Structural properties and photophysical behavior of conformationally constrained hexapeptides functionalized with a new fluorescent analog of tryptophan and a nitroxide radical quencher

The influence of the conformational properties on the photophysics of two de novo designed hexapeptides was studied by spectroscopic measurements (IR, NMR, steady-state and time resolved fluorescence) and molecular mechanics calculations. The peptide sequences comprise two nonproteinogenic residues: a beta-(1-azulenyl)-L-alanine (Aal) residue, obtained by formally functionalizing the Ala side chain with the azulene chromophore, and a C(alpha)-tetrasubstituted alpha-amino acid (TOAC) incorporating a nitroxide group in a cycloalkyl moiety. Aal represents a new fluorescent, quasi-isosteric Trp analog and TOAC a stable radical species, frequently used as a paramagnetic probe in biochemical studies. The peptide chains differ in the sequence position of the two probes and are heavily based on Aib (alpha-aminoisobutyric acid) residues to generate conformationally restricted helical structures, as confirmed by both spectroscopic and computational results. The conformationally controlled, excited state interactions, determining the photophysical relaxation of the Aal(*)/TOAC pair, are also discussed.