8 W cm⁻²). c Plot of I/I₀ (green curve) or temperature (red curve) of BPBBT in THF:water (5%, v/v) versus HSA/BPBBT ratio. I and I₀ represent the fluorescence intensity of BPBBT calculated from (a). Temperature of BPBBT at 10 min in (b). d–f Isothermal titration calorimetry (ITC) analysis of the interaction between HSA and BPBBT. The thermodynamic parameters are determined during every injection of BPBBT into HSA aqueous solution containing 5% THF. d Raw data of the integrated heat after correction for heat of dilution. e The integrated heat against molar ratio (HSA:BPBBT). The solid line is the fitted curve. f The analysis of ITC results from (d, e). The binding of BPBBT to HSA is exothermic in nature (ΔH < 0) and characterized by a negative entropic change (ΔS < 0). The free energy change of binding is negative (ΔG < 0). g Overview of the molecular modeling of BPBBT binding to HSA (PDB code: 5ID7); h The active pocket; i The inside binding residues, Lys195 (orange) and Ser454 (cyanine). Yellow dashed lines, hydrogen bonds between HSA and BPBBT. j Optimized ground-state (S₀) geometry in the gas phase, and calculated HOMO and LUMO of BPBBT at B3LYP/6-31G(d) level. The dihedral angles are presented in the figures. To reduce the computational requirements, side chains are replaced by methyl groups. k Geometry of BPBBT binding to HSA in g with dihedral angles presented