Spectroscopic properties and conformational features of short linear peptides in solution: A fluorescence and molecular mechanics investigation

The ground- and excited-state properties of two conformationally constrained hexapeptides of general formula Boc-Bin-A(1)-A(2)-T-A(1)-A(2)-OtBu, where A(1) and A(2) are alpha-aminoisobutyric acid (Aib) or L-alanine (Ala), Bin is an optically pure, axially chiral 1,1'-binaphthyl-substituted Aib, and T (Toac) is a stable nitroxide free radical-containing Ac6c analog, were investigated in methanol solution. These peptides are denoted as (R)-Bin/Toac and (S)-Bin/Toac, depending on the chirality of the binaphthyl moiety. Electronic spectra in methanol indicate the occurrence of intramolecular exciton interaction between the naphthyl moieties of Bin, and time-resolved fluorescence measurements show a biexponential decay for both peptides examined. According to infrared (IR) absorption data in the NH stretching frequency region, and to earlier X-ray diffraction results on (S)-Bin/Toac in the crystal state, both (R)-Bin/Toac and (S)-Bin/Toac populate a 3(10)-helix in solution with opposite screw sense, the helical handedness being determined by the chirality of binaphthyl and not by that of the Ala residues in the main chain. The combination of molecular mechanics calculations with fluorescence decay data indicate that the two observed lifetimes for each peptide arise from two conformations having different interprobe distance and orientation, in which electronic energy transfer from excited Bin to Toac takes place.