Kenji Okuyama

Structural diversity of chitosan and its complexes

Abstract The lattice parameters for a number of chitosan specimens were classified into two, hydrated and anhydrous, groups. Polymers in them have the same extended two-fold helical structure, while the packing arrangements and water contents are quite different. Chitosan complexes with acid or metal salts can be classified into two types. Type I has a 10.3xa0A axial repeat similar to hydrated and anhydrous chitosan, while Type II has a 40xa0A axial repeat; by using X-ray data from highly oriented specimens, plausible crystal structures have been obtained in each case. The Type II conformation corresponds to a less-extended two-fold helical model with a tetrasaccharide repeat in a helical asymmetric unit.

Structural study of anhydrous tendon chitosan obtained via chitosan/acetic acid complex

The molecular structure and packing arrangement of anhydrous tendon chitosan was determined by the X-ray fibre diffraction method together with the linked-atom least-squares refinement technique. The specimen was prepared from chitosan/acetic acid complex which was obtained by exposing tendon chitosan to acetic acid vapour at room temperature for several days. There is high degree of orientation and crystallinity compared with the specimen obtained by the annealing method. Two chitosan chains are present in an orthorhombic unit cell of dimensions a = 8.26(2), b = 8.50(1), c (fibre axis) = 10.43(2) A and space group P2(1)2(1)2(1). The 2-fold helical chain is stabilised by O3 triple bond O5 hydrogen bond with the gt orientation of O6. There are direct hydrogen bonds (N2 triple bond O6) between adjacent chains along the a-axis, which makes a sheet structure parallel to the ac-plane. On the other hand, no hydrogen bond is found between the sheets.

Crystal transition mechanisms in poly(ethylene succinate)

Abstract Poly(ethylene succinate) [PES] showed a crystal transition between the α (T3GT3Ḡ) and β (T8) form, where T, G and Ḡ denoted trans, gauche and minus gauche, respectively. Here, the β form appeared only under the strain. We have investigated the mechanisms of this crystal transition by FT-IR and X-ray diffraction. In the FT-IR spectrum, the absorbance peaks at 841 and 873xa0cm−1, corresponding to the α form, started decreasing at strain of ϵ∼20%, while the absorbance at 805xa0cm−1, corresponding to the β form, appeared at ϵ∼20%, then increased with strain. In addition, the isosbestic point was observed at 820xa0cm−1, indicating that the crystal transition occurred only between the α and β form, where no amorphous part contributed. In the X-ray diffraction, streaks appeared in layer lines in the β form, indicating that the β form had some disorder along the fiber axis. The equatorial reflection of α (at 2θ=20.4°) started decreasing at ϵ∼15%. On the other hand, the reflection of β (at 2θ=21.9°) appeared at ϵ∼15%, then increased with ϵ, consistent with FT-IR results. The molar fraction of the β form, χβ, was determined as a function of stress, σ, by X-ray, where χβ showed a drastic increase at a critical value of σ ∗ =190 MPa . The thermodynamic first-order phase transition was hence the operative mechanism of the transition as similar to poly(butylene terephthalate) [PBT] and poly(tetramethylene succinate) [PTMS]. The free energy difference between the α and β form, ΔG, was determined to be ΔG∼2.3 (kJ/mol of monomer unit), being larger than the reported value of PBT (ΔG∼1.4) and PTMS (ΔG∼1.6). This difference would arise from the locus of conformational change upon the transition. In the case of PES, the conformational change was observed in both alcohol and acid units, while in the cases of PTMS and PBT, it was observed in the alcohol unit.

Refined molecular and crystal structure of silk I based on Ala-Gly and (Ala-Gly)(2)-Ser-Gly peptide sequence.

The molecular and crystal structure of one of the crystalline modifications of Bombyx mori, silk I, was determined by x-ray diffraction method. Cell dimensions are essentially the same as those found in the synthetic model peptide poly(L-Ala-Gly). The (straight phi, psi) values of L-Ala and Gly in the repeating unit are (-112 degrees, -6 degrees ), and (71 degrees, -99 degrees ) respectively, which are in the Bridge and the forth quadrant regions of the Ramachandran map, respectively. The observed molecular conformation in the present study has a crank-shaft or a S-shaped zigzag arrangement, leading to a remarkable agreement of observed and calculated structure amplitudes for both dipeptide and hexapeptide sequences, and has a reasonable hydrogen bond networks. Obtained (straight phi, psi) values are quite different from those reported by Lotz and Keith, even though overall appearances are quite similar to each other. In spite of intra- and intermolecular hydrogen-bond networks, silk I structure changes easily to the silk II by a mechanical deformation. This fragility may be due to the above peculiar crank-shaft conformation deduced from the alternating structure of alanine and glycine.

Two different molecular conformations found in chitosan type II salts.

The type II structure of chitosan acidic salts prepared from crab tendon in solid state was studied using an X-ray fiber diffraction technique together with the linked-atom least-squares (LALS) technique. The cylindrical Patterson method was applied to confirm the molecular conformation of the chitosan. It was shown that there are two different helical conformations for type II salts. One is the relaxed twofold helix having a tetrasaccharide as an asymmetric unit as found in chitosan.HCl salt, which was previously reported as a conformation of chitosan.HCOOH salt. The other is the fourfold helix having a disaccharide as an asymmetric unit newly found in chitosan.HI salt.

Crystalline Transformation of Chitosan from Hydrated to Anhydrous Polymorph Via Chitosan Monocarboxylic Acid Salts

ABSTRACT Spontaneous removal of monocarboxylic (formic, acetic, propionic or butyric) acids accompanying dehydration of the corresponding chitosan salts was observed from X-ray fiber diffraction diagrams obtained during the storage of these salts for a given period of time. The first three salts were prepared by immersing a tendon chitosan (a hydrated crystal) in an aqueous solution of respective monocarboxylic acid and 2-propanol. The salts showed similar fiber patterns not only to one another but also to the “Eight-fold” polymorph of the original chitosan, indicating that they are Type II salts, hydrated crystals, where the backbone chitosan molecule takes up an eight-fold helical conformation. The temperature required for the salt formation depended on the hydrophobicity of the acid, e.g., the chitosan formic acid salt could be prepared at room temperature, whereas, formation of the propionic acid salt was carried out at 4 °C. All the acids spontaneously evaporated accompanied by dehydration during sto...

Structure Analysis of Fibrous Polymers

繊維状高分子のX線回折測定に放射光を用いることにより,回折像のS/N比が上がり強度データの質と量の向上が期待できる。本稿では,放射光を利用して収集した精度の高い繊維回折データに基づいた高分子の結晶構造解析に関する最近の研究成果について,筆者らの研究も含め紹介する。

Fiber diffraction studies on polymers using synchrotron radiation source

The advent of a high-flux and sharp X-ray beam with a small divergence makes it possible to record fiber diffraction patterns from thin filamentous specimens of muscle, collagen, silk and other systems and well-oriented sols of virus and bacterial flagellar filament. The advantage of synchrotron radiation is also useful for the data collection of the bundles of fibers that have been used in a laboratory. In this study, we collected fiber diffraction data of synthetic polyesters [poly(tetramethylene succinate)(PTMS) and poly(tetramethylene adipate)(PTMA)] and chitosan obtained from a crab tendon whose size were comparable to those in laboratory experiments (< 0.5 mm). Fiber diffraction patterns were collected at the BL40B2 of SPring-8 using an imaging plate area detector (Rigaku, R-AXIS IV++) with an X-ray wavelength of 1.0 Å. Synchrotron radiation experiments enabled us to obtain very good diffraction data with high S/N ratio and well-separated diffraction spots. In order to get sufficiently high S/N ratio we needed 3 minutes exposure. Crystal structures of PTMS, PTMA and chitosan were analyzed based on synchrotron radiation data using linked-atom least squares refinement technique. Although the resultant structures were essentially same as those analyzed with intensity data collected in our laboratory, R-values of PTMS(0.13), PTMA(0.11) and chitosan(0.11) were lower than those based on laboratory data (0.18, 0.12 and 0.16, respectively). It is suggested that exact and reliable intensities can be obtained from synchrotron radiation data even for weak diffraction spots.

Structural study of collagen model peptide, (Pro-Hyp-Gly)11 at high resolution

Since the 7/2-helical model was proposed by our group in 1977 based on the structure of (Pro-Pro-Gly)10, the single crystal structures of model peptides have contributed to obtain important information on the triple-helical structure of collagen. In this study, we report the high resolution crystal structure of collagen model peptide, (Pro-Hyp-Gly)11 using synchrotron diffraction data (SPring-8) at 100K. The structure has been refined to an R-factor of 0.15 and an Rfree-factor of 0.19 using 1960 and 217 reflections respectively, extending up to 1.5 Å resolution on the layer lines based on the repeating period of 20 Å. Different from (Pro-Hyp-Gly)10 case, only a few satellite spots were observed above and below the strong reflections on the 20 Å layer lines. The obtained molecular conformation and hydrogen bonding schemes of peptide chain are essentially the same as those found in the previous analyses of (Pro-Pro-Gly)n (n=9,10) and (Pro-Hyp-Gly)10, which suggests that the peptide molecule in this crystal takes the 7/2-helical conformation. Compared with that of (ProHyp-Gly)10 case where intensity data were collected at room temperature, a fairly large number of water molecules bound to the triple-helical structure were found in this analysis. Most of the newly found water molecules maybe attributed to the data collection temperature together with the analysis at high resolution. All the proline rings of Hyp were found to be up-puckering, while those of Pro showed both upand down-puckering.