Fret studies of conformational changes in heparin-binding peptides

FRET (Förster Resonance Energy Transfer) was applied to study structural properties of heparin-binding peptides containing the sequence XBBBXXBX where 'X' represents hydropathic or uncharged and 'B' represents basic amino acids. Internally quenched fluorogenic peptides were synthesized containing the fluorescent donor oaminobenzoic acid (o-Abz) and the acceptor dinitrophenyl ethylenediamine (Eddnp) group. Using the CONTIN computational package, distance distributions were recovered from time resolved fluorescence data, associated to end-to-end distances of the peptides. The peptides containing three or four repeat units have random structure in aqueous medium, and the interaction with low molecular weight heparin stabilized short end-to end distances. Experiments in water/trifluoroethanol (TFE) mixtures showed changes in distance distributions compatible with compact conformations stabilized above 40% volume content of TFE in the mixture. Similar changes in distance distributions were also observed for the peptides in interaction with SDS micelles in aqueous suspensions and circular dichroism data revealed alpha-helix formation in the peptides in interaction with heparin, SDS micelles or the co-solvent TFE. The process is dependent on electrostatic and hydrogen bond interactions and the end-to-end distances obtained are smaller than expected for the peptides in linear α-helix conformation, indicating the occurrence of structural arrangements leading to additional decrease in the distances.