L-Aktiiyl-L-alanine crystallizes in the tetragonal space group, 14, with a = 17.985 i : 0.005 and c = 5.154 rt 0.003 8. The crystal structure of this dipeptide was solved using noncentrosymmetric direct methods on 1396 reflections measured with Mo Ka! radiation on an automatic four-circle diffractometer; details of the phasing procedure are briefly discussed. Full-matrix least-squares refinement of' the structure yielded a conventional R = 0.866, with standard deviations of 0.005 8 and 0.3" in the bond lengths and bond angles. Comparison of the peptide bonding parameters with a weighted average set of available data presented by Marsh and Donohue reveals small but statistically significant differences. The conformation of the molecule in this zwitterion structure differs markedly from the more nearly planar form found in the hydrochloride derivative. The intricate hydrogen bond network, atypical of other known peptide crystal structures, interlocks tehragonally-packed columnar arrays of four-membered rings of molecules. IntroductionThe application of direct statistical methods to noncentrosymmet rlc structures typically involves space groups of c~rtliorhornbic or Iomer symmetry and the structures usually include a moderately heavy atom or a substantial planar group. This report on L-alanyl-L-alanine describes a unique and curious equal-atom dipeptide structure in a remarkably, rare tetragonal space group, 14, In addition to its intrinsic interest as an unusual example of a zwitterion dipeptide structure unperturbed by heavy atom or salt interactions, it provides an interesting comparison with the recently !;tricture of ~-alanyl-~-alanine hydrochloride; significant differences are revealed in the conformation of the rnolecule in the two states. Such comparisons are of fundamental importance to any analysis of the effects of hydrogen bonding and ionic interactions on the conformation of peptide linkages. The ultimate succehs of u p~i o r i calculations of peptide and protein conformations must surely depend upon the successful analysis of such perturbations in suitable model systems.

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Crystals of the title compound, CaaHxxO2N, are orthorhombic, space group Fdd2, Z= 8, a=27.289 + 0.004, b = 13.108 + 0.005, c = 9-165 + 0.002/~, Dx = 1.362 g.cm -3. Data for 454 reflections were collected at room temperature on a Picker automated diffractometer. The structure was solved by direct methods, using a computerized multiple-solution procedure, and it was refined by full-matrix least-squares methods to a final R on F of 0"046, with 377 observed reflections. The average e.s.d.'s of the bond lengths and angles not involving hydrogen are about 0.008 A and 0.5 °, respectively. Bond lengths and angles were corrected for thermal motion. Some C-C single bonds are found to be significantly shorter than the normal C-C single bond length of 1.54 A. The shortening of the C-C bonds is explained by the conjugation of the C=O bond and C=C bond with the three-membered ring.