X-ray crystallographic structure of the cyclic di-amino acid peptide: N,N '-diacetyl-cyclo(Gly-Gly)

The cyclic di-amino acid peptide N,N;-diacetylcyclo(Gly-Gly), C8H10N2O4, crystallizes in the triclinic space group P (1) over bar with unit cell parameters a = 9.4855(4) angstrom, b = 10.0250(3) angstrom, c = 10.0763(4) angstrom, alpha = 73.682(2)degrees, beta = 82.816(2)degrees, gamma = 81.733(2)degrees, V = 906.40(6) angstrom(3), Z = 4 (2 molecules, A and B, per asymmetric unit), D-c = 1.452 g cm(-3) and linear absorption coefficient 0.118 mm(-1). The crystal structure determination was carried out with MoK alpha X-ray data measured at 120(2) K. In the final refinement cycle the data/restraints/parameter ratios were 4124/0/258 and goodness-of-fit on F-2 = 1.0008. Final R indices for [I > 2 sigma(I)] were R-1 = 0.0501, wR(2) = 0.1007 and R indices (all data) R-1 = 0.0864, wR(2) = 0.11180. The largest electron density difference peak and hole were 0.241 and -0.232 e angstrom(-3), respectively. The DKP rings in both molecules A and B have boat conformations with pseudo mm2 (C-2v) symmetry if the N atoms and CH2 groups are considered identical. In each case, the prow and stern of the boat are the alpha-carbons C(3) and C(6). The overall molecular symmetry of molecules A and B is approximately C-2 with the twofold symmetry axis of the DKP boat being maintained through the centre of the DKP ring. Details of the molecular geometry are compared with that of the parent compound cyclo(Gly-Gly) in which the DKP ring is planar with exact symmetry (1) over bar (C-i).