Threefold Helix Research Articles

Manufacture of strong melt-spun fibers derived from α-1,3-glucan esters and determination of their crystal structures and crystalline elastic moduli

Melt-spun fibers derived from α-1,3-glucan esters were successfully manufactured without a plasticizer. Fibers derived from α-1,3-glucan butyrate had a tensile strength of over 1000 MPa. The crystalline structures of the α-1,3-glucan esters were determined as monoclinic systems by X-ray diffraction. The linear esters comprised threefold helices and the branched esters comprised five- or eightfold helices. The crystalline elastic moduli (El) parallel to the fiber axes were determined by time-resolved X-ray diffraction with tensile tests. Specifically, α-1,3-glucan propionate, α-1,3-glucan butyrate, and α-1,3-glucan valerate—which had threefold helices—had El values of 24.0, 12.4, and 4.2 GPa, respectively. In contrast, α-1,3-glucan 2-methyl valerate—which had a fivefold helix—had an El of 1.0 GPa. Therefore, the El decreased as the length of the side chain and the helix inclination increased. We concluded that El values of polysaccharide esters are not dependent on the glycosidic linkages, but rather on helix structures.

Crystal polymorphism of polylactides and poly(Pro-alt-CO): The metastable beta and gamma phases. Formation of homochiral PLLA phases in the PLLA/PDLA blends

Among the various crystalline phases of polylactides, an elusive orthorhombic gamma phase (γPLLA) had been obtained so far only by epitaxial crystallization on hexamethylbenzene. A recent work by J. Shao et al. (Macromolecules, 2013, 46, 6933) reports on two “novel modified crystallites” of PLLA obtained “under the confinement of stereocomplex PLLA/PDLA crystallites” produced on cooling blends of PLLA and PDLA from the melt at specific rates. These novel modified crystallites are metastable and transform to the stable αPLLA on heating. Reinterpretation of Shao et al.'s DSC and X-ray data suggests that one novel modified crystallite is the elusive γPLLA. The second form could be a disordered hexagonal phase that blends PDLA and PLLA three-fold helices, although the structural evidence is less compelling. In a parallel line of analysis, a crystal modification of the related alternating copolymer of propene and carbon monoxide (poly(Pro-alt-CO)) described as an orthorhombic structure very similar to γPLLA by Anokhin et al. (Polymer Science, Ser. A, Vol. 46, No. 1, 2004, pp. 52–60) must be reinterpreted as a trigonal frustrated phase, a phase that has already been observed in single crystals of this polymer.

The structure of uniaxially stretched isotactic polypropylene sheets: Imaging with frequency-modulation atomic force microscopy

Isotactic polypropylene sheets were uniaxially stretched and observed with a frequency-modulation atomic force microscope operated in phenyloctane liquid. Crystalline lamellae were seen in fibrils with their axis parallel to the stretched direction. Individual CH3 side-chains of three-fold helices were identified in the lamellae. Fragmentation of the lamellae was induced by further stretching. The real-space features observed with the microscope were successfully compared with X-ray scattering results obtained in a synchrotron radiation facility.

Role of cosolutes in gelation of high-methoxy pectin. Part 2. Anomalous behaviour of fructose: calorimetric evidence of site-binding

Gels of high-methoxy pectin (DE 70; 1.0 wt%; pH 3.0) in the presence of fructose at concentrations of 50, 55, 60 and 65 wt% showed an intense endotherm followed immediately by an intense exotherm on heating. These transitions occurred over approximately the same temperature-range as initial gelation on cooling (characterised by low-amplitude oscillatory measurements of G′ and G″) and increased in magnitude with increasing concentration of fructose. The displacement of both transitions, and particularly the exotherm, to progressively higher temperature as the rate of heating was increased was much greater than anticipated from simple thermal lag, indicating that the underlying structural changes are slow. The proposed interpretation is that fructose is capable of site-binding to pectin in both the ordered (threefold helix) and disordered state; the endotherm is attributed to helix melting and displacement of fructose; the subsequent exotherm is attributed to re-attachment of fructose to disordered chains, and the slow kinetics of this process to the conformational mobility of disordered pectin. On cooling over the same temperature range, a single exotherm was observed; the absence of detectable splitting is attributed to rapid re-attachment of fructose to conformationally-rigid helices. The magnitude of this endotherm (Δ H≈20 J/g) is close to the value found for cosolutes that show no evidence of site-binding, and to the net change in enthalpy for the endothermic and exothermic processes observed on heating, suggesting that the values of Δ H for displacement and re-attachment of fructose are essentially equal and opposite, with the net change coming from formation or melting of threefold helices. A smaller thermal process at higher temperature (endothermic on heating and exothermic on cooling) is attributed to hydrophobic association, which was also seen as an increase in G′ and G″ on heating in two consecutive cycles of temperature change.

The frustrated structure of poly(l-lactide)

The crystal structure formed upon stretching or stroking of poly(l-lactide) is determined by electron diffraction and conformational energy analysis. It rests on a frustrated packing of three three-fold helices in a trigonal unit-cell of parameters a=b=1.052nm,c=0.88nm, space group P32. The frustrated packing is of the type described as North–South–South (NSS). This structure appears to be formed to accommodate the random up–down orientation of neighbor chains associated with rapid crystallization conditions. This randomness introduces structural disorder (c-axis shifts and azimuthal setting of neighbor helices). The resultant streaking of the diffraction pattern is modeled. Frustrated packings observed in polymeric systems that depart from three-fold symmetry, and in pseudo-racemates of low molecular weight compounds are discussed.

X-ray crystallographic study of xylopentaose binding to Pseudomonas fluorescens xylanase A.

The structure of the complex between a catalytically compromised family 10 xylanase and a xylopentaose substrate has been determined by X-ray crystallography and refined to 3.2 A resolution. The substrate binds at the C-terminal end of the eightfold betaalpha-barrel of Pseudomonas fluorescens subsp. cellulosa xylanase A and occupies substrate binding subsites -1 to +4. Crystal contacts are shown to prevent the expected mode of binding from subsite -2 to +3, because of steric hindrance to subsite -2. The loss of accessible surface at individual subsites on binding of xylopentaose parallels well previously reported experimental measurements of individual subsites binding energies, decreasing going from subsite +2 to +4. Nine conserved residues contribute to subsite -1, including three tryptophan residues forming an aromatic cage around the xylosyl residue at this subsite. One of these, Trp 313, is the single residue contributing most lost accessible surface to subsite -1, and goes from a highly mobile to a well-defined conformation on binding of the substrate. A comparison of xylanase A with C. fimi CEX around the +1 subsite suggests that a flatter and less polar surface is responsible for the better catalytic properties of CEX on aryl substrates. The view of catalysis that emerges from combining this with previously published work is the following: (1) xylan is recognized and bound by the xylanase as a left-handed threefold helix; (2) the xylosyl residue at subsite -1 is distorted and pulled down toward the catalytic residues, and the glycosidic bond is strained and broken to form the enzyme-substrate covalent intermediate; (3) the intermediate is attacked by an activated water molecule, following the classic retaining glycosyl hydrolase mechanism.

Thermally reversible acid-induced gelation of low-methoxy pectin

Gelation of low-methoxy pectin (DE 31.1) on cooling under acidic conditions in the absence of Ca 2+ has been investigated by rheological measurements under low-amplitude oscillatory shear. The mechanical spectra obtained after 60 min at 5°C showed a progressive increase in solid-like response (increasing G′; decreasing tan δ; increasing frequency-dependence of η ∗) as the pH was reduced from 4.0 to 1.6, with formation of a critically crosslinked network at ∼pH 3.0 (for a polymer concentration of 3.0 wt%). By extrapolation from X-ray fibre diffraction analysis of pectic acid, it is suggested that crosslinking occurs by association of three-fold helices. At pH values between ∼3.5 and ∼2.5 there is no detectable thermal hysteresis between the sol–gel transition on cooling and gel–sol transition on heating, and both are accompanied by a sigmoidal change in optical rotation (attributed to formation and melting of three-fold order). Substantial hysteresis is, however, observed at lower and higher pH, and is attributed to extensive aggregation as electrostatic repulsion is suppressed (below ∼pH 2.5) and slow formation of intermolecular hydrogen bonds by protonated carboxyl groups (above ∼pH 3.5), respectively. The transition enthalpy from DSC heating scans has a maximum value of ΔH≈11 J/g at ∼pH 3.0, but decreases sharply at lower and higher pH, with accompanying loss of a detectable transition in optical rotation. It is suggested that the chain conformation in solution at low pH is predominantly three-fold with, therefore, little conformational change on adoption of the ordered, intermolecular structure, whereas at high pH the solution conformation is predominantly two-fold, with only limited conversion to the three-fold (acid) form on cooling.

Frustration and single crystal morphology of isotactic poly(2-vinylpyridine)

The crystal structure of isotactic poly(2-vinylpyridine) (iP2VP) established in 1977 by Puterman et al. is shown to conform to a recently proposed frustrated packing scheme which involves three isochiral three-fold helices packed in a trigonal unit-cell, and observed in a number of polymers and biopolymers. Single crystals of iP2VP grown from thin films at high temperature ( Tc ≈ 200° C) display highly unusual morphologies with six (100) growth sectors but a three-fold overall symmetry apparent through different lamellar thicknesses in adjacent growth sectors, and oblique growth facets of three of the sectors. These features are morphological manifestations of the frustration of the helix packing, and support the P31 or P3 2 space group of the unit-cell. The different lamellar thicknesses are mainly due to an isothermal thickening process, which has different impacts depending on the nature of the growth faces. Analysis of the growth features of frustrated structures can be of help when dealing with finer details of theories of polymer crystal growth.

The α‘‘ “Superstructure” of Syndiotactic Polystyrene: A Frustrated Structure

The crystal structure of syndiotactic polystyrene (sPS) in its stable α form (a so-called “superstructure” named α‘‘) is reevaluated. As shown by all previous investigations, the trigonal unit cell contains three triplets of extended sPS chains. The azimuthal orientation of the three triplets is shown to differ from those assumed so far, which generates a clear-cut example of frustrated packing. The azimuthal settings and relative shifts of the triplets indicate that two triplets maximize their interactions, while the third one interacts less favorably with its neighbors. This frustrated packing scheme fully justifies the three-triplets unit cell and constitutes an interesting variant of related frustrated packings previously described for isochiral three-fold helices.

Molecular and crystal structure of cellulose acetate dipropanoate (CADP, 6-O-acetyl-2,3-di-O-propanoyl cellulose)

The molecular and crystal structure of cellulose acetate dipropanoate (CADP, 6-O-acetyl-2,3-di O-propanoyl cellulose) has been determined by using a constrained linked-atom least-squares refinement method, combined with X-ray and electron diffractograms and stereochemical refinement. The diffraction analysis indicated that CADP crystallized in a P2 1 monoclinic space group with unit cell parameters:a =1.088 nm,b (unique axis)=1.593 nm,c (fibre axis)=1.509 nm and β =94.1°. The best model derived from combining the stereochemical refinement with the diffraction intensities gave R=0.217 (R″=0.195) for the three-dimensional information from the X-ray fibre diagram and R=0.198 for the base plane data resulting from electron diffraction analysis. In the model, the crystal structure of CADP consisted of a system of right-handed threefold helices packed in an antiparallel fashion, with two molecules passing through the unit cell.

Conformation of an arabinoxylan isolated from the rice endosperm cell wall by X-ray diffraction and a conformational analysis.

The chain conformation of an arabinoxylan isolated from the rice endosperm cell wall, which was composed of the linear backbone of a 1,4-linked beta-xylose residue highly substituted at the C3 position with alpha-L-arabinofuranose, was studied by X-ray fiber diffraction and a conformational analysis. From the X-ray results the polysaccharide was suggested to take an extended left-handed, three-fold helical structure which is not a desirable conformation to make a complex firmly associated with cellulose or xyloglucan that was elucidated to be present in the cell wall. A conformational analysis, using the PFOS force field for the O-alpha-L-arabinofuranose-(1-->3)-beta-D-xylose residue and O-alpha-L-arabinofuranose-(1-->3)-[(1-->4)-beta-D-xylose residue]3, in which the arabinose residue was linked to the C3 position of the middle residue of the xylotriose residue, and using the PS79 computer program for the arabinoxylan indicated that the backbone xylan chain could not take a two-fold helix similar to that of cellulose because of significant steric interaction between the arabinose side chain and the backbone. Thus, the left-handed, three-fold helix was found to be reasonable.

Conformational study of digalacturonic acid and sodium digalacturonate in solution

The solution conformation of digalacturonic acid and its sodium salt have been analyzed using nuclear magnetic resonance data and molecular mechanics calculations. The flexibility around the glycosidic linkage was characterized by calculation of the relaxed (Φ, Ψ) potential surfaces for the isolated molecule, and also for dimethyl sulfoxide and aqueous solutions using the CHARMM and SOLVOL programs. The one-bond and three-bond proton-carbon couplings were measured and H-1′–H-4 distances were estimated from NOESY experiments. The calculated potential surfaces were used to determine theoretical ensemble averages of NMR data. The agreement between the experimental and theoretical data is very satisfactory. The calculations show a strong effect of solvent on the solution behavior of both compounds. The vacuum lowest energy conformer of digalacturonic acid is stabilized by solvation, while for sodium digalacturonate the solvent induces a conformational change. An extrapolation of the stable conformers to polysaccharide chains implies that poly(galacturonic acid) occurs in solution as a three-fold helix and sodium poly(galacturonate) as a two-fold helix.

Molecular and crystal structure of (1 → 3)-α- d-glucan triacetate

A crystal and molecular structure for GTA I, the low temperature polymorph of (1 → 3)-α- d-glucan triacetate, is proposed on the basis of X-ray diffraction analysis of well-oriented films, combined with stereochemical model refinement. The unit cell is monoclinic with parameters a = 30.17 A ̊ , b = 17.42 A ̊ , c (fibre axis) = 12.11 A ̊ , and β = 90° C. The probable space group is P2 1 with b axis unique. Six molecular chains pass through the unit cell with alternating polarity and with three independent chains comprising the asymmetric unit. The chain axes are located in a hexagonal packing arrangement. The chain backbone conformation is a left-handed, three-fold helix, but all nine O(6) acetyl groups of the asymmetric unit are in non-equivalent rotational positions. The most probable structure is indicated by X-ray residuals R = 0.261 and R ″ = 0.283, based on 62 reflection intensities (41 observed and 21 unobserved).

X-Ray Diffraction Study of Glucomannans and Their Acetates

Crystalline polymorphs of four glucomannans with a different mannose/glucose ratio from 1.5 to 4.0 and their acetyl derivatives was studied by X-ray diffraction measurements. The glucomannans showed an amorphous or similar crystal structure (Mannan I or Mannan II polymorph) of β-1,4-d-mannan depending on their mannose content, indicating the occurrence of isomorphous replacement of mannose by glucose during glucomannan crystallization. When these glucomannans were annealed, however, Konjac glucomannan (mannose/glucose ratio = 1.6) showed a different powder pattern from both Mannans I and II. The fiber pattern of the annealed Konjac glucomannan indicated that it took an extended two-fold helical structure. Among the acetylated glucomannans, Higanbana glucomannan with a high mannose content (mannose/glucose = 4.0) showed a similar diffraction pattern to that of β-1,4-d-mannan triacetate, indicating that the isomorphous replacement also occurred in the acetyl derivative. However, glucomannan triacetate with a low mannose content (1.5 or 1.6) showed a different crystalline form from that of mannan triacetate. The fiber diffraction pattern of Konjac glucomannan acetate suggested that it took a three-fold helix.

Three-dimensional structure analysis of the crystalline moiety of A-starch

Despite the fact the first first X-ray diffraction diagrams of starch were reported in the 1930s (I) , unambiguous description of the native crystalline arrangement is not yet complete. Kainuma and French (2) were the first to suggest a double-stranded helix for the B polymorph (commonly found in tubers). This origianl proposal was extensively considered by Wu and Sarko (3,4) who arrived at the conclusion that both B and A types (found in cereals) can be described as right-handed, parallel-stranded double helices, packed in an antiparallel fashion. However, A and B polymorphs differ considerably in the crystalline packing of the duplexes. Current models derived from biochemical data (5,6) rely only on parallel arrangements of molecules. Similar arguments about chain polarity also arise from biosynthetic considerations; it would be more reasonable to expect parallel packing of parallel-stranded double helices since all the chains would have the same polarity. There is therefore a discrepancy between the currently accepted models (7). The present paper is concerned with the threedimensional structure analysis of the A-type polymorph . New crystallographic data were obtained by using two complementary methods: electron diffraction on single, micron-sized crystals (8) and deconvolution of X-ray powder patterns (9) into individual peaks. Wide-angle X-ray diffraction diagrams recorded were similar to those displayed by native cereal starch granules and those corresponding to fibrallar samples. They all can be indexed using an orthorhombic-like unit cell having a volume of -2.2 nrrr'. According to the measured density of 1.48, 12 glucose residues, together with a few water molecules, can be accommodated in the unit cell. Computer modelling of IX (1-4)-linked glucose units indicated that the observed repeat of c = 1.069 nm can either correspond to an extended conformation of a maltotriose unit or to a shallow conformation of a maltoheaxose unit. Therefore, three structural hypotheses had to be tested against their packing constraints: (i) four single three-fold helices (n = 3, h = 0.355 nm); (ii) two six-fold helices (n == 6, h = 0.178 nm); and (iii) two double six-fold helices repeating in 2c = 2.139 nm [in order to restore the observed repeat of 1.069 nm, each double helix has to be parallel stranded, the single strands (n = 6, h = 0.355 nm) being associated to its counterpart through a two-fold axis]. All cases were thoroughly examined and only the latter was Shown to be a valid candidate. Further molecular modelling showed that the conformation at the glycosidic linkage corresponds to the intersectionof n = 6 and h -0.355 nm (left-handedchirality) (Figure I). Such a situation corresponds to one of the stable energy minima (10) and is reminiscent of the observed conformation of a crystalline amylose oligomer (11). Assembling two such parallel single strands through a two-fold symmetry operation permits forma-

Quasi-periodic primary structructures of core proteins of human T-lymphotropic leukemia retroviruses

Fourier analysis, previously introduced by Liquori et al. (1983, 1986), has been applied to the primary structures of two core proteins of human T-lymphotropic leukemia retroviruses HTLV-I and HTLV-II. The resulting autocorrelation functions display striking patterns that can be interpreted in terms of an approximately fourfold quasi-periodicity of the primary structures. Self-alignments of the amino acid sequences containing a few gaps are consistent with the above finding and suggest that the tertiary structure of these two homologous core proteins contains alpha-helical and delta-helical segments, the latter being characteristic of the threefold helix present in collagen structure.

Crystal structure of nylons 2/3/3 and 1,3

AbstractThe crystal structures of two polyamides, poly(glycyl‐β‐alanyl‐β‐alanine) (nylon 2/3/3) and poly(methylene malonamide) (nylon 1,3), have been investigated by x‐ray diffraction and electron microscopy. Crystallization of nylon 2/3/3 from a solution in a mixture of water and formic acid yields lamellar single crystals exhibiting a triangular habit. Doughnut‐shaped morphologies diffracting as single crystals are obtained in the crystallization of nylon 1,3. A helical structure of the type known as polyglycine II is found for both polyamides. In such a structure, chains are intermolecularly linked by hydrogen bonds giving a hexagonal lattice of a = 4.79 Å. Insufficient data are available to determine precisely the conformation of the chains. We assume a threefold helix having c = 35.2 Å and c = 18.0 Å for nylon 2/3/3 and nylon 1,3 respectively. No sign of the layered structure familiar in polyamides has been detected for these polymers throughout the experiments made in the present study.

X-ray fibre diffraction study of conformational changes in hyaluronate induced in the presence of sodium, potassium and calcium cations

Hyaluronate purified from all cations by ion exchange chromatography was introduced to the cations sodium, potassium and calcium in a controlled way. The conformations formed in the presence of these ions were studied as a function of ionic strength, hydrogen ion activity, humidity and temperature using X-ray fibre diffraction. In sodium hyaluronate above pH 4.0 a contracted helix is found which approximates to a four-fold helix with an axial rise per disaccharide of 0.84 nm. There is no requirement for water molecules in the unit cell as the Na + can be coordinate by the hyaluronate chains alone. On crystallizing hyaluronate below pH 4.0 an extended 2-fold helix with an axial rise per disaccharide of 0.98 nm is formed. In the presence of potassium above pH 4.0 a conformation similar, but not identical, to that of sodium was found where the helix backbone is again four-fold with an axial rise per disaccharide h=0.90 nm. To maintain the coordination of the potassium ion, four water molecule/disaccharide are required and on removal of these the conformation is destabilized going to a new helix where n = 4 and h = 0.97 nm. Below pH 4.0 the conformation is a contracted 4-fold helix with h = 0.82 nm. In this structure two antiparallel chains intertwine to form a double helix. The packing of the double helical units is stabilized by water molecules, the unit cell requiring 8 water molecules/disaccharide. Formation of the calcium hyaluronate complex above pH 3.5 yields a three-fold helix with h = 0.95 nm. The requirement for water in the unit cell to maintain full crystallinity is high, at 9 water molecules/disaccharide; however, on removal of this water, though the crystallinity is disrupted, the conformation remains constant. The acid form of calcium-hyaluronate yields an equivalent conformation to that of sodium under the same condition, i.e. a helix with n = 2, h = 0.98 nm. The presence of small quantities of calcium in what are otherwise potassium or sodium solutions of hyaluronate yield the 3-fold conformation for hyaluronate. Thus calcium has an important role to play in deciding the dominating conformation present in hyaluronate. The variety of conformations yielded by the different cations indicates a subtle interaction between hyaluronate and its environment, in which the balance between the cations will control to some degree the interactions between hyaluronate chains and thus affect the mechanical properties of the matrix which they form. The conformations of individual chains are all stabilized in varying degrees by intra-chain hydrogen bonds.

Crystalline conformation and structure of lichenan and barley β-glucan

An investigation based on X-ray fiber diffraetion and conformational analysis methods has provided a proposed chain conformation and crystalline structure for lichenan, a poly β(1 → 3) cellotriose. The analysis of the fiber diagram of the hydrated form of lichenan led to a trigonal unit cell of dimension a = b = 9.92 Å and c (fiber axis) = 42.03 Å, with cellotriose as the asymmetric unit and six cellotriose residues per unit cell. A unit cell of the dry form is also trigonal with a = b = 9.36 Å and c = 42.03 Å. A right-handed threefold helix made up of three cellotriose residues was derived from a comparison of experimental helical parameters with isoenenergy conformational maps made by plotting dihedral angles ([Formula: see text], ψ) at the β(1 → 3) linkage. A virtual bond approach was also used to arrive at the proposed conformation which is stabilized by intramolecular hydrogen bonds between all carbohydrate units. The refinement of geometrical and packing parameters produced results consistent with an antiparallel arrangement of the two chains corresponding to a space group symmetry of P31. The naturally occurring copolysaccharides known as mixed (1 → 3) and (1 → 4) β-glucans from the cell walls of barley endosperm have been shown to crystallize in the same fashion as lichenan.

Extended and threefold helical conformations in gels of isotactic polystyrene

Gels of isotactic polystyrene (iPS) were prepared, using solvents of different molecular geometries, by rapid quenching of the solutions formed at high temperatures. The extended conformation was observed in gels prepared with bulky, cyclic hydrocarbon solvents, whereas the aromatic solvents promoted the threefold helical structure. The gels containing the threefold helix were more elastic compared to those with the extended conformation. The effect of the solvent geometry is explained qualitatively.