Conducting gramicidin channels form predominantly by the transmembrane association of monomers, one from each side of a lipid bilayer. In single-channel experiments in planar bilayers the two gramicidin analogs, [Val1]gramicidin A (gA) and [4,4,4-F3-Val1]gramicidin A (F3gA), form dimeric channels that are structurally equivalent and have characteristically different conductances. When these gramicidins were added asymmetrically, one to each side of a preformed bilayer, the predominant channel type was the hybrid channel, formed between two chemically dissimilar monomers. These channels formed by the association of monomers residing in each half of the membrane. These results also indicate that the hydrophobic gramicidins are surprisingly membrane impermeant, a conclusion that was confirmed in experiments in which gA was added asymmetrically and symmetrically to preformed bilayers.

The crystal structure of 2,4,6-triamino-1,3,5-triazine has been investigated with extensive three-dimensional X-ray (sin 0/,t < 0.91) and neutron (sin 0/h < 0.84 A -I) diffraction data. The structure is significantly nonplanar with the amine groups deviating by up to 0-10 A from the mean plane through the ring. The molecule does not behave like a rigid body, the out-of-plane mean-square amplitudes of the ring nitrogens being about 12% greater than those of the C atoms. The positional parameters for the C and N atoms from the X-ray refinement are generally similar to the neutron values, but the thermal parameters for these atoms show a systematic bias as a result of the neglect of the non-spherical symmetry of the valence density. The X-ray H positions are displaced along the N-H bonds by amounts which, while generally similar to those reported for similar systems, are also related to the internal modes of vibration in the structure. The internal motions of the H atoms, as indicated by the differences between thermal ellipsoids for the H and amine N atoms, are related to the hydrogen bonding. The amplitudes for the hydrogen-bonded atoms are relatively lower normal to the N--H bonds, and larger in the directions corresponding to the N-H stretching mode. This exaggerates the foreshortening of the N-H bonds, as has been predicted by Coulson

The crystal structure of/~-sulphanilamide has been refined from three-dimensional photographic X-ray data and two-dimensional neutron diffraction data. Positional and anisotropic thermal parameters of the non-hydrogen atoms and positional and isotropic thermal parameters of the hydrogen atoms were refined in the X-ray analysis to give a final R index of 4"9 %. Positional and isotropic thermal parameters of the hydrogen atoms were refined in the neutron study. The final residual factors were R(hOl) = 8-0 %, R(hkO)= 9.1%. The mean standard deviation in the C-C bonds is 0.0028 A and the e.s.d.'s in the hydrogen atom positional parameters are approximately 0-035 A. The bond lengths suggest that there is a small but significant contribution of a quinonoid resonance form to the structure of the molecule. The distribution of residual electron density within the benzene ring and in the tetrahedral sulphamide group is explained in terms of effects resulting from electron redistribution at bonding. The hydrogen bond system closely resembles that found in e-sulphanilamide and the N-H...O bonds are about 0.25/~ longer than in related zwitterion compounds.

The crystal structure of the methyl ester of N-acetylneuraminic acid monohydrate has been determined from three-dimensional X-ray diffraction data. The crystals are orthorhombic, space group P212121, with unit-cell constants a=7.954+ 1, b= 11.675+4, c= 16.974+2 ~. The structure was solved using the tangent formula and refined by block-diagonal least-squares techniques to R=0.042. The three bulky substituent groups, the N-acetyl, the glycero and the methyl ester groups, are all equatorially bonded to the pyranose ring while the hydroxyl group at the anomeric carbon atom C(2) is in the axial position. An extensive system of seven inter-and one intramolecular hydrogen bonds holds the molecules quite rigidly in the crystal lattice, resulting in an average overall isotropic temperature factor coefficient of only 2.7 ,~2.

The crystal structure of triacetylsphingosine C24H43OsN) has been determined by direct methods. The crystals are orthorhombic, P212121, with a = 5.002, b = 8"709 and c = 60.62/~. Positional and isotropic thermal parameters of the non-hydrogen atoms were refined to give a final R index of 0"109. The molecules are arranged head-to-tail in layers within which the carbon chains pack according to the common orthorhombic subcell, O A.. The chain axis forms an angle of 58 ° with the end group planes. Adjacent layers show opposite tilt of the chains. In spite of the bulky acetyl branches the molecules adopt a very effective packing (Din = 1.07 g.cm-3). The molecules are connected by a continuous system of N-H---O hydrogen bonds parallel to a, and there is also evidence for two weaker C-H---O type interactions.