Structures and IR spectra of the Gramicidin S peptide: pushing the quest for low-energy conformations.

An extensive molecular modeling study was carried out on the doubly protonated cyclic decapeptide Gramicidin S following several recent gas-phase experiments. Our computational strategy includes replica-exchange molecular dynamics simulations with the new generation force field AMOEBA for exploration and density functional calculations using several functionals for refinement of structures and computation of IR spectra. This procedure yields low-energy structures of which three are proposed to correspond to the three conformers detected in low-temperature IR experiments. The most stable structure has C(2) symmetry and four strong β-sheet interactions between Orn and Val residues. Furthermore, all the other peptidic N-H bonds are involved in seven-membered C(7) motifs. The computed IR spectra of the three conformers are in good agreement with the experimental ones in the 1400-2000 cm(-1) range. In the 3000-3600 cm(-1) region, the computed spectrum is also in good agreement with experiment for the main conformer, and predictions are made of structure-specific signatures for the other two conformers. The accuracy of several density functionals is discussed in detail. These results point out that efficient potential energy surface explorations coupled to appropriate density functional theory (DFT) calculations are able to reveal the structures of molecules as large and flexible as decapeptides.