Abstract
The chemical synthesis and single‐crystal X‐ray diffraction analysis of a model peptide, Boc–Thr–Thr–NH~2~ (1) comprised of proteinogenic residues bearing an amphiphilic C^β^‐stereogenic center, has been described. Interestingly, the analysis of its molecular structure revealed the existence of a distinct conformation that mimics a typical β‐turn and Asx‐turns, i.e., the two Thr residues occupy the left‐ and right‐corner positions. The main‐chain torsion angles of the N‐ and C‐terminal residues i.e., semiextended: ϕ = −68.9°, ψ = 128.6°; semifolded: ϕ = −138.1°, ψ = 2.5° conformations, respectively, in conjunction with a gauche^−^ disposition of the obligatory C‐terminus Thr C^γ^H~3~ group, characterize the occurrence of the newly described β‐turn‐ and Asx‐turns‐like topology. The preferred molecular structure is suggested to be stabilized by an effective nonconventional main‐chain to side‐chain C~i~O · · · HC‐type intraturn hydrogen bond. Noteworthy, the observed topology of the resulting 10‐membered hydrogen‐bonded ring is essentially similar to the one perceived for a classical β‐turn and the Asx‐turns, stabilized by a conventional intraturn hydrogen bond. Considering the signs as well as magnitudes of the backbone torsion angles and the orientation of the central peptide bond, the overall mimicked topology resembles the type II β‐turn or type II Asx‐turns. An analysis of Xaa–Thr sequences in high‐resolution X‐ray elucidated protein structures revealed the novel topology prevalence in functional proteins (unpublished). In view of indubitable structural as well as functional importance of nonconventional interactions in bioorganic and biomacromolecules, we intend to highlight the participation of Thr C^γ^H in the creation of a short‐range CO · · · HC^γ^‐type interaction in peptides and proteins. © 2006 Wiley Periodicals, Inc. Biopolymers 81: 440–449, 2006
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