Structural, energetic and dynamical properties of sodiated oligoglycines: relevance of a polarizable force field

Oligoglycine peptides (from two to ten residues) complexed to the sodium ion were studied by quantum chemical and molecular mechanics calculations to understand their structural and energetic properties. Modeling such systems required the use of a polarizable force field and AMOEBA, as developed by Ren and Ponder [J. Comput. Chem., 2002, 23, 1497], was chosen. Some electrostatic and torsional parameters were re-optimized using a rigorous procedure and validated against both geometric and energetic ab initio data in the gas phase. Molecular dynamics simulations were performed on seven sodiated octa-glycine (G(8)) structures. Structural transitions were generally observed (with the notable exception of the a-helix), leading to new structures that were further proved by ab initio calculations to be of low energies. The main result is that for G(8)-Na(+), there is a compromise between sodium peptide interactions and multiple hydrogen bonding. The accuracy achieved with AMOEBA demonstrates the potential of this force field for the realistic modeling of gaseous peptides.