π‐Turns in proteins and peptides: Classification, conformation, occurrence, hydration and sequence

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Abstract
The i + 5 → i hydrogen bonded turn conformation (7r‐turn) with the fifth residue adopting αL conformation is frequently found at the C‐terminus of helices in proteins and hence is speculated to be a “helix termination signal.” An analysis of the occurrence of i + 5 → i hydrogen bonded turn conformation at any general position in proteins (not specifically at the helix C‐terminus), using coordinates of 228 protein crystal structures determined by X‐ray crystallography to better than 2.5 Å resolution is reported in this paper. Of 486 detected π‐turn conformations, 367 have the (i + 4)th residue in αL conformation, generally occurring at the C‐terminus of α‐helices, consistent with previous observations. However, a significant number (111) of π‐turn conformations occur with (i + 4)th residue in αR conformation also, generally occurring in α‐helices as distortions either at the terminii or at the middle, a novel finding. These two sets of 7r‐turn conformations are referred to by the names παL and παR‐turns, respectively, depending upon whether the (i + 4)th residue adopts αL or αR conformations. Four π‐turns, named π'αL ‐turns, were noticed to be mirror images of παL‐turns, and four more π‐turns, which have the (i + 4)th residue in β conformation and denoted as πβ‐turns, occur as a part of hairpin bend connecting twisted β‐strands. Consecutive π‐turns occur, but only with παR‐turns. The preference for amino acid residues is different in παL and παR‐turns. However, both show a preference for Pro after the C‐termini. Hydrophilic residues are preferred at positions i + 1, i + 2, and i + 3 of παL‐turns, whereas positions i and i + 5 prefer hydrophobic residues. Residue i + 4 in παL‐turns is mainly Gly and less often Asn. Although παR‐turns generally occur as distortions in helices, their amino acid preference is different from that of helices. Poor helix formers, such as His, Tyr, and Asn, also were found to be preferred for παR‐turns, whereas good helix former Ala is not preferred. π‐Turns in peptides provide a picture of the π‐turn at atomic resolution. Only nine peptide‐based π‐turns are reported so far, and all of them belong to παL‐turn type with an achiral residue in position i + 4. The results are of importance for structure prediction, modeling, and de novo design of proteins.

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