CONFORMATIONAL DISTRIBUTION OF A TETRAPEPTIDE IN SOLUTION USING A COMBINED RANDOM SEARCH AND CONTINUUM DIELECTRIC APPROACH

The gas-phase and solution energy minima of the tetrapeptide Arg-Cys-Gly-Val were searched in its dihedral angle conformational space. Conformations were generated randomly and then energy minimized in vacuum and solution (using explicit representation of water molecules). The relative free energies of the solvated conformations were calculated using the continuum dielectric method. A combination of random search and continuum dielectric methods yields not only the energy minima but also their probable conformational distribution. No preferred conformation is predicted for the RCGV peptide in solution. This is due to a fine balance between the electrostatic free energy, which favors compact conformations with opposite charges close together, and the solvation free energy, which favors extended conformations with well separated charged groups that can be optimally solvated. Thus, in solution low-energy structures are found with opposite charges both close to each other and well separated. In vacuum or an organic solvent with low dielectric constant, compact conformations with opposite charges close to each other are expected. In general, solvation reduces the tendency for opposite charges to come close together and vastly increases the number of favorable conformations.