George A. Jeffrey

Carbohydrate liquid crystals

Abstract There has been a marked increase during the past 5 years in the number of mesogenic compounds synthesized from the common, naturally occurring carbohydrates, particularly from glucose and glucitol. The structure of the mesophases depends on the shape of the molecules. Cyclic and acyclic carbohydrates with single n-alkyl or acyl chains of more than six carbon atoms are calamitic amphiphiles which form thermotropic smectic Ad phases. The more soluble of these mesogens are used as non-ionic surfactants for solubilizing and crystallizing membrane proteins. They form gels and lyotropic phases with water. The lamellar phase is most common, but some carbohydrate amphiphiles exhibit all of the classical lyotropic phases associated with the ionic surfactants, i.e. lamellar, cubic and hexagonal. Both the smectic Ad and the lamellar Lα phases are believed to be bilayers with interdigitized alkyl chains in the interior of the molecular clusters. The crystal structures of the alkylated cyclic sugars hitherto ...

Hydrogen bonding in nucleosides and nucleotides

Abstract An analysis of the hydrogen bonding in 76 nucleoside and 11 nucleotide crystal structures shows that the hydrogen bond lengths fall into well-defined categories according to the nature of the donor or acceptor groups. The shortest bonds are those involving POH or OP groups. For donor groups, the sequence in bond lengths is P—OH w (H)—H There are ten examples of two centre H—H…O bonds, which are comparable in length with POH …O bonds. The acceptor seqeunce is O=P 2 2 2 )C … The number of three-centre bonds, about 24%, is comparable to that observed in the carbohydrates and the amino acids. Most hydrogen bonds are involved in short finite chains. Only in the nucleotides are cyclic hydrogen bonding schemes observed.

A survey of hydrogen bond geometries in the crystal structures of amino acids

An analysis of the geometries of the hydrogen bonds observed by neutron diffraction in thirt-two crystal structures of amino acids shows the following results. Of the 168 hydrogen bonds in the data set, 64 involve the zwitterion groups  and CO2. Another 18 are from to sulphate or carbonyl oxygens. The majority, 46, of these H … O bonds are three-centered (bifurcated). Nine are four-centered (trifurcated). The geometry in which the three-centered hydrogen bond involves both oxygens of the same carboxylate group is not especially favoured. When it does occur, one hydrogen bond is generally shorter and the other longer, than when the bonding involves oxygens on different carboxylate groups. The shortest hydrogen bonds are the OH … O C, from a carboxylic acid hydroxyl to a carboxylate oxygen, and NH … OC when the nitrogen is the ring atom in histidine or proline. Carboxylate groups, on average, accept six hydrogen bonds, with no examples of less than four bonds. The reason for the large number of three-centered H … OC bonds is therefore a proton deficiency arising from the disparity between the tripled donor property of the groups and the sextuple, on average, acceptor property of the carboxylate groups. There is good geometrical evidence for the existence of H … O and H … Cl− hydrogen bonds, especially involving the hydrogen atoms on α-atoms.

Hydrogen-Bonding: An Update

This article focusses on the crystallographic research aimed at a better understanding of hydrogen bonds which has been published since January 1990. In the interest of continuity, earlier work is quoted when it relates to that published during this period.

The crystal structure of deuterated benzene

The crystal structure of deuterated benzene has been refined by single-crystal neutron diffraction analysis at 15 and 123 K. The unit-cell dimensions were also measured at 52.6 and 80 K, and the thermalexpansion coefficients at all four temperatures were calculated. The molecules have C3v symmetry with a small chair-distortion from D6h, which is possibly significant for the carbon atoms and significant for the deuterium atoms. The mean observed bond lengths at 15 K [123 K] are C-C = 1.3972(5) Å [1.3940(9) Å] (1 Å = 10-10 m = 10-1 nm); C-D = 1.0864(7) Å [1.0838(10) Å]. When corrected for molecular libration, the corresponding rest values are 1.3980 Å [1.3985 Å]; 1.088 Å [1.088 Å]. Ab initio molecular orbital calculations at the MP-2/6-31G* level gave energy-optimized bond lengths of 1.395 and 1.087 Å for the isolated molecule at rest, in agreement with the corrected values from the crystal structure within the experimental errors.

The application of ab initio molecular orbital theory to structural moieties of carbohydrates

Abstract Ab initio molecular orbital calculations were made on methoxymethanol, a model for the hemiacetal and acetal moieties in aldopyranoses and methyl aldopyranosides, thereby improving on the previous calculations using methanediol. The new calculations confirmed the favored conformations already deduced, and gave, for the conformational-energy differences and CO bond-length variations, values more appropriate to the carbohydrate systems, as confirmed by a re-examination of the experimental data from crystal-structure determinations. From the results, it was predicted that the OCH3 bond in methyl aldopyranosides is lengthened; this is supported by the experimental data. An examination of the angles and bond-lengths in the pyranoid ring and of the linkage bonds of oligosaccharides indicated that similar electronic effects involving the oxygen lone-pair electrons apply to oligo- and poly-saccharides.

The crystal structure of anhydrous α,α-trehalose at −150°

Abstract The crystal structure of α,α-trehalose (α- d -glucopyranosyl α- d -glucopyranoside), C12H22O11, is monoclinic, P21, Z = 2, with unit cell dimensions at −150° [20°] of a = 12.971(5) [13.003(5)], b = 8.229(4) [8.252(4)], c = 6.789(3) [6.799(3)] A, β = 98.12(4) [98.33(4)]. The crystal structure was solved by using MULTAN, and refined to R = 0.059, Rw = 0.048 for 1564 intensities, measured with MoKα radiation. The molecular structure is very similar to that observed in the dihydrate crystals. It has approximate C2 symmetry. Both glucopyranosyl groups are in the 4C1 conformation. The linkage torsion angles, O-5—C-1—O-1—C-1, are +60.8° and +60.1°. The primary alcohol groups are oriented gauche/gauche and gauche/trans, as in the dihydrate structure. A comparison of the cross-polarization, magic-angle-spinning (c.p.-m.a.s.), 13C-n.m.r. spectra for powders of the crystalline anhydrous and dihydrate forms shows differences in resonances assigned to C-1 and C-4 that would not be anticipated from the results of the crystal-structure analyses.

Carbohydrate liquid-crystals☆

Abstract Certain alkyl glycosides and 1-thioglycosides are shown to have thermotropic, liquid-crystal phases between room temperature and their melting points. In the alkyl 1-thio-β- d -xylopyranoside series, liquid-crystal phases occur with heptyl and octyl chains, but not with hexyl, or lower, members of the series. Heptyl 1-thio-α- d -mannopyranoside forms liquid crystals, but the substitution of terminal OH, Cl, and CN groups on the alkyl chains inhibits liquid-crystal formation. Octyl, nonyl, and decyl α- and β- d -glucopyranosides form liquid crystals. This property is associated with crystal structures in which the carbohydrate moieties are hydrogen-bonded and the alkyl chains intercalated.

Carbohydrate Liquid Crystals

Abstract Certain alkyl l-O and l-S glycosides with hydrocarbon chain lengths greater than hexyl have been shown to form thermotropic liquid crystals at temperatures between 60 and 100[ddot]C. These are believed to be members of a large class of alkyl and acyl carbohydrate mesogens which, apart from their intrinsic value as potentially useful solid-state materials, could provide a variety of structurally accessible model systems for studying the phase transitions in the cell-membrane glycolipids.

THE UPS AND DOWNS OF C-H HYDROGEN BONDS

Abstract C–H⋯O hydrogen bonding has been known for more than sixty years. At one time it was a controversial subject. For the past sixteen years, its credibility has steadily increased until now it is very fashionable. Weak C–H⋯O interactions appear to be non-directional. Does this disqualify them as hydrogen bonds?