Showing papers on "Hydrogen bond published in 1974"

Structural Chemistry of 1,2 Dilauroyl-DL-phosphatidylethanolamine: Molecular Conformation and Intermolecular Packing of Phospholipids

Abstract: Crystals of 1,2 dilauroyl-DL-phosphatidyl-ethanolamine:acetic acid are monoclinic with a = 462, b = 777, c = 995 A, β = 920°; space group P21/c The structural analysis, based on the visual estimates of 1467 reflection intensities, was achieved by direct methods, and least squares analysis convergence was to R1 = 028 There are marked differences between the observed molecular conformation and those that have been predicted theoretically The mean planes containing the lipid chains are essentially parallel to one another; the phosphodiester moiety has a double gauche conformation, while intermolecular hydrogen bonding modifies the conformation that could be anticipated for an isolated phosphatidylethanolamine molecule The intermolecular packing produces the classical lipid bilayer structure, adjacent lipid bilayers being separated by acetic acid molecules of crystallization The hydrocarbon chain packing can be considered either as a quasi-hexagonal type or as a complex orthorhombic subcell arrangement One-dimensional electron density profiles across the lipid bilayer at increasing resolution clearly demonstrate the origin of features present on the low resolution profiles of both model and natural membranes

Structures for polyinosinic acid and polyguanylic acid

Abstract: X-ray-diffraction analysis of oriented, partially crystalline fibres of polyinosinic acid has resulted in a new molecular model. This model consists of four identical polynucleotide chains related to one another by a fourfold rotation axis. The coaxial helices are righthanded (screw symmetry 23(2)) and have an axial translation per residue h=0.341nm and a rotation per residue t=31.3 degrees . Incorporated in the model are standard bond lengths, bond angles and C-2-endo furanose rings. The nucleotide conformation angles, determined by linked-atom least-squares methods, are orthodox and the fit with the X-ray intensities is good. Each hypoxanthine base is linked to two others by hydrogen bonds involving O-6 and N-1. Further stability may arise from intrachain hydrogen bonds between each ribose hydroxyl group and the phosphate oxygen O-3. If guanine were to be substituted for hypoxanthine in an isogeometrical molecular structure, additional hydrogen bonds could be made between every N-2 and N-7.

Model of interaction of polar lipids, cholesterol, and proteins in biological membranes.

Abstract: Membranes are proposed to consist of a hydrophobic core, two hydrogen belts, and two polar zones The hydrogen belts consist of hydrogen bond acceptors, ie the carbonyl groups of phospholipids and sphingolipids, and hydrogen bond donors, ie the labile hydrogens of cholesterol, sphingosine, proteins, and water The density of anhydrous hydrogen bonding and the impermeability of the membrane increase with increasing concentrations of cholesterol, sphingolipids, α-hydroxy acyl residues, plasmalogens, and ether phospholipids Cholesterol owes its membrane-closing properties to its rigid longitudinal orientation in the membrane combined with the latitudinal orientation of the O−H bond It is suggested that the intrinsic proteins of membranes are held in position by hydrogen bonding, as well as by hydrophobic and electrostatic forces, and that hydrogen bonding also mediates the penetration of membranes by proteins

Depolarized Rayleigh scattering and hydrogen bonding in liquid water

Abstract: Depolarized Rayleigh scattering spectra have been obtained for water over essentially its entire liquid range at atmospheric pressure. Three principal components of the Rayleigh line were identified and these were described in terms of molecular reorientation of the molecules, the formation and breaking of hydrogen bonds between molecules, and polarizability anisotropies resulting from the collisions or interactions among several molecules. For the region of the spectrum 0 to 50 cm−1, the most intense component of the spectrum is that which we have ascribed to hydrogen bond kinetics; from it we have determined the mean lifetime of a hydrogen bond in water as a function of temperature.

Hydrogen bonding in adsorption on silica

Abstract: A spectral and calorimetric investigation was carried out into hydrogen bonding between the surface hydroxyl groups of silica and molecules of n -hexane, cyclohexane, carbon tetrachloride, benzene, nitromethane, acetonitrile, acetone, diethyl ether, dioxane, tetrahydrofuran, pyridine, and triethylamine. The integral intensity of the absorption band due to the stretching vibration of free hydroxyl groups on the surface of aerosil and the integral intensities of the absorption bands of these hydroxyl groups perturbed by adsorbed molecules were determined. From the difference in the heats of adsorption on hydroxylated and strongly dehydroxylated surfaces of a pure macroporous silica the heats of formation of the hydrogen bond between adsorbed molecules and free surface hydroxyl groups were evaluated. They were compared with the change in the integral intensity of the stretching vibration band of the hydroxyl groups resulting from adsorption and with their frequency shifts. The relation between the spectral and energetic characteristics of the hydrogen bond resulting from adsorption is approximately the same as that which occurs when the hydrogen bond is formed in solution.

A Proposed Model for Interaction of Polypeptides with RNA

Abstract: Pairs of antiparallel β polypeptide-chain segments in known protein structures are usually observed to form right-handed double helixes with helix parameters in the same range as those of nucleic acids We have constructed a model containing only standard bond lengths, bond angles, and dihedral angles in which such a polypeptide double helix fits precisely into the minor groove of an RNA double helix with identical helix parameters The geometry of the RNA portion is essentially a hybrid between those of the A and A′ forms Hydrogen bonds can be made between the ribose 2′-hydroxyls and polypeptide carbonyl oxygens Since such precise complementarity between the stable conformations of RNA and polypeptides is unlikely to be merely coincidental, we propose that it played a fundamental role in the initiation of precellular evolution Specificially, we propose that the two double-helical structures are mutually catalytic for assembly of one another from activated precursors in the prebiotic soup, and moreover that they provide some degree of genetic coding

Non-linearity of hydrogen bonds in molecular crystals

Abstract: IN recent studies of the non-linearity of hydrogen bonds1,2, histograms have been shown of the O–H ⋯ O angles, ψ The distributions reflect the energy dependence of the hydrogen bond on the angle ψ. In such distributions maxima are found at values of ψ approximately 15° from the linear configuration. Model calculations on water dimer interactions3 indicate, however, that the configuration of minimum energy involves a linear hydrogen bond.

Crystal and molecular structure of V‐anhydrous amylose

Abstract: The conformation and crystalline packing of V-anhydrous amylose has been investigated by a combination of linked atom model building and X-ray diffraction analysis. The unit cell, the P212121 space group, the left-handed sixfold helical conformation with all O(6) in gt rotational positions, and the intrahelical O(2)---O(3) and O(2)---O(6) hydrogen bonds are substantially in agreement with previous studies. A new model for packing of the chains in the unit cell and the presence of crystallographic water is proposed. Packing appears to be stabilized by corner-to-center chain O(2)---O(2) hydrogen bonds. The nature of the transition from the amylose–DMSO complex to Va-amylose was considered and it is shown that the transition involves translation of the amylose chains parallel to the a and b unit cell axes with only slight changes in the orientation of the helix. No significant conformational changes result from the transition.

The Crystal and Molecular Structure of β-Lactose

Abstract: The crystal structure of β-lactose (C12H22O11) was solved by direct phasing methods with integrated photographic intensity data. The structure was refined by the anisotropic least-squares method to give a final R value of 0.068 for the observed reflections. The space group is P21 with Z=2 and with unit cell dimensions of a=10.839 (6), b=13.349 (6), c=4.954 (5) A, and β=91.31 (9)°. All the hydrogen atoms were located on different syntheses. The crystals are nearly isostructural with those of β-cellobiose except for the axial O(4) atom. The exocyclic C(5)–C(6) and C(5′)–C(6′) bonds are significantly shorter than the other C–C bonds. The lengths of the two ring C–O bonds are unequal in the galactose unit and equal in the glucose unit. The C(1)–O(1) and C(1′)–O(1′) bond lengths are shorter by 0.015 A and 0.030 A than the mean exocyclic C–O length of 1.417 A. The structure contains an intramolecular hydrogen bond and exhibits an asymmetrical twist about the bridge bond. All the oxygen atoms, except the bridge ...

A 13C-NMR and IR study of isocyanides and some of their complexes

Abstract: 13C chemical shifts and 1J(14N13C) coupling constants as well as stretching frequencies of the isocyano group are reported for some representative aliphatic, unsaturated and aromatic isocyanides and for two copper(I) isocyanide complexes. The results are discussed in terms of the inductive and mesomeric substituent effects on the polarisation and charge density of the CNC bonds. The marked solvent effect on the chemical shifts of the isocyano carbon hampers comparison of our data with previously reported data. The hydrogen bonding shift of this carbon in water or methanol is much smaller than previous data suggest.

Hydrogen bond strengths and acidities of hydroxyl groups on silica–alumina surfaces and in molecules in solution

Abstract: The properties of hydroxyl groups on mixed silica–alumina gels (0–50 % Al2O3) have been studied by adsorbing weak H bond accepting molecules onto the gels and recording the OH stretching infrared band.Additional measurements on silica gel and compilation of numerous literature data have shown that frequency shift measurements, or series of Δtext-decoration:overlinevOH measurements condensed into BHW (Bellamy, Hallam and Williams) slopes, provide a reliable estimate of the OH acidity. The resulting scale is independent of the atom to which the hydroxyl is attached; it may be used for surfaces of solids as well as for solutions.The results obtained for silica–alumina show that two distinct populations of hydroxyl groups are present. Hydroxyl groups of type 1 are identical with those present on the surface of silica. The spectra show the presence of hydroxyl groups of a second type, the proportion of which increases as the Al content of the gel rises; their acidity corresponds to a pKa between –4 and –8 and they are presumably the sites responsible for the protonation observed upon adsorption of pyridine or ammonia.

Crystal structure of NH4ClO4 at 298, 78, and 10 °K by neutron diffraction

Abstract: The crystal structure of ammonium perchlorate has been studied at 298, 78, and 10°K by means of neutron diffraction. The NH4ClO4 crystal has an orthorhombic unit cell, space group Pnma, with four formula units per cell at all three temperatures. The unit cell dimensions at 298°K are a=9.20, b=5.82, c=7.45 A, as determined previously; at 78°K, a=9.02, b=5.85, c=7.39 A; and a=8.94, b=5.89, c=7.30 for 10°K. Initial positions of H atoms were determined from a difference Fourier map of the room temperature data. However, a least‐square refinement of these data did not converge. The thermal motions and orientations of the ammonium groups at low temperature (10 and 78°K) were determined by means of a constrained refinement which treated the ammonium group as a rigid body; these results are compared with the results obtained by the conventional least‐squares method. Both refinements show that the librational motions about one of the three principal axes have particularly large rms amplitudes: 21° for the 10°K structure and 30° for the 78°K structure. Each ammonium group is surrounded by 10 oxygen atoms with short N ⋯ O distances ranging from 2.9 to 3.25 A. The ClO4− group and NH4+ group each have essentially ideal tetrahedral structure. They are linked together by N–H ⋯ O type hydrogen bonds, one for each hydrogen, to form a three‐dimensional network. Examination of the rms amplitudes for libration and the hydrogen bonding of the NH4+ ions indicates that two of the four hydrogens are bound identically, one hydrogen is bound more rigidly, and the fourth more weakly. These results suggest that the rotational motions of the ammoniums are quite complex even at 10°K.

On the Hydrogen Bond Breaking Ability of Fluorocarbons Containing Higher Halogens

Abstract: Infrared measurements show that fluorocarbons containing higher halogens are able to open O—H---O, N—H---N, S—H---S, N—H---O=C, type hydrogen bonds. This is probably due to a competitive mechanism ...

Cluster expansions for hydrogen bonded fluids. II. Dense liquids

Abstract: The cluster expansion theory of Mayer is formulated in a way that makes it useful for application to dense hydrogen bonded fluids, or, in general, to fluids with very strong attractive intermolecular potentials that are very dependent upon the relative orientations of the molecules. In the resulting diagrammatic series, the hydrogen bond appears only in a renormalized form that is much weaker than the Mayer f function for the original unrenormalized hydrogen bond potential. The diagram series for the pair correlation function is divided into various parts according to the topological structure and physical interpretation of the diagrams, and exact relationships among these parts are derived. These exact relationships can become the starting point for approximate calculations of the pair correlation function for hydrogen bonded liquids such as water.