Poly unit cell data

French500 has collected together unit-cell data for maltose hydrate and some poly-O-acylsaccharides in the hope that some packing information might be obtained which could be applied to the problem of starch structure. 

The orthogonal projection of the epitaxial poly(DMDA) could not be indexed using the unit cell data for the bulk polymerized crystal (8). However, poly(DMDA) cannot usually be polymerized to completion or to high crystallinity in the bulk due to cross-linking. The use of an epitaxial substrate may have controlled the polymerization process that led to oriented single crystals. 

Intensity data were collected using a Picker Facs-I computer controlled four-circle single crystal diffractometer. Unit cell data for poly(5,7-dodecadiinediol-l,12-bis-phenylurethane) are a=6.229A, =39.03A, =4.909(chain axis), 3=106.85 degrees, space group P2,/c, and Z=4. The observed intensity data consisted of 724 independent reflections. 

Figure 11.9 Poly(ET-c6>-EN) statistical copolymer melting point and unit-cell data, shown as a function of composition (a) copolymer melting temperature, (b) projection of the a unit cell length onto the plane normal to the chain axis, a (c) projection of the b unit cell length onto the plane normal to the chain axis, b(d) c unit-cell length. In panels (b-d), filled symbols indicate a PEN-like crystal structure, while open symbols correspond to a PET-like structure. Reprinted from Reference [86] with permission of Elsevier, Copyright 1995.

Abstract. Three independent determinations have been made of the crystalline structure of the a-phase of poly (tetra-methylene terephthalate). The data on which these determinations have been based are used to asses the contributions to the uncertainties of the structural parameters caused by errors in the unit cell parameters, structure factors, and bond parameters. The effects of differences in the model from which refinement is started are also assessed. The major contribution to uncertainty arises from errors in the structure factors (the "R-factor" between structure factor sets from two different laboratories can be greater than 20%) but errors in bond parameters also make a sizeable contribution. Hamilton s test indicates that one of the structure factor sets used in this study is less inaccurate than the other two and using this the best model satisfying all the other data is estimated together with the uncertainties in its parameters. 

Full circles denote observed reflections, open circles absent reflections that would be observed if all orientations about the fiber axis were present (fiber type orientation). While the 100 reflection is observed in poly(i.-methionine), it is weaker than would be expected if the orientation were fiber type, and it appears that the (100) planes develop preferentially parallel to the surface and (110) planes perpendicular (5). The diagram for poly(y-methyl-ia-glutamate) is constructed from earlier data (12) with a very similar unit cell a = 11.8 A, c = 27.0 A, compared with a = 11.46 A, c = 26.8 A for poly(i.-methionine). The cell size and the positions of the reflections are consistent with the presence of an a-helix containing 18 residues in five turns. The hatched areas denote the position of layer line streaks. The meridional 00,18 reflections at about 1.5 A are not shown. 

The structure and spectra of /3-poly(L-glutamic acid) [ -(GluH) ] and its salts have been studied in some detail. X-Ray and electron-diffraction studies (Keith et al., 1969a,b), particularly on the Ca salt [)3-(GluCa) ], have indicated that this polypeptide forms an APPS structure. A model has been presented for the structure, and coordinates have been given for the atoms in the unit cell (Keith et al., 1969a), but a detailed test was not possible because of the paucity of diffraction data. 

As a consequence of the a to P transition which accompanies orientation, fibre patterns of the a-form are difficult to obtain. Melt extruded fibres of poly(a,a-dimethyl-P-propiolactone), known as poly(pivalolactone) (PPL), do, however, crystallize in the a-form and retain their orientation upon high temperature annealing under tension . Furthermore, oriented samples of poly(a-methyl-a-n-propyl-B-propiolactone) (PMPPL) have been prepared which show x-ray layer lines corresponding to the two phases. The a-form of both PPL and PMPPL is characterized by a fibre repeat distance of about 6 A, which has been identified as the periodicity of a 2j helix. Although oriented samples of poly(a-methyl-a-ethyl-B-propiolactone) (PMEPL) in the a-phase have never been reported, x-ray powder data have been fitted with a monoclinic unit cell with c = 6.1 A. 

A prerequisite for this stability is the formation of hydrogen bonded tmd oriented beta sheets. Synthetic peptides containing the GAGAGS sequence have been analyzed in their crystalline states (Lucas et al., 1958). X-ray diffraction studies of poly(alanylglycine) have prowded model unit cell crystal dimensions for beta sheet structure (Fraser et al, 1965). Films created from manually stroked evaporating solutions of these peptides were also studied using FTIR dichroism (Fraser et al., 1965). Li sing these data for comparison, it has been possible to demonstrate the physical similarity between BetaSilk protein polymiers and these model peptides (Cappello et al, 1990). 

Fig. 20. In panel A diffraction patterns on different positions inside a spherulite of poly(3-hydroxybutyrate) are shown. The diffraction patterns are mapped onto the position inside the spherulite. The spherulite was mapped with a lO- im beam on a microfocus beamline at the ESRF. The data presented here shows approximately 150 ixrc of the spherulite. Panel B shows one of the diffraction patterns. It could be indexed to an orthorombic unit cell with dimensions a = 0.576, b = 1.32, and c = 0.596 nm. The degree of disorder increases toward the center of the spherulite. Courtesy of A. Mahendrasingam.

The geometrical parameters, V/Lm, cr, and I, required for this computation were obtained as follows. jLm has already been shown to be 0.41 for the polypeptide chain in the a-helical and fully extended forms. Since we have assumed that there is one amino acid residue per statistical element for simplicity, the value of V is 3.6 X 10 cm. A reasonable value for square centimeter cross section of the fiber, is obtained from X-ray data on poly-7-benzyl-L-glutamate. Pauling and Corey (1951) interpreted the X-ray data for this synthetic polypeptide in terms of a parallel array of a-helices with two helices per unit cell of cross section 25.0 X 14.42 A. Dividing the number of chains per unit cell by the cross-sectional... 

The Sanchez-Eby and Wendling-Suter models, which differ mainly in the limit of large e, have been successfully applied to describe experimental copolymer crystallization data, especially when counit inclusion in the crystal unit cell is substantial (i.e., e is small) [88,101]. In one such example, Marchessault and colleagues studied the isodimorphic crystallization behavior in poly(j8-hydroxybutyrate-C(9-/3-hydroxyvalerate) statistical copolymers [87, 88]. The experimentally measured copolymer melting temperatures were found to be well described by Sanchez and Eby s model with the assumptions that the crystals were of finite thickness (i.e., Eq. 11.12) and defect inclusion was uniform (i.e., Xcb = b) [88]. 

Since the solid-state structure formed in block copolymers with homogeneous melts is driven by crystallization, the kinetics of lamellar-scale structure formation might be expected to parallel those of crystallization at the unit-cell level. In the same poly(ethylene-6-(ethylene- / -propylene)) diblock copolymer system, the time evolution of the copolymer crystallinity calculated based on the observed SAXS peaks, as illustrated in Figure 11.16, overlaps with that calculated based on the WAXS data. Since SAXS measures the development of diblock copolymer microstructure on the tens-of-nanometers scale, while WAXS measures polyethylene crystallization on the angstrom scale, the observation that the SAXS data track the WAXS data indicates that the formation of the lamellar microstructure in these diblock copolymers is indeed driven by crystallization, rather than by microphase separation between chemically incompatible blocks [115]. 

A xylomannan (xylose to mannose ratio, 1 1.2), obtained from Poly-porus tumulosus cell-wall by extraction with alkali followed by Fehl-ing precipitation, consists of a (l- 3)-linked a-D-mannopyranosyl main-chain principally substituted by xylobiosyl side-chains. Methyl-ation data showed end units that were principally xylopyranosyl, with... 

In the data shown in Fig. 1, soluble chromatin was applied to a poly(ADP-ribose) antibody column in PBS, and washed extensively. In earlier studies, nucleosomes were labeled in vitro with NAD prior to immunofractionation. In the experiments described [1-3], no in vitro synthesis of exogenous poly(ADP-ribosylation) was performed and absorption to the antibody column relied upon the presence of in vivo synthesized poly(ADP-ribose) at the sites in chromatin where it presumably exists naturally. This laboratory has recently shown the natural presence of 15 units of poly(ADP-ribose) attached to histone HI isolated from intact, unlabeled cells using the same antibody [5]. 

This raises the question how much charge per unit length of the polymer backbone may be trans-fered in the redox process. The answer to this problem is very relevant discussing the possible use of poly(acetylene) in storage batteries where the capacity of the cell is determined by the exact stoichiometry of the redox reaction. The analysis of the X-ray structural data, despite their limitations due to the limited accuracy of the X-ray patterns, seems to indicate that the charge density on the chain is essentially limited by the packing density of the counterions X in the case of the oxidized polymer. The disproportionation reaction may then be formulated by... 

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