An experimental Ramachandran plot for retropeptide derivatives: Conformational features of derivatives of GEM‐diamino and malonyl amino acids

Malonic and diaminomethane residues, equivalent to the two possible retro modifications of a glycine unit, with an inverted peptide group, present particular conformations that differ from those found in glycine and, in general, in α‐amino acids. In both cases the φ i and ψ i torsional angles have restricted values as deduced from inspection of the Cambridge Structural Data Bank and from compounds studied by us. Thus, both ψ i angles tend to be equal to 115° (or −115°) in the malonyl residues, whereas the φ i angles tend to be equal to 88° (or −88°) in the diaminomethane residues. These results are in agreement with previous experimental data on polymers, but in the case of malonyl residues they differ from theoretical calculations on isolated molecules. The experimental data for both residues can be represented in a way similar to the usual Ramachandran plot, which will be useful in analyzing the incorporation of these residues into proteins. When side chains are present in either type of residue, they are similar to conventional α‐amino acids, although the orientation of the peptide groups is different. In such cases they acquire conformations similar to those found in peptide residues in the α‐helix and β‐sheet conformations, although other conformations are also possible. © 1998 John Wiley & Sons, Inc. Biopoly 45: 149–155, 1998