Crystal structure and conformation of a cyclic tetrapeptide cyclo ( L ‐Pro‐Sar) 2 containing all‐ cis peptide units

The crystal structure and conformation of the synthetic cyclic tetrapeptide, cyclo ( L ‐Pro‐Sar) 2 , was determined by x‐ray analysis. The peptide crystallizes in the orthorhombic space group P 2 1 2 1 2 1 with cell parameters a = 9.277(1), b = 12.884(1), and c = 15.581(2) Å. The crystal structure was solved by the symbolic addition procedure for direct phase determination and least‐squares refinement using 1796 reflections, which led to the final R value of 0.043. This structure provides the first example observed in a crystal of a cyclic tetrapeptide in which all four peptide units have been found in the cis conformation with ω angles deviating slightly by 2°–10° from the ideal value of 0°. It was also found that the two Pro C α ‐CO single bonds assumed a trans ′ (ψ = 159.6° and 158.4°) conformation. Adjoining average planes of the peptide groups fall at nearly right angles to each other. The pyrrolidine ring conformations of the two prolyl residues are in the envelope form, with C γ carbon out of the least‐squares planes for the remaining four atoms.