Bz. Packard et al., INTRAMOLECULAR RESONANCE DIPOLE-DIPOLE INTERACTIONS IN A PROFLUORESCENT PROTEASE SUBSTRATE, JOURNAL OF PHYSICAL CHEMISTRY B, 102(4), 1998, pp. 752-758
In this study NorFES, an undecapeptide containing an amino acid sequen
ce 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 inte
ractions. To this end using all possible combinations from the group o
f xanthenes including fluorescein, tetramethylrhodamine, and rhodamine
-X, three heterobichromophoric and three homobichromophoric NorFES der
ivatives were synthesized; their absorption and fluorescence spectra w
ere measured both before and after cleavage by elastase. In the hetero
bichromophoric substrates the fluorescence of the fluorophore that wou
ld be the nominal donor in a Forster 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 Forster m
odel. Rather, spectra for all six doubly labeled peptides could be exp
lained by delocalization of excitation over each substrate's two fluor
ophores. Thus, by taking into account dipole-dipole interactions betwe
en two dyes placed in close proximity to each other, the spectral prop
erties observed could not be ascribed to the monomeric components but
were the unique optical signature of each ground-state dimer.