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We report magnetic birefringence measurements up to high fields (17.5 T) of dilute aqueous suspensions of rod-like cellulose nanocrystals with well characterized distributions of lengths, widths and thicknesses. We compare these data with three models, one with colinear (1), one with perpendicular cylindrically symmetric tensors for diamagnetic susceptibility and refractive index (2) and one with biaxial diamagnetic anisotropy (3). We find that taking into account polydispersities of length, width, and thickness is essential for accurate fitting and that model 1 is the most appropriate, presumably because of the twisting of the suspended nanocrystal along their long axis. The best-fitted susceptibility anisotropy was Δχz(xy) = χzz–(χxx+χyy)/2 = −2.44 × 10–6 when considering only the crystalline core of nanocrystals and, more appropriately, Δχz(xy) = −0.95 × 10–6 when including crystalline core and skin. The latter value is slightly higher than Δχz(xy) = −0.68(5) × 10–6 deduced from estimations using Pasca...

International audience; Single crystals of amylose V2-propanol, i.e., amylose co-crystallized with water and 2-propanol, were prepared from synthetic linear amylose. When observed under frozen hydrated conditions, these crystals yielded base-plane electron diffraction diagrams containing more than 100 independent diffraction spots, whose intensities were used to solve the crystal structure of this complex. The molecular and crystal structure of amylose V2-propanol clearly indicated that the amylose molecules were organized in 7-fold left-handed helices, with 2-propanol and water molecules located as guests both within and between the helices. The V2-propanol unit cell contains 4 helices, distributed in two antiparallel pairs of parallel helices organized along the P212121 symmetry. The helices are organized along alternating motifs of 4 helices in a close-packed hexagonal arrangement together with 4 others in a nearly square organization surrounding a central column of water and 2-propanol. Whereas the location of the amylose helices is well established in the unit cell, the coordinates of the guest molecules could not be defined with certainty, most likely due to a positional disorder. A tentative model of the guest molecule distribution is presented, which consists of 2 2-propanol and 2 water molecules within the helical cavity together with 4 water molecules and 2 2-propanol molecules between the helices. The mobility of the guest and its description as a continuum, rather than at fixed crystallographic positions, explain why so many structures isomorphous to V2-propanol can be obtained with different guest molecules, while yielding similar electron diffraction diagrams.

The crystal and molecular structure, together with the hydrogen-bonding system in ammonia-mercerized cellulose IIII, has been determined using synchrotron X-ray and neutron fiber diffraction data. The structure has a one-chain monoclinic unit cell with an asymmetric unit that contains only one glucosyl residue and with the hydroxymethyl group in the gt conformation. The hydrogen-bonding system is well-defined with no evidence of disorder. A bifurcated hydrogen bond links a donating secondary alcohol O3 atom to a ring O5 atom (major) and a primary alcohol O6 atom (minor) of an adjacent residue in the same chain. Two hydrogen bonds are present between neighboring chains, perpendicular to the chain direction. A detailed comparison of the crystal structure and hydrogen-bonding system reported here for cellulose IIII and those reported previously for the other cellulose polymorphs is given. The conformation of the chain in cellulose IIII has features similar to that of the center chain in the highly stable cel...

The crystal and molecular structure and hydrogen bonding system in cellulose II have been revised using new neutron diffraction data extending to 1.2 {angstrom} resolution collected from two highly crystalline fiber samples of mercerized flax. Mercerization was achieved in NaOH/H{sub 2}O for one sample and in NaOD/D{sub 2}O for the other, corresponding to the labile hydroxymethyl moieties being hydrogenated and deuterated, respectively. Fourier difference maps were calculated in which neutron difference amplitudes were combined with phases calculated from two revised X-ray models of cellulose II. The revised phasing models were determined by refinement against the X-ray data set of Kolpak and Blackwell, using the LALS methodology. Both models have two antiparallel chains organized in a P2{sub 1} space group and unit cell parameters: a = 8.01 {angstrom}, b = 9.04 {angstrom}, c = 10.36 {angstrom}, and {gamma} = 117.1{degree}. One has equivalent backbone conformations for both chains but different conformations for the hydroxymethyl moieties: gt for the origin chain and tg for the center chain. The second model based on the recent crystal structures of cellotetraose, has different conformations for the two chains but nearly equivalent conformations for the hydroxymethyl moieties. On the basis of the X-ray data alone, themore » models could not be differentiated. From the neutron Fourier difference maps, possible labile hydrogen atom positions were identified for each model and refined using LALS. The second model is significantly different from previous proposals based on the crystal structures of cellotetraose, MD simulations of cellulose II, and any potential hydrogen-bonding network in the structure of cellulose II determined in earlier X-ray fiber diffraction studies. The exact localization of the labile hydrogen atoms involved in this bonding, together with their donor and acceptor characteristics, is presented and discussed. This study provides, for the first time, the coordinates of all of the atoms in cellulose II.« less

Individual dendrimer molecules of poly(amidoamine) (PAMAM) from generation 10 (G10) to G5 were imaged by conventional transmission electron microscopy (TEM) after staining with aqueous sodium phosphotungstate. The dendrimers were resolved as separate, beam-stable entities, and their sizes and distribution of sizes were statistically analyzed and compared with data from small-angle X-ray scattering in solution. Aqueous G10 dendrimers were also characterized by examination in the frozen, hydrated state after quench-freezing in liquid ethane (cryo-TEM) and the data compared with the results of the staining experiments. To a first approximation, the dendrimers appeared circular in projection and the diameters conformed to a Gaussian distribution which broadened somewhat with increasing generation number. Cryo-TEM, in general, confirmed the staining experiments but suggested that, in the native state, there is more variability in the shapes of the dendrimers, with polyhedral shapes occurring quite frequently.

With the aim of improved understanding of the mechanism of depolymerase action, folded chain lamellar single crystals were partially degraded with PHB-depolymerase A from Pseudomonas lemoignei. Enzymatically degraded single crystals of bacterial poly(3-hydroxybutyrate), PHB, were observed by transmission electron microscopy and were found to be splintered parallel to their long axes. Prior to degradation, the crystals were lamellar with macroscopic dimensions of approximately 2.5 μm by 20 μm and the classical baseplane single crystal diffraction pattern corresponding to bacterial PHB was recorded. When observed by TEM, the partially degraded crystals had been splintered longitudinally to a needlelike morphology. The needlelike fragments of PHB still yielded the same crystalline baseplane diffraction pattern. These results support an “edge attack” model for the degradation of PHB single crystals and explain, at a molecular level, the lack of decrease in molecular weight during the degradation since the dir...

13 C NMR, IR and electron diffraction results show that when annealing conditions ranging from 260°C and 30 min to 280°C and 60 min in a series of mediums ranging from helium gas to various solvents were selected, around 80 % of of the Iα phase could be converted into thr Iβ, whereas a residual 20 % remained unmodified. With polar solvents the transformation occurred at temperatures lower than with nonpolar mediums. Samples of lower crystallinity (bacterial cellulose) were converted more easiliy than highly crystalline samples (Valonia).

Cellulose from the cell wall of the green alga Microdictyon tenuius exhibits two distinct crystalline phases. The major phase has a one-chain, triclinic (P1) structure with unit cell parameters of a=0.674 nm, b=0.593 nm, c (chain axis)=1.036 nm, α=117°, β=113°, and γ=81°. The crystal unit cell of the minor component has two chains and is monoclinic (P2 1 ), with a=0.801 nm, b=0.817 nm, c (chain axis)=1.036 nm, and γ the monoclinic angle=97.3°. The triclinic phase is metastable, and annealing it in dilute alkali at 260°C converts it into the monoclinic form. The triclinic and the monoclinic structures correspond to the Iα and Iβ spectra, respectively

Cellulose samples of various origins were subjected to heat annealing. The initial samples contained various proportions of Iα (triclinic) and Iβ (monoclinic) phases. They could be classified into two families: a family I where the amount of the phase Iα was either small or below detection and a family II where this phase was the major component. The annealing treatment revealed that the Iα phase was metastable as it could be converted totally to the stable Iβ phase

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Ce polymere a une maille unitaire monoclinique de dimensions a=6,486 A, b+9,482 A, c=13,51 A et β=45,9° avec un groupe spatial P2 1 /n