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 α-amino acid residues. More recently, the discovery of well-folded structures in oligopeptides containing β-amino acids has focused a great deal of current interest on the conformational properties of peptides
constructed from higher homologues (ω) of α-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 β-turn, a ubiquitous structural feature formed by two residue (αα) 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 αα counterparts. The available crystallographic information on hydrogen-bonded hybrid turns is surveyed in this review. Several recent examples demonstrate that individual ω-amino acid residues and hybrid dipeptide segments may be incorporated into the regular structures of α-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 α-residue sequences. The use of stereochemically constrained ω-residues promises to expand the range of peptide design strategies to include ω-amino acids. This approach is exemplified by well-folded structures like the C12 (αγ) and C14 (γγ) helices formed in short peptides
containing multiply substituted γ-residues. The achiral γ-residue gabapentin is a readily accessible building block in the design of peptides
containing γ-amino acids. The construction of globular polypeptide structures using diverse hybrid sequences appears to be a realistic possibility.
Author(s): Raghavender US, Bhaswati B, Indranil S, Rajagopal A, Shamala N, et al.
Author(s): Awasthi SK, Raghothama S, Balaram P
Author(s): Hutchinson EG, Thornton JM
Author(s): Gunasekaran K, Ramakrishnan C, Balaram P
Author(s): Mahalakshmi R, Raghothama S, Balaram P
Author(s): DeGrado WF, Schneider JP, Hamuro Y