摘要:

Fibrous polynucleotide duplexes are very polymorphic. Detailed x-ray analysis of the D forms of poly d(AT) ·poly d(AT) and poly d(IC) ·poly d(IC) and of the B form of poly d(GC) ·poly d(GC) show that 5′RpY3′ dinucleoside monophosphates in these molecules have eithert,tort,gconformations at C3′–03′, 03′-P which lead to similar orientations of phosphate groups. On the other hand, 5′YpR3′ fragments commonly haveg,tconformations and hence a markedly different orientation of the phosphate group. Studies of the more complex structures of the C form of poly d(ACC) ·poly d(GGT) and of a pleiomeric D form of poly d(AT) ·poly d(AT) where the repeated structural motifs are hexanucleotides reveal further details of the surface “wrinkles” on DNA. In the case of the H form of poly d(A) ·poly d(T), the sugar rings in each strand are puckered differently-C2′-endo in poly d(T) and C3′-endo in poly d(A). Such heteromerous duplexes have unusually pronounced directional properties. Similar structure ma be present in some DNA-RNA hybrids. These surface feature might enable regulatory proteins and enzymes to recognize their host DNA sequences more easily in the blinding process.

ISSN:0022-2348    

DOI:10.1080/00222348508248014

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

Glycosaminoglycans are prominent components of mammalian connective tissue and possess important ion-binding properties. X-ray fiber diffraction studies have demonstrated that glycosaminoglycans show different conformational responses to mono-and divalent cations.

ISSN:0022-2348    

DOI:10.1080/00222348508248015

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

This paper describes x-ray analysis of the structure of a group of wholly aromatic thermotropic liquid crystalline copolyesters, focusing on two systems: condensation copolymers of 4-hydroxy-benzoic acid (HBA) and 2-hydroxy–6-naphthoic acid; and of HBA, 2,6-dihydroxy-naphthalene, and terephthalic acid. The fiber diagrams of these copolymers contain a series of aperiodic meridional maxima that shift in position with the monomer ratio. We have shown that these characteristics are predicted by a structure consisting of an array of chains of completely random monomer sequence. The positions of the maxima are predicted by a simple model in which the monomers are represented by points, separated from their neighbors by the appropriate monomer lengths. Use of atomic models for the monomers leads to prediction of both the positions and the intensities of the maxima. Calculations for nonrandom sequence distribution show that all but minimal blockiness can be ruled out. From the breath of the meridional maximum at d = 2.1 Å it is possible to estimate the correlation or persistence length for the stiff chain conformation in the solid state.

ISSN:0022-2348    

DOI:10.1080/00222348508248016

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

The hard segments in MDI/diol-based polyurethanes form crystalline domains, and these have been found to exist in different polymorphic structures depending on processing history. Structures of the crystal structures of low molecular weight urethane model compounds possess a number of common features that can be used to predict models for the polymers. Specifically, information is derived concerning the conformation of the diphenylmethane, phenyl-urethane, and chain extender region of the chain. This information has been used to predict the possible conformation of poly (MDI/diol) chains in which the diol region is fully extended (all trans). Less information is available that is relevant to the structures that have a more contracted conformation, but the options to be considered are discussed.

ISSN:0022-2348    

DOI:10.1080/00222348508248017

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

Calculated defect diffuse scattering patterns, using the kink defect model employed earlier to explain changes in Bragg electron diffraction intensities for heated n-paraffins, are shown to account best for observed diffuse data from solution-grown and epitaxially grown micro-crystals. The presence of significant diffuse scattering below 15°K, moreover, indicates that such chain kinks are stable in the crystal lattice.

ISSN:0022-2348    

DOI:10.1080/00222348508248018

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

Epitaxial growth produces oriented crystal projections where a long unit cell axis is parallel to the major crystal face rather than perpendicular to it and thus provides a favorable condition for electron diffraction data collection which minimizes perturbations due to elastic crystal bending. Even though the crystals are not so perfect as those produced by solution growth, the intensity data from them have been useful for the quantitative crystal structure analysis of several molecular organic compounds and linear polymers as discussed in this review.

ISSN:0022-2348    

DOI:10.1080/00222348508248019

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

Meridional x-ray reflections of Kevlar (TM) aramid fibers have been analyzed to provide direct information about fine structure in the direction of the fiber axis. In particular, these reflections have been studied to provide evidence for a one-dimensional paracrystalline structure in Kevlar and to seek quantitative structure-property relationships based on a paracrystalline structure model. In a series of Kevlar fibers an excellent correlation is found between paracrystalline distortion factor gIIand tensile modulus, with gIIdecreasing from 2.33% to 1.69% as modulus increases from 75 GPa to 125 GPa. Surprisingly, however, there is no systematic increase in axial crystallite length within this series in spite of heat treatment.

ISSN:0022-2348    

DOI:10.1080/00222348508248020

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

The dependence of the unit cell parameters of several polymers on temperature from room temperature to the melting point was measured by x-ray diffraction using an automated Rigaku - diffractometer. The anisotropic thermal expansion was characterized, and in the cases of polytetrafluoroethylene and 66 nylon, additional changes were found below their melting points. These data were combined with the results of pressure-volume-temperature experiments to calculate heats of fusion using the Clapeyron equation.

ISSN:0022-2348    

DOI:10.1080/00222348508248021

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

The light diffraction phenomenon from Kevlar 49 fibers exhibits unusual polarization effects and its angular and wavelength dependence cannot be described by the grating equation. This unexpected behavior is analyzed by modeling the fiber as an anisotropic grating of finite thickness in which the diffraction arises from a periodic change in the orientation of the molecular dipoles. Theoretical predictions on this basis explain all available experimental observations. These findings are discussed in regard to currently proposed structural models for the fiber.

ISSN:0022-2348    

DOI:10.1080/00222348508248022

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

The determination of the crystalline structure of oriented fibers of poly-2,5-benzoxazole (ABPBO) and poly-2,6-ben-zothiazole (ABPBT) is described. Both unit cells are metrically orthorhombic, with the parameters: a = 6.061 (17), b = 3.384 (13), c (fiber axis) = 11.575 (6) Å for ABPBO; and a = 6.044 (6), b = 3.417 (7), c (fiber axis) = 12.194 (18) Å for ABPBT. The fiber repeat consists in each structure of two fused ring groups arranged in a planar, zigzag conformation. The conformational torsion angle and orientation of chains within the unit cells are derived from a linked-atom least squares refinement technique. Polymer chains pack laterally through van der Waals interactions. A plausible disorder model which involves defects in chain direction is presented. Refinement of a static disorder model for ABPBO in which 50% of the chains have their chain directions reversed leads to a lower R residual and sum of constraints.

ISSN:0022-2348    

DOI:10.1080/00222348508248023

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor