Transferability of atomic volumes and charges in the peptide bond region in the solid state

Bader's partitioning scheme that makes use of the zero flux surfaces (ZFS) in the electron density gradient vector field has been applied to the peptide bond regions of three oligopeptides (two dipeptides, one hexapeptide), yielding eight peptide bond regions to compare. From integration over the atomic volume, very reproducible atoms in molecules (AIM) charges were calculated which agree within the given atom types by 0.04-0.08 e. The polarization of the peptide bond atoms is high compared to the charges normally used in force field parametrization. The positive charges of the C-alpha, C', and H atoms sum up to approximate to +1.6 e, while the negative charges of N and O amount to approximate to -1.85 e, so that for each peptide bond region an excess of -0.25 e has to be compensated by the C-alpha hydrogen and the side chains.