Structural features of linear (alphaMe)Val-based peptides in solution by photophysical and theoretical conformational...

In continuation of our studies on the determination of the structural features of functionalized peptides in solution by combining time-resolved fluorescence data and molecular mechanics results, the conformational features of a series of linear, L-(alphaMe)Val-based peptides have been investigated in methanol. These foldamers have the general formula F[(alphaMe)Val](r)-T-[(alphaMe)Val](2)NHtBu, where (alphaMe)Val = C(alpha)-methylvaline and r = 0-3, while F [= fluoren-9-ylmethoxycarbonyl (Fmoc)] and T [= 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-carboxylic (Toac)] are a fluorophoric N(alpha)-protecting group and a nitroxide-based alpha-amino acid quencher, respectively. According to ir and CD spectra, the longest term of the series (r = 3) attains a 3(10)-helical structure, while the other peptides populate an intramolecularly H-bonded, 3(10)-helix-like conformation affected by dynamic helical distortions, which are enhanced by the shortness of the backbone chain. Such distortions are reflected in both the energy of the stretching mode and the molar extinction coefficient of the H-bonded N-H groups, the former being higher and the latter smaller than those of a stable 3(10)-helix. Steady-state and time-resolved fluorescence measurements in methanol show a strong quenching of Fmoc by the Toac residue, located at different helix positions, depending on the r value. Comparison of quenching efficiencies and lifetime preexponents with those theoretically obtained from the deepest energy minimum conformers, assuming a Förster mechanism, is satisfactory. The computed structures exhibit a rather compact arrangement, which accounts for the few sterically favored conformations for each peptide, in full agreement with the time-resolved fluorescence data. Orientational effects between the probes must be taken into account for a correct interpretation of the fluorescence decay results, implying that interconversion among conformational substates involving the probes is slower than the energy transfer rate.