Separation and analysis of dynamic stokes shift with multiple fluorescence environments: Coumarin 153 in bovine β-lactoglobulin A

We use time-dependent fluorescence Stokes shift (TDFSS) information to study the fluctuation rates of the lipocalin, β-lactoglobulin A in the vicinity of an encapsulated coumarin 153 molecule. The system has three unique dielectric environments in which the fluorophore binds. We develop a method to decompose the static and dynamic contributions to the spectral heterogeneity. This method is applied to temperature-dependent steady-state fluorescence spectra providing us with site-specific information about thermodynamic transitions in β-lactoglobulin. We confirm previously reported transitions and discuss the presence of an unreported transition of the central calyx at 18°C. Our method also resolves the contributions to the TDFSS from the coumarin 153 centrally located in the calyx of β-lactoglobulin despite overlapping signals from solvent exposed dyes. Our experiments show dynamics ranging from 3-1200 ps. The analysis shows a decrease in the encapsulated dye's heterogeneity during the relaxation, which is taken as evidence of the breakdown of linear response.