DIRECT OBSERVATION OF BETTER HYDRATION AT THE N-TERMINUS OF AN ALPHA-HELIX WITH GLYCINE RATHER THAN ALANINE AS THE N-CAP RESIDUE

The structural basis for the stability of N termini of helices has bee n analyzed by thermodynamic and crystallographic studies of three suit ably engineered mutants of the barley chymotrypsin inhibitor 2 with Se r, Gly, or Ala at the N-cap position (residue 31). Each mutant has a w ell-organized shell of hydration of the terminal NH groups of the heli x. The three structures are virtually superimposable (rms separations for all atoms, including the common water molecules, are 0.15-0.17 ang strom) and show neither changes in conformation at the site of substit ution nor changes in the crystal packing. The only changes on going fr om Ser-31 to Ala-31 to Gly-31 are in the position of a water molecule (Wat-116). This is bound to the Ser-O(gamma) atom in the Ser-31 struct ure but is in a weak hydrogen bonding position with the NH of residue 34 (O...N = 3.28 angstrom) in the Ala-31 mutant, partly replacing the strong Ser-31-O(gamma)...N34 hydrogen bond (O...N = 2.65 angstrom). Th e corresponding water molecule completely replaces the Ser hydroxyl hy drogen bond to N34 on mutation to Gly (2.74 angstrom). The only other change between the three structures is an additional water molecule in the Ala-31 structure (Wat-150) that partly compensates for the weak W at-116...N34 hydrogen bond. Perturbation of solvation by the side chai n of Ala is consistent with earlier hypotheses on the importance of ex posure of the termini of helices to the aqueous solvent.