  Protein solubility and conformational stability are a result of a balance of
interactions both within a protein and between protein and solvent. The
electrostatic solvation free energy of oligoglycines, models for the peptide
backbone, becomes more favorable with an increasing length, yet longer peptides
collapse due to the formation of favorable intrapeptide interactions between CO
dipoles, in some cases without hydrogen bonds. The strongly repulsive solvent
cavity formation is balanced by van der Waals attractions and electrostatic
contributions. In order to investigate the competition between solvent
exclusion and charge interactions we simulate the collapse of a long
oligoglycine comprised of 15 residues while scaling the charges on the peptide
from zero to fully charged. We examine the effect this has on the
conformational properties of the peptide. We also describe the approximate
thermodynamic changes that occur during the scaling both in terms of
intrapeptide potentials and peptide-water potentials, and estimate the
electrostatic solvation free energy of the system.
