Expanding the polypeptide backbone: hydrogen-bonded conformations in hybrid polypeptides containing the higher homologues of alpha-amino acids.

Half a century has passed since the hydrogen-bonded secondary structures of polypeptides and proteins were first recognized. An extraordinary wealth of conformational information is now available on peptides and proteins, which are formed of alpha-amino acid residues. More recently, the discovery of well-folded structures in oligopeptides containing beta-amino acids has focused a great deal of current interest on the conformational properties of peptides constructed from higher homologues (omega) of alpha-amino acids. This review examines the nature of intramolecularly hydrogen-bonded conformations of hybrid peptides formed by amino acid residues, with a varying number of backbone atoms. The beta-turn, a ubiquitous structural feature formed by two residue (alphaalpha) segments in proteins and peptides, is stabilized by a 10-atom (C10) intramolecular 4-->1 hydrogen bond. Hybrid turns may be classified by comparison with their alphaalpha counterparts. The available crystallographic information on hydrogen-bonded hybrid turns is surveyed in this review. Several recent examples demonstrate that individual omega-amino acid residues and hybrid dipeptide segments may be incorporated into the regular structures of alpha-peptides. Examples of both peptide helices and hairpins are presented. The present review explores the relationships between folded conformations in hybrid sequences and their counterparts in all alpha-residue sequences. The use of stereochemically constrained omega-residues promises to expand the range of peptide design strategies to include omega-amino acids. This approach is exemplified by well-folded structures like the C12 (alphagamma) and C14 (gammagamma) helices formed in short peptides containing multiply substituted gamma-residues. The achiral gamma-residue gabapentin is a readily accessible building block in the design of peptides containing gamma-amino acids. The construction of globular polypeptide structures using diverse hybrid sequences appears to be a realistic possibility.