The effect of resonance energy homotransfer on the intrinsic tryptophan fluorescence emission of the bothropstoxin-I dimer

Bothopstoxin-I (BthTX-I) is a homodimeric Lys49-PLA(2) homologue from the v enom of Bothrops jararacussu in which a single Trp77 residue is located at the dimer interface. Intrinsic tryptophan fluorescence emission (ITFE) quen ching by iodide and acrylamide has confirmed that a dimer to monomer transi tion occurs on reducing the pH from 7.0 to 5.0. Both the monomer and the di mer showed an excitation wavelength-dependent increase in the fluorescence emission maximum, however the excitation curve of the dimer was blue-shifte d with respect to the monomeric form. No differences in the absorption or c ircular dichroism spectra between pH 5.0 and 7.0 were observed, suggesting that this curve shift is due neither to altered electronic ground states no r to exciton coupling of the Trp residues. We suggest that fluorescence res onance energy homotransfer between Trp77 residues at the BthTX-I dimer inte rface results in excitation of an acceptor Trp population which demonstrate s a red-shifted fluorescence emission. (C) 2001 Academic Press.