Proline lacks an amide proton when found within proteins. This precludes hydrogen bonding between it and hydrogen bond acceptors, and thus often restricts the residue to the first four positions of an α-helix. Helices with proline after position four have a pronounced kink [(1988) J. Mol. Biol. 203, 601-619]. In these cases, we find that the proline residue almost always occurs on the solvent exposed face of each helix. This positioning facilitates the compensatory hydrogen bonding between solvent and residues P-3 and P-4 (relative to proline, P), through the formation of the kink. Further, it aids in the packing of long helical structures around globular protein structures.
