Hydrophobicity of the peptide C=O...H-N hydrogen-bonded group.

The hydrophobicity of the peptide C=O ... H-N hydrogen-bonded group is an important parameter that determines the structure of proteins in water and in biological membranes, and therefore the free energy of transferring this group from water to non-polar solvents should be determined accurately. The essential work on this problem was carried out by Klotz and co-workers, and has been summarized elsewhere. Using N-methylacetamide as a model peptide, the free energies of the following processes were determined; (1) formation of the C=O ... H-N bond in water, (2) formation of the C=O ... N-N bond in CCl4, and (3) transfer of N-methylacetamide from water to CCl4. (4) From (3), the free energy of transferring the non-hydrogen bonded (C=O, H-N) group from water to CCl4 was calculated. When the free energies of (1), (2) and (4) are combined, one finds that the free energy of transferring the C=O ... H-N group from water to CCl4 is a surprising -1.4 kcal/mol (1 cal = 4.184 J). This number does not seem reasonable, since it implies that the C=O ... H-N group is about as hydrophobic as an isopropyl group, i.e. the side-chain of valine. In the present report, it is shown that this apparent hydrophobicity results from an underestimation of the free energy contribution that the methyl groups make to the transfer of N-methylacetamide from water to CCl4. When appropriate methyl group transfer free energies are used, one finds that the free energy of transferring the C=O ... H-N group from water to CCl4 is +0.62 kcal/mol. Therefore, this group is relatively insensitive to solvent polarity. A similar calculation shows that the free energy of transferring the C=O ... H-O hydrogen-bonded group from water to benzene is +0.55 kcal/mol.