Intramolecular Resonance Dipole−Dipole Interactions in a Profluorescent Protease Substrate

Abstract

In this study NorFES, an undecapeptide containing an amino acid sequence recognized by the serine protease elastase, was covalently labeled with two xanthenes, one on each side of its cleavage site, to serve as a tool for examination of intramolecular resonance dipole−dipole interactions. To this end using all possible combinations from the group of xanthenes including fluorescein, tetramethylrhodamine, and rhodamine-X, three heterobichromophoric and three homobichromophoric NorFES derivatives were synthesized; their absorption and fluorescence spectra were measured both before and after cleavage by elastase. In the heterobichromophoric substrates the fluorescence of the fluorophore that would be the nominal donor in a Förster model system was quenched. Since the fluorescence intensity of the nominal acceptor in these substrates was also decreased, these data were not consistent with the Förster model. Rather, spectra for all six doubly labeled peptides could be explained by delocalization of excitation over each substrate's two fluorophores. Thus, by taking into account dipole−dipole interactions between two dyes placed in close proximity to each other, the spectral properties observed could not be ascribed to the monomeric components but were the unique optical signature of each ground-state dimer.