Stability and solvation of Thr/Ser to Ala and Gly mutations at the N-cap of alpha-helices.

The solvation of polar groups at the N-terminal end of alpha-helices was studied by comparing the crystal structures of T4 lysozyme, barley chymotrypsin inhibitor 2 (CI2), barnase and their respective N-cap mutants. Whether or not the N3 residue is solvated on mutating the N-cap Thr/Ser to Ala or Gly appears to be related to the identities and the side-chain conformations of the N2 and N3 residues. When these two residues are alanines, as is in the pseudo-wild-type CI2 (E33A/E34A), the main-chain NH at the N3 position is exposed to the solvent and can be solvated. If the N2 residue is an Asp or a Glu, it is more likely that the side-chain of these residues will form a surrogate N-cap with the amide NH at N3 to compensate for the lost -OH group. In this case, no additional solvation will be observed. In general, Gly can be more stable than Ala at the N-cap because its small side-chain allows nearby polar groups to form hydrogen bonds with optimal geometry with solvent molecules or other polar groups.