The conformations of peptides and proteins can be restricted by combining unusual amino acid sidechains with thiopeptides. These conformational diagrams depict the φ, ψ energy surface calculated for the penicillamine residue (which has a bulky sidechain) juxtaposed with peptide and/or thiopeptide bonds.The penicillamine residue can occupy three potential conformations, but when the peptide bond carbonyl group is thiolated, the conformational space available to the penicillamine residue is calculated to be restricted even further.The calculations presented by Tran et al. (see pp. 401–408) predict that other combinations of bulky sidechains and peptide backbone methylation or thiolation can lead to unique local conformations for the peptide chain.
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