Guanidyl‐carboxylate interactions: crystal structures of arginine dipeptides*

The crystal structures of the hydrated dipeptides l‐arginyl‐l‐aspartic acid and l‐arginyl‐l‐glutamic acid have been determined from three‐dimensional X‐ray diffraction data. Each peptide crystallizes as a double‐zwitterion with both the main and side‐chain carboxyl groups ionized and the amino and guanidyl termini protonated. The peptide backbone conformation in both peptides is remarkably similar. Both peptides adopt a trans conformation for the peptide linkage with the guanidyl and acidic side‐chains extended on opposite sides of the peptide backbone. The arginyl side‐chain conformations differ between peptides; the conformation observed for arginyl aspartic acid is unique. Extensive intermolecular hydrogen bonding networks are observed in both structures; however, in neither structure is there evidence of intramolecular hydrogen bonding. The intermolecular guanidyl‐carboxylate interactions are detailed. These interactions include a modified Type A interaction which models the possible bridging of adjacent peptide carbonyl oxygens in an α‐helix by the guanidinium moiety.