First peptide-based system of rigid donor - rigid interchromophore spacer - rigid acceptor: a structural and photophysical study

The results of X-ray diffraction analysis, fluorescence experiments and molecular mechanics calculations on the terminally protected hexapeptide -(S)Bin-Ala-Aib-TOAC-(Ala)(2)- are presented. This is the first peptide investigated photophysically that is characterized by a) a rigid, binaphthyl-based alpha-amino acid (Bin) fluorophore, b) a rigid interchromophore bridge, the -Ala-Aib-sequence, and c) a rigid, nitroxide-based alpha-amino acid quencher (TOAC). In the crystal state the backbone of the spectroscopically critical 1-4 segment of both independent molecules in the asymmetric unit of the hexapeptide is folded in a regular, left-handed 3(10)-helix. The steady-state fluorescence spectra show a remarkable quenching of Bin emission by the TOAC residue located one complete turn of the helix apart. Time-resolved fluorescence measurements exhibit a biexponential decay with solvent-dependent lifetime components ranging from 0.5 to 1.5 ns and from 3 to 5 ns. Time-decay data combined with molecular mechanics calculations allowed us to assign these lifetimes to two left-handed 3(10)-helical conformers in which an intramolecular electronic energy transfer from excited Bin to TOAC takes place. For a given solvent the difference between the two lifetimes primarily depends on a different relative orientation of the two chromophores in the conformers, which is in turn related to a different puckering of the TOAC cyclic system.