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Classical molecular dynamics (MD) studies of trialanine in aqueous solution are presented. The investigations have been inspired by recent 2D vibrational spectroscopy experiments of Woutersen and Hamm (J. Phys. Chem. B 2000, 104, 11316), who determined the structure and the conformational fluctuations of trialanine. The MD studies include various unbiased 20-ns simulations as well as umbrella sampling calculations of the potential of mean force along the central dihedral angles of trialanine. By employing the GROMOS96 force field, it is predicted that solvated trialanine is predominantly (∼80%) found in the extended conformations β and P_II and is also (∼16%) found in the helix conformation α_R. The results are explained by analyzing the free-energy contributions of intra- and intermolecular interactions, calculating the absolute entropy of the trialanine molecule, investigating the solvation and hydrogen …