Letter pubs.acs.org/macroletters
Crystal Polymorphism of Curdlan Propionate: 6‑Fold versus 5‑Fold Helices Hironori Marubayashi,†,‡,§ Kazuyori Yukinaka,† Yukiko Enomoto-Rogers,† Takaaki Hikima,∥ Masaki Takata,‡ and Tadahisa Iwata*,†,‡ †
Science of Polymeric Materials, Department of Biomaterial Sciences, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan ‡ Structural Materials Science Laboratory and ∥Research Infrastructure Group, RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan S Supporting Information *
ABSTRACT: The molecular and crystal structures of curdlan propionate (CDPr) were examined by the X-ray fiber diffraction methods combined with energy calculations. CDPr forms two different crystal structures (CDPr forms I and II) depending on annealing conditions: solvent-annealing yields CDPr form I, whereas thermal-annealing gives CDPr form II. In CDPr form I, the 6/1 helix is packed in the hexagonal unit cell with a = b = 1.154 nm, and c (fiber axis) = 2.287 nm. In the case of CDPr form II, the 5/1 helix is packed in the pseudohexagonal cell with a = b = 1.175 nm, and c (fiber axis) = 1.859 nm. The crystal transition from CDPr forms I to II occurs by thermal-annealing at temperatures ≥ 160 °C. lobal concerns about finite fossil fuels, global warming, and ecosystem destruction strongly and urgently demand the paradigm shift from petroleum-based to biobased polymer products in the polymer industry toward a sustainable society. Unlike cellulose, starch, and chitin, the great majority of natural polysaccharides are not well understood and utilized. Recently, bacterial exopolysaccharides have attracted increasing attention because they can be easily harvested and prepared in bulk through biotechnological routes from biomass.1 Curdlan is a microbial β-(1 → 3)-D-glucan without branching2 and mainly used as a thickening agent in the food field because of its gelling ability and nontoxicity. Curdlan forms three crystal modifications depending on preparation conditions. Curdlan form I consists of a right-handed 6/1 single helix,3 while curdlan forms II and III are the triple-helical structures composed of righthanded 6/1 helices.4 Chemical modification of polysaccharides such as esterification is an effective way to obtain thermoplastics.5 Esterification of hydroxyl groups in polysaccharides can improve organic solubility and thermal stability due to the hindrance to forming hydrogen bonding. Polysaccharide esters exhibit widely varied structure and properties depending on acyl carbon numbers, as seen in esters of cellulose,6 amylose,7 konjac glucomannan,8 curdlan,9 pullulan,10 xylan,11 and so on. Hence, adjustment of side-chain structure is needed to obtain desired material properties. Furthermore, it is of great importance to understand substituent effects on the molecular and crystal structures of crystalline polysaccharide esters12 for the material design.
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© 2016 American Chemical Society
As for curdlan esters, Okuyama et al. found that the helical conformation of curdlan triacetate (CDAc) is the right-handed 6/1 helix,13 whose backbone conformation is the same as that of curdlan form I.3 We have recently reported that some of curdlan triesters with longer acyl groups have a crystalline nature.9 Especially, curdlan propionate (CDPr, Figure 1)
Figure 1. Chemical structure of curdlan propionate (CDPr).
possessed high crystallinity (>50%) and melting point (>200 °C) with melt-processability (unlike CDAc). Thus, CDPr has a great potential as biobased crystalline thermoplastics with high thermal stability. However, very little is known about its molecular and crystal structures, which are absolutely important to understand the structure−property relationship and maximize material properties. Received: March 4, 2016 Accepted: April 23, 2016 Published: April 28, 2016 607
DOI: 10.1021/acsmacrolett.6b00186 ACS Macro Lett. 2016, 5, 607−611
Letter
ACS Macro Letters In this study, the molecular and crystal structures of CDPr were investigated using the X-ray fiber diffraction methods for its uniaxially oriented samples. We demonstrate here two different crystal structures of CDPr in which the crystallized chains take a different helical sense and the crystal transition between them. Figure 2a shows wide-angle X-ray diffraction (WAXD) curves of solution-cast films and thermally annealed (i.e.,
Figure 3. WAXD profiles (Cu Kα) of various CDPr films. Thermalannealing treatments for cast films were conducted at the prescribed temperature (Ta = 130−180 °C) for 1 day. Data of solution-cast film (CHCl3) and thermally annealed one (Tc = 180 °C) are shown as reference standards.
Tm1. After thermal-annealing at Ta = 160 °C (≈Tm1), new diffraction peaks appeared (e.g., the peak marked with an asterisk) and coexisted with peaks derived from the cast film (CDPr form I). With increasing further temperature, WAXD profiles changed to become that of thermally annealed samples (CDPr form II), and a single melting endotherm was observed (Figure S1). These results clearly indicate that the thermally induced transition from CDPr forms I to II occurs around Tm1. In order to reveal the crystal structure of CDPr form I, a drawn film (not fully crystallized) was exposed to acetone vapor in the fixed state (method 1), because cast films of CDPr were too brittle to be stretched. As a result, fiber diffraction of CDPr form I was successfully obtained, as shown in Figure 4a. Sharp
Figure 2. (a) WAXD profiles (Cu Kα) and (b) DSC thermograms (first run) of solution-cast films and thermally annealed ones of CDPr. Solution-casting was done at room temperature and atmospheric pressure. Thermal-annealing treatments of amorphous films were conducted at the prescribed temperature (Tc = 140−180 °C) for 1 day.
cold-crystallized) ones of CDPr. Solution-cast films showed essentially the same diffraction profiles, irrespective of solvent species (chloroform, dichloromethane, and tetrahydrofuran). Previously, we reported WAXD data of the solution-cast film of CDPr from its chloroform solution.9 On the other hand, WAXD profiles of thermally annealed CDPr films were completely different from those of solution-cast films. As a clear indicator, the diffraction peak at 2θCuKα = 13.2° was observable only for thermally annealed films, not for solutioncast films. In addition, diffraction peaks in a higher-angle region (2θCuKα > 20°) were seen only in WAXD profiles of solutioncast films. It was confirmed that thermal-annealing treatments of the amorphous films in air gave almost the same WAXD curves, regardless of the crystallization temperature (Tc). These results clearly suggest that the crystal structure of thermally annealed CDPr differs from that of solution-cast CDPr. Namely, CDPr possesses crystal polymorphism. The crystal structure of solution-cast CDPr is hereafter called the “CDPr form I” and that of thermally annealed CDPr is named the “CDPr form II”. Figure 2b shows DSC first heating runs of solution-cast films and thermally annealed ones of CDPr. Interestingly, two separated melting peaks can be seen in solution-cast films, where their peak tops are about 160 (Tm1) and 210−230 °C (Tm2). On the contrary, thermally annealed samples showed a single melting peak around 210 °C. When the solution-cast film was thermally annealed below Tm1 (Ta = 130−150 °C), there were almost no changes of WAXD profiles, as shown in Figure 3. On the other hand, the situation became quite different for thermal-annealing at Ta ≥
Figure 4. (a) Fiber diagram of CDPr crystallized in acetone vapor in the oriented state (CDPr form I; method 1). Cu Kα (λ = 0.1542 nm). The strong reflection on the sixth layer line is marked by a blue arrow. (b) Fiber diagram of CDPr thermally annealed in the oriented state (CDPr form II; method 2). Synchrotron X-ray (λ = 0.1000 nm). The strong meridional reflection at the fifth layer line is marked by a red arrow. Drawing direction is vertical for both diagrams.
and well-separated reflections with a relatively high degree of orientation (0.95) were observed. Note that this oriented film contained almost no acetone molecules (i.e., dried state). It is noteworthy that diffraction intensity is relatively high on the layer lines with l = 0 (equator), 1, 2, 3, and 6, indicating the formation of 6-fold helices of CDPr, which can be understood by a dependence of structure factors of helical polymers on the layer line order.14 The fiber period was 2.287 nm, which is 608
DOI: 10.1021/acsmacrolett.6b00186 ACS Macro Lett. 2016, 5, 607−611
Letter
ACS Macro Letters comparable to that of 6-fold helices of CDAc (2.291 nm)13 and curdlan form I (2.28 nm).3 Therefore, it is considered that the helical conformation of CDPr form I is similar to that of CDAc and curdlan form I (right-handed 6/1 helix). From the fiber diagram, a total of 27 unique reflections were extracted and successfully indexed by the hexagonal system with a = b = 1.154 nm, and c (fiber axis) = 2.287 nm, where the calculated density (1.25 g/cm3 for Z = 1) agreed well with the observed one (1.23 g/cm3 for solution-cast films). Observed and calculated dspacings of CDPr form I are listed in Table S1. The space group is expected to be P61, which is reasonably supported by the conformational analysis mentioned later. Since CDAc forms the hexagonal cell (P61) with a = b = 1.100 nm, and c (fiber axis) = 2.291 nm,13 the area of ab plane of CDPr form I (1.154 nm2) is larger than that of CDAc (1.048 nm2), which would be reasonable in terms of an increase in the side-chain carbon numbers. Figure 4b depicts the X-ray fiber diffraction pattern of CDPr form II, which was prepared by thermal-annealing a drawn film at 165 °C in the fixed state (method 2). As is the case with CDPr form I (Figure 4a), we can see sharp and well-separated reflections. A high degree of orientation, 0.98, was achieved. It should be noted that the strong meridional reflection was seen on the fifth layer line. This result clearly indicates that CDPr molecules form the 5-fold helices in CDPr form II. Namely, the helical conformation of CDPr form II (5-fold helix) is clearly different from that of CDPr form I (6-fold helix). In other words, CDPr chains can take two different backbone conformations (i.e., 5- and 6-fold helices). Since curdlan form I3 and CDAc13 take the 6-fold helical conformation, this is the first report on the 5-fold helix of curdlan backbone. From a distance between neighboring layer lines, the fiber period was calculated to be 1.859 nm. Totally 15 unique reflections were satisfactorily indexed by the hexagonal lattice with a = b = 1.175 nm, and c (fiber axis) = 1.859 nm, where the calculated density (1.23 g/cm3 for Z = 1) was in good agreement with the observed one (1.21 g/cm3 for thermally annealed films). Observed and calculated d-spacings of CDPr form II are exhibited in Table S2. Strictly speaking, a pseudohexagonal unit cell (P1) would be better rather than a hexagonal cell because 5-fold CDPr chains have no 6-fold symmetry. Although the helical nature of CDPr form II is different from that of CDPr form I, shapes of unit cells are essentially the same and areas of ab plane are similar (1.154 and 1.196 nm2 for CDPr forms I and II, respectively). Further analyses were carried out to clarify the helical structure of CDPr forms I and II. Figure 5a shows atomic numbering and definition of dihedral angles for CDPr. Molecular building is based on CDAc13 and details are shown in Tables S3−S5. The fiber period (f p,n) and the helical sense of helical polymers with n residues per period can be obtained by the Miyazawa equation.15 As shown in Figure 5b, two torsional angles around the glycosidic linkage (φ, ψ) satisfying f p,6 = 2.287 nm, which corresponds to CDPr form I (6-fold helix), trace a hyperbolic curve, which is defined as the fiber-period curve ( f p,6 = 2.287 nm). Also, (φ, ψ) points meeting a prescribed helical sense draw hyperbolic curves, which are named the 6/1-, 6/2-, 6/3-, 6/4-, and 6/5-helix curves for 6/1, 6/2, 6/3, 6/4, and 6/5 helices, respectively. 6/ 1-, 6/2-, and 6/3-helix curves intersect with the fiber-period curve (f p,6 = 2.287 nm). Among a total of 12 intersection points, the point with (φ, ψ) ≈ (169°, −113°) gives the most energetically stable helix (6/1 helix with f p,6 = 2.287 nm). The
Figure 5. (a) Atomic numbering and definition of dihedral angles for CDPr. θ1 = C1−C2−O2−C21, θ2 = C2−O2−C21−C22, θ3 = O2− C21−C22−C23, θ4 = C3−C4−O4−C41, θ5 = C4−O4−C41−C42, θ6 = O4−C41−C42−C43, θ7 = C4−C5−C6−O6, θ8 = C5−C6−O6− C61, θ9 = C6−O6−C61−C62, θ10 = O6−C61−C62−C63. (b) Ramachandran plots as functions of two torsional angles around glycosidic linkage (φ, ψ) for CDPr forms I and (c) II, where Eintra mapping by MMFF94 is common. Plots of Eintra ≥ 0 kcal/mol are shown by a white color. Hyperbolic curves representing observed fiber periods and a given helical sense are exhibited. (θ1, θ4, θ7, θ8) = (95°, 95°, 174°, −174°). (θ2, θ3, θ5, θ6, θ9, θ10) are set to be 180° (trans conformation).
obtained (φ, ψ) values agree well with those of CDAc [(φ, ψ) = (168.9°, −113.3°)].13 Figure 6a,b shows the obtained helix model of CDPr form I (6/1 helix with f p,6 = 2.287 nm), where the side-chain torsional angles are optimized. Figure 5c shows the fiber-period curve (f p,5 = 1.859 nm) and the 5-fold-helix curves on the intramolecular interaction energy (Eintra). Only 5/ 1- and 5/2-helix curves intersect with the fiber-period curve ( f p,5 = 1.859 nm). Among a total of 8 intersection points, the point with (φ, ψ) ≈ (165°, −100°) yields the most stable 5/1 helix having f p,5 = 1.859 nm. Figure 6c,d depicts the resultant helix model of CDPr form II (5/1 helix with f p,5 = 1.859 nm). The fiber periods per residue are 0.381 and 0.372 nm for CDPr forms I and II, respectively. In addition, the helix radii are about 0.95 nm for both CDPr forms I and II. Thus, the 5/1 helix (CDPr form II) is more compact in the fiber-axis direction, as compared to the 6/1 helix (CDPr form I). Here, values of Eintra per residue are −96 and −82 kcal/mol for CDPr forms I and II, respectively, where a cutoff distance is 1.5 nm. Differences of both intra- and intermolecular interactions between CDPr forms I and II would be responsible for a great change in Tm (160 and 210−230 °C, respectively). The intermolecular interaction energy (Einter) will be obtained from information on chain packing of 6/1 and 5/1 helices in hexagonal unit cells, which is currently being analyzed. Furthermore, it was found that the crystal transition from CDPr forms I to II is accompanied by conformational changes in the glycosidic linkage (|Δφ| = 4.4°; |Δψ| = 13.3°). In summary, we have revealed the molecular and crystal structures of CDPr by using the fiber diffraction methods combined with energy calculations. It was found that CDPr possesses crystal polymorphism (CDPr forms I and II): solvent-annealing gives CDPr form I, whereas thermal609
DOI: 10.1021/acsmacrolett.6b00186 ACS Macro Lett. 2016, 5, 607−611
Letter
ACS Macro Letters
atmospheric pressure, followed by vacuum drying. Chloroform, dichloromethane, and tetrahydrofuran were used as solvents. Thermal-annealing (i.e., cold-crystallization) treatments for the amorphous films were done in air at a constant temperature (Tc = 140−180 °C) for 1 day (enough time for crystallization). The amorphous films were prepared by hot-pressing at 240 °C under 3 MPa for 1 min, followed by quenching by ice−water. As a separate method, thermal-annealing treatments of cast films were conducted at the prescribed temperature (Ta = 130−180 °C) for 1 day. Preparation of Oriented-Crystallized Films. The orientedcrystallized films of CDPr were prepared in two different ways: (i) drawing the melt-quenched (i.e., amorphous) film around its glass transition temperature (120 °C), followed by exposure to acetone vapor in the fixed state at room temperature for 1 week; (ii) drawing the melt-quenched film at 120 °C, followed by annealing in air at 165 °C for 1 day in the fixed state. Wide-Angle X-ray Diffraction (WAXD). X-ray fiber diagrams were obtained by WAXD measurements at the beamline BL45XU of SPring-8 (Harima, Japan), where synchrotron X-ray was monochromatized by the diamond (111) plane to give the wavelength of 0.1000 nm. Each WAXD diagram was recorded on an imaging plate (Rigaku R-Axis IV++; 3000 × 3000 pixels, 100 × 100 μm2 pixel−1) with a sample-to-detector distance of about 190 mm. A helium-gas-filled path was set up between a sample holder and the detector. Twodimensional WAXD measurements were also done in laboratory using a Rotaflex RU-200BH (Rigaku Corp.) operating at 50 kV and 100 mA with Ni-filtered Cu Kα radiation (λ = 0.15418 nm). Each Xray fiber diagram was recorded on an imaging plate (Fujifilm Corp.; 2540 × 2540 pixels, 50 × 50 μm2 pixel−1) and read by a RAXIA-Di (Rigaku Corp.). A sample-to-detector distance was set to about 43 mm. A sample holder and the detector were set in the vacuum chamber. For both cases, silicon powder was used as a standard sample, and measurements were carried out at room temperature. Fiber diagram analysis was performed by using a handmade GUI software.16 Differential Scanning Calorimetry (DSC). Crystal transition and melting behaviors of CDPr were examined using a DSC 8500 (Perkin Elmer Co., Ltd.). About 1.5 mg sample packed in an aluminum pan was heated from 25 to 250 °C at 20 °C/min under a nitrogen gas atmosphere. Conformational Analysis. MMFF9417 was used as a force field. In the main-chain conformational analysis, Eintra was calculated only for f p,n > 1.5 nm. After determination of the main-chain torsional angles (φ, ψ), the side-chain torsional angles (θ1−θ10) were refined. Details are described in the Supporting Information.
Figure 6. Helix models of CDPr forms I [(a) top and (b) side views of 13 residues] and II [(c) top and (d) side views of 13 residues]. CDPr form I: (φ, ψ, θ1, θ4, θ7, θ8) = (168.9°, −113.3°, 85°, 123°, 180°, 125°). CDPr form II: (φ, ψ, θ1, θ4, θ7, θ8) = (164.5°, −100.0°, 79°, 157°, 169°, −164°). Fiber periods are exhibited in side views.
annealing yields CDPr form II. CDPr form I is the hexagonal structure with a = b = 1.154 nm, and c (fiber axis) = 2.287 nm, in which the 6/1 helix [(φ, ψ) = (168.9°, − 113.3°)] is packed. On the contrary, CDPr form II is the pseudohexagonal structure with a = b = 1.175 nm, and c (fiber axis) = 1.859 nm, in which the 5/1 helix [(φ, ψ) = (164.5°, −100.0°)] is packed. Thermal-annealing at temperatures ≥ 160 °C brings about the transition from CDPr forms I to II, during which conformational changes occur in the glycosidic linkage (|Δφ| = 4.4°; |Δψ| = 13.3°). Thus, it was found that the helical nature of curdlan triesters has a distinct dependence on carbon numbers of acyl substituents and the surrounding atmosphere (air or organic solvents). The molecular and crystal structures of crystalline curdlan triesters with bulkier substituents will be reported elsewhere. In addition, the formation mechanism of CDPr form I (6/1 helix) in the presence of solvent molecules (i.e., a role of solvents) is of special interest.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00186. DSC thermograms of solution-cast and reannealed films of CDPr, observed and calculated d-spacings of CDPr forms I and II, molecular parameters of CDPr forms I and II, and details of conformational analysis (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
EXPERIMENTAL SECTION
Present Address §
Materials. CDPr used in this study was the same as that used in our previous study.9 Mw = 6.0 × 105, PDI = 1.8, and the degree of substitution of ester groups for hydroxyl groups (DS) = 3 (fully substituted). Preparation of Unoriented-Crystallized Films. Solution-casting from 5 mL of CDPr solution (ca. 50 mg/mL) was done on a glass Petri dish (diameter of 42.5 mm) at room temperature and
Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2−12−1 Ookayama, Meguro-ku, Tokyo 152− 8552, Japan (H.M.). Notes
The authors declare no competing financial interest. 610
DOI: 10.1021/acsmacrolett.6b00186 ACS Macro Lett. 2016, 5, 607−611
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ACS Macro Letters
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T. A. J. Comput. Chem. 1996, 17, 553−586. (d) Halgren, T. A.; Nachbar, R. B. J. Comput. Chem. 1996, 17, 587−615. (e) Halgren, T. A. J. Comput. Chem. 1996, 17, 616−641.
ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (A) (No. 26248044) from JSPS and an ALCA Project from JST. Synchrotron radiation experiments were performed at BL45XU of SPring-8 with the approval of RIKEN (Proposal Nos. 20140030 and 20150080).
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REFERENCES
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