Abstract
The solvent effects on the conformational equilibrium of the Glycine dipeptide are studied using a Quantum Mechanics/Molecular Mechanics method based on the mean field approximation. Glycine is the structurally simplest amino acid found in nature and is usually considered the amino acid that best conforms to the basic assumptions of the random coil model. It has been experimentally found, however, that in gas phase the conformational landscape of the glycine dipeptide is dominated by only two structures (C5 and C7) characterized by the formation of an intramolecular hydrogen bond (HB). In this work we show that a similar situation is found in water solution: the conformational equilibrium is also dominated by only two structures (PPII and α) that, furthermore, are not stable in gas phase and in which the intramolecular HB is replaced by an intermolecular HB. This behavior mimics that of other peptides which display less conformational freedom. We show that relative stability results from the interplay of two free energy components that operate in opposite directions: internal energy and dipeptide-water interaction energy. The influence exerted by the solvent structure and the dipeptide charge distribution on the relative stability of the different conformers is also analyzed.
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