The effect of N-methylation on helical peptides

In N-methyl amino acids, the hydrogen of the N-H group is replaced with a bulky methyl group. While this change is expected to destabilize helical structures, the amount of destabilization is not known. Here the N-methyl group is placed into several positions of the helical peptides, acetyl-WGG(EAAAR)4A-amide and acetyl-WGG(RAAAA)4R-amide, and the melting of the peptides followed using CD. When analyzed using a simple two-state model, the destabilization associated with the H to CH3 substitution at 0 degree C is between 0.3 to 1.7 kcal/mole and is position dependent. The melting data may also be analyzed using a modified form of the Lifson-Roig statistics that should more correctly model the helix-coil transition in this small peptide. This analysis fails, however, apparently because the destabilization energy is greater than the energy that can be attributed to a single residue in this model.