ISSN:
0006-3525
Schlagwort(e):
Chemistry
;
Polymer and Materials Science
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
Notizen:
Small- and wide-angle x-ray scattering is used for structural characterization of amylose solutions and gets. Recently published coordinates determined by x-ray fiber structure analysis and electron diffraction [A. Imberty and S. Perez: (1988) Biopolymers, Vol. 27. pp. 1205-1212: H. C. Wu and A. Sarko (1978) Carbohydrate Research, Vol. 6.1, pp. 7-40]. X-ray crystallography [W. Hinrichs & W. Saenger (1990) Journal of the American Chemical Society, Vol. 112. pp. 2789-2796], and theoretically calculated atomic coordinates for energy-minimized conformers of amylose molecules in solution and crystals served to simulate small- and wide-angle x-ray scattering curves. The simulation of scattering curves renders possible a quick screening and detection of special features in experimental curves and the decision of whether they are significant or not. The scattered intensities of the models were calculated using the atomic scattering factors and van der Waals radii within the framework of the improved cube method [J. J. Müller (1983) Journal of Applied Crystallography, Vol. 16, pp. 74-82]. All model data and the scattering curves are stored for a fast information retrieval in the database OBIOSCAT controlled by the ORACLE management system.In the context of a mixture of diflerent structures existing in an amylose solution or gel, theparallel-stranded left-handed B-form double helices (Imberty and Perez) do not scatter in a way that is significantly different from that of the parallel-stranded right-handed duplex proposed by Wu and Sarko. The structure of the energy-minimized left-handed parallel-stranded double helix is very similar to that of the canonical B form, but energy-minimized right-handed duplexes with parallel or antiparallel strands have structures that produce new scattering features. Up to now, such features have not been experimentally detected. Extended or collapsed single helices, too, can be discriminated by their scattering features from double helices for scattering vectors larger than 5 nm-1, and the energy-minimized left-handed single helices are nearly identical with the V-forms experimentally found in fibres [G. Rappenecker and P. Zugenmaier (1981) Carbohydrate Research, Vol. 89, pp. 11-19.].Because the investigated amylose gels contain crystallites. The growing of V- and B-form nanocrystallites up to dimensions of 20 nm was simulated with atomic resolution. The scattering curves of independently scattering nunocrystals hold information about crystallite shape, size, surrounding, and the structure, factors of the asymmetric unit in the unit cell, hence, they differ remarkably from the recently published fiber-structure factors and provide this structure information at an early stage of crystallization.Experimental scattering data of wheat amylose recorded during the gelation process can explained by a remarkable amount of V-helices with 6-12 glucopyranosyl residues in solution at 70°C. Extended single helices probably exist also under these conditions. A mixture of independently scattering V- and B-form nanocrystallites is detectable in freshly cooled samples (40°C), and pure B-form nanocrystallites embedded in a matrix of an electron density comparable to that in the crystallites exist together with amorphous material afterjve weeks of aging at 21°C. From the scattering of the amylose sample during the gelation process it follows that Gidley's gelation model [M. J. Gidley (1989) Macromolecules, Vol. 22, pp. 351-358], which assumes crystallization during the phase separation, is prejerred to the model proposed by Miles et al. [M. J. Miles, V. J. Morris, and S. G. Ring (1985) Carbohydrate Research, Vol. 135, pp. 257-269], where crystallization is preceded by phase separation. © 1995 John Wiley & Sons, Inc.
Zusätzliches Material:
14 Ill.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1002/bip.360350303
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