X-ray crystallographic structure and absolute configuration of the cyclic di-amino acid peptide:cyclo(l-homoCySH-l-homoCySH)

The cyclic di-amino acid peptide cyclo(l-homoCySH-l-homoCySH) [(3S, 6S)-3, 6-bis(2-sulfanylethyl) piperazine-2,5-dione, C8H14N2O2S2, crystallizes in the orthorhombic space group P2(1)2(1)2(1) with unit cell parameters a = 6.1509(2) , b = 18.0217(9) , c = 29.6166(14) , V = 3283.0(2) (3), Z = 12 (3 molecules, A, B and C, per asymmetric unit), D (c) = 1.422 g cm(-3) and a linear absorption coefficient of 0.464 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 5595/0/385 and goodness-of-fit on F (2) = 1.084. Final R indices for [I > 2sigma(I)] were R1 = 0.0746, wR2 = 0.1356 and R indices (all data) R1 = 0.1092, wR2 = 0.1529. The largest electron density difference peak and hole were 0.526 and -0.445e (-3). The DKP rings in all three molecules are essentially, and unusually, planar and the C=O oxygen atoms are co-planar with the ring in each case. Ring atom rms deviations, including the =O groups, are 0.0668, 0.0658 and 0.0656 in molecules A, B and C, respectively. Details of the molecular geometry are compared with the compound cyclo(Gly-Gly) (Degeilh R, Marsh RE Acta Cryst 12:1007, 1959) and in addition some cyclic di-amino acid peptides in which the DKP rings have more puckered boat conformations.