Fluorescence spectral resolution of tryptophan residues in bovine and human serum albumins

Static quenching and time-resolved emission spectra of tryptophan residues of BSA (2 Trp residues) and HSA (1 Trp residue) were performed in the presence of high concentrations of calcofluor white, a fluorophore that is specific to both carbohydrate residues and to hydrophobic sites in proteins. In the absence of calcofluor white, BSA and HSA emit with a maximum at 340 and 330 nm, respectively. Also, tryptophan residues in both proteins fluoresce with three identical lifetimes. Time-resolved spectra of HSA show that the three lifetimes emit at a maximum equal to 330 nm while spectra obtained from BSA show different peak positions for the three lifetimes.At high calcofluor concentrations, steady-state fluorescence emission spectrum of BSA displays a maximum at 330 nm instead of 340 nm in the absence of calcofluor. Fluorescence excitation spectra of the protein recorded in the absence and presence of calcofluor indicate the absence of protein conformational modification upon calcofluor white binding. Time-resolved emission spectra of the three lifetimes show identical peaks equal to 330 nm. Steady-state and time-resolved emission spectra performed on HSA in the presence of calcofluor do not show any modification in the emission peak (330 nm) indicating the absence of any conformational change and confirming the fact that the shift observed for tryptophan residues emission in BSA is the result of fluorescence quenching of Trp-134 residue.