Energy transfer photophysics from serum albumins to sequestered 3-hydroxy-2-naphthoic acid, an excited state intramolecular proton-transfer probe.

The steady-state and time-resolved studies of the sensitized emission of the excited-state proton transfer (ESIPT) probe 3-hydroxy-2-naphthoic acid (3HNA) when bound to bovine serum albumin (BSA) and human serum albumin (HSA) indicate that the nonradiative dipole-dipole Förster type energy transfer from Trp singlet state of proteins to the ESIPT singlet state of 3HNA is greater in the case of HSA. This is supported by the distance and the orientation of the donor-acceptor pair obtained from the protein-ligand docking studies. The docking studies of the complex of BSA-3HNA also indicate that Trp 134 rather than Trp 213 is involved in the energy transfer process. The local environment of Trp 134 in BSA rather than that of Trp 213 is perturbed because of interaction with 3HNA as revealed by the optical resolution of Trp 134 phosphorescence in the complex at 77 K. Docking studies support the larger rotational correlation time, thetac (approximately 50 ns), observed for Trp residue/residues in the complexes of HSA and BSA compared with that in the free proteins.