Secondary Structures in Phe-Containing Isolated Dipeptide Chains: Laser Spectroscopy vs Quantum Chemistry

The intrinsic conformational landscape of two phenylalanine-containing protein chain models (-Gly-Phe- and -Ala-Phe- sequences) has been investigated theoretically and experimentally in the gas phase. The near UV spectroscopy (first pp* transition of the Phe ring) is obtained experimentally under jet conditions where the conformational features can be resolved. Single-conformation IR spectroscopy in the NH stretch region is then obtained by IR/UV double resonance in the ground state, leading to resolved vibrational spectra that are assigned in terms of conformation and H-bonding content from comparison with quantum chemistry calculations. For the main conformer, whose UV spectrum exhibits a significant FranckCondon activity in low frequency modes involving peptide backbone motions relative to the Phe chromophore, excited state IR spectroscopy has also been recorded in a UV/IR/UV experiment. The NH stretch spectral changes observed in such a pi pi* labeling experiment enable us to determine those NH bonds that are coupled to the phenyl ring; they are compared to CC2 excited state calculations to quantify the geometry change upon pi pi* excitation. The complete and consistent series of data obtained enable us to propose an unambiguous assignment for the gallery of conformers observed and to demonstrate that, in these two sequences, three conceptually important local structural motifs of proteins (beta-strands, 27 ribbons, and beta-turns) are represented. The satisfactory agreement between the experimental conformational distribution and the predicted landscape anticipated from the DFT-D approach demonstrates the capabilities of a theoretical method that accounts for dispersive interactions. It also shows that the flaws, inherent to a resonant two-photon ionization detection scheme, often evoked for aromatic chromophores, do not seem to be significant in the case of Phe.