Threefold Helix Research Articles

Conformation of crystalline type III pneumococcal polysaccharide

The conformation of crystalline Type III pneumococcal polysaccharide, poly[(1→3)-β- d-Glc p-(1→4)-β- d-Glc p], has been studied by X-ray diffraction and stereochemical analysis. The X-ray pattern, recorded at 80 % relative humidity, led to a trigonal unit-cell, with a  b  1.028 nm, c  2.77 nm, and γ  120°. Meridional reflections on the third and the sixth layer-lines show that the chain consists of a three-fold helix, with a repeat of 0.923 nm. Conformational analysis, by the virtual-bond method, shows that one right-handed and two left-handed helices are equally favorable, on the basis of energies of isolated chains. Packing of the three helix models in the unit cell, by contact-distance criteria, ruled out the possibility of the right-handed helical conformation. One of the left-handed helices has intramolecular hydrogen-bonds O-3′...O-5 and O-2... O-6′ between the residues in the (1→4) linkage, and O-4′...O-5 between those in the (1→3) linkage. In the other left-handed helix, the O-4′...O-5 hydrogen bond is replaced by an O-2...O-2′ hydrogen bond. The choice between the two left-handed helix models is inhibited by the paucity of X-ray intensity data.

The molecular details of collagen hydration

AbstractNuclear magnetic resonance and dielectric data on hydrated collagen are interpreted in terms of Ramachandran's hydration model. It is found that all data are compatible with this model, indicating two specific binding sites per three amino acids in the threefold collagen helix. Sorption data have been interpreted according to the multilayer theory of Guggenheim and used to derive the fraction of bound water in the primary sites. From magnetic resonance anisotropies structural details of the position of the water molecules can be derived under the assumption that both sites are equally occupied. The residence time of a water molecule in one of these sites in moderately hydrated collagen (45 g H2O/100 g collagen) is 1.2 × 10−6 sec. The remainder of the water is weakly bound and consists of rapidly exchanging species with rotational correlation time shorter than 10−10 sec. The sites are 50% occupied at a water content of 10 g/100 g collagen and may contribute significantly to the stability of the collagen threefold helix.

Molecular structures for microbial polysaccharides: x-ray diffraction results from Klebsiella serotype K57 capsular polysaccharide

Fibre X-ray diffraction patterns have been obtained from oriented, semi-crystalline films of the Klebsiella capsular polysaccharide secrotype K57. K57 is a polytetrasaccharide that contains galactosyluronic acid, mannosyl, and galactosyl residues in the backbone, and an additional mannosyl group as a side-appendage. The simplest interpretation of the diffraction pattern is that the molecule crystallizes as a three-fold helix with an axially projected repeat of 1.143 nm which correlates directly with the chemical repeat. The chain is highly extended, even though it incorporates a 1,2-diaxial linkage in the main backbone. Molecular models have been built using least-squares techniques to minimise interatomic compression and simultaneously meet the observed helical parameters. These models have been compared with the experimental data by using cylindrically averaged, Fouriertransform calculations.

Conformation of the gummy polysaccharide from corm sacs of Watsonia pyramidata

X-ray diffraction patterns have been obtained from drawn fibres of the gummy polysaccharide found in the corm sacs of Watsonia species. The polysaccharide consists of 1,4 linked β- d-xylose units, highly substituted with very short side chains. The X-ray reflections index on a trigonal unit cell and the evidence suggests that the molecule crystallizes as a left-handed, three-fold helix with an axially projected rise of 0.495 nm. The unit cell dimensions are dependent on relative humidity and contract from a = b = 1.40 nm at high relative humidity, to a = b = 1.34 nm when dried. Little variation is observed for the chain axis repeat, c. Contraction is accompanied by a loss of crystallinity.

X‐ray structure determination of isotactic poly(2‐vinylpyridine)

AbstractThe structure of isotactic poly(2‐vinylpyridine) has been determined from x‐ray fiber data utilizing rigid‐body least‐squares refinement techniques. The polymer exists as a threefold helix with three chains passing through a hexagonal unit cell having the dimensions a = b = 15.49 Å and c = 6.56 Å (fiber axis) and space group P31 or P32. The asymmetric unit consists of three monomeric units (one in each chain), resulting in three crystallographically independent chains in the unit cell. The results show that two of the chains are interlocked through pyridine ring interactions whereas the third chain is less restricted. A statistical structure is proposed in which any two chains in the unit cell can be interlocked, provided they have the same sense. Such a situation permits a completely statistical structure in which half the chains in any crystallite are oriented upward and the other half downward.

The conformation of the mucopolysaccharides. Hyaluronates.

X-ray-diffraction patterns of hyaluronate fibres from a variety of sources were obtained. Sodium hyaluronate gives well-defined patterns which index on a hexagonal unit cell with dimensions a=1.17+/-nm and a fibre repeat-distance of 2.85+/-0.03nm. A further form of sodium hyaluronate is produced by annealing at 60 degrees C in 75% relative humidity. This stable state indexes on a hexagonal unit cell of unchanged fibre repeat-distance but with a=1.87nm. The chain conformation is a threefold helix. Analysis of these diffraction patterns led to two tentative structures for sodium hyaluronate, involving different packing of the polysaccharide chains. The significance of side-chain interaction is discussed. Hyaluronic acid produces an X-ray pattern different from that obtained with the sodium salt. The fibre repeat-distance is 1.96+/-0.02nm and the unit cell appears to be monoclinic. The chain conformation is a twofold helix and conformational change between free acid and monovalent salt is discussed. These findings, together with model-building experiments, are interpreted as indicating a highly ordered structure, and the physical properties of hyaluronate solutions with regard to molecular shape and polyelectrolyte behaviour are rationalized.

Crystal structure of isotactic polyvinylethylsilane

Isotactic polyvinylethylsilane crystalizes in a rhombohedral unit cell whose constants referred to hexagonal axes are: a = b = 21·60 ± 0·20 A ̊ , c = 6·50 ± 0·05 A ̊ (fibre axis), space group R3̄. The unit cell contains 18 monomeric units, C 4SiH 10, and the calculated density of the crystalline polymer is 0·98 g/cm 3. The conformation of the macromolecule in the crystal state, is a three-fold helix. Each molecule is surrounded by three enantiomorphous anticlined molecules.

Isotactic polyvinylethylsilane. A polymer containing active bonds Si—H in its side groups

The polymerization of vinylethylsilane (CH 2 = CH)SiH 2(C 2H 5) by the catalyst system VCl 3/Al(C 2H 5) 3, acting heterogeneously, is described. The resulting polymer is a white fibrous solid exhibiting crystallinity by X-ray examination. On the basis of the i.r. spectra, the polymer was assigned a hydrocarbon chain structure with side groups containing two Si-H bonds. The fibre spectra revealed a three-fold helix conformation of the macromolecules with an isotactic succession of the tertiary carbon atoms.

Crystal structure of isotactic poly (vinyl methyl ether)

AbstractThe crystal structure of isotactic poly(vinyl methyl ether) has been defined on the basis of x‐ray fiber spectra. Isotactic poly(vinyl methyl ether) crystallizes in a rhombohedral unit cell whose constants referred to hexagonal axes are: a = b = 16.25 ± 0.15 A., c = 6.50 ± 0.05 A. (chain axis) Z = 18, dcalc. = 1.168 g./cm.3. Probable space group \documentclass{article}\pagestyle{empty}\begin{document}$ \[R\overline 3 \] $\end{document}. Themagnified imagemonomeric units repeat along a threefold helix. Atomic positions were deduced by trial and error. Right‐handed and left‐handed macromolecules are contained in the unit cell and each macromolecule is surrounded by three enantiomorphous, anticlined macromolecules. The internal rotation angles around the CC bonds, defined with respect to the main chain bonds, have been found to be 280° and 160° for the right‐handed helix (the macromolecule is growing anticlockwise) and 80° and 200° for the enantiomorphous macromolecule. A comparison between the packing of the macromolecules of isotactic poly(vinyl methyl ether), of isotactic poly‐α‐butene (modification 1), and of other isotactic polymers is carried out.

Chain structure of isotactic polyvinyl cyclohexane

Many types of helical chains present in the crystals of different isotactic polymers have been described in the past few years. They all correspond to successions of more or less distorted trans and gauche bonds. The chains of the polymers, which have an identity period along the fiber axis comprised between 6·4 and 6·7 A, generally correspond to successions of undistorted trans (σ 1 = 180°) and gauche (σ 2 ∼ 60°) bonds, repetition being achieved along a three-fold helix 2 [(AB) 3 or (AC) 3 ]. This chapter presents an evaluation of the structure and mode of packing of the isotactic polyvinyl cyclohexane. The isotactic structure of polyvinyl cyclohexane is confirmed by the fact that through hydrogenation of isotactic polystyrene, and hydrogenation and dehalogenation of isotactic amorphous polychloro-styrenes, the products obtained exhibit the same X-ray diffraction lines as crystalline polyvinyl cyclohexane. All X-ray reflections given by a fiber of this polymer may be indexed on the basis of a tetragonal unit cell, with a = 21·9 A.