COMPUTER-SIMULATIONS OF X-RAY-SCATTERING CURVES - GELATION AND CRYSTALLIZATION PROCESS IN AMYLOSE SOLUTIONS

Small- and wide-angle x-ray scattering is used for structural characte rization of amylose solutions and gels. 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. 61, pp. 7-40] , x-ray crystallography [W. Hinrichs & W. Saenger (1990) Journal of th e American Chemical Society, Vol. 112, pp. 2789-2796], and theoretical ly calculated atomic coordinates for energy-minimized conformers of am ylose molecules in solution and crystals served to simulate small- and wide-angle x-ray scattering curves. The simulation of scattering curv es renders possible a quick screening and detection of special feature s in experimental curves and the decision of whether they m-e signific ant or not. The scattered intensities of the models were calculated us ing the atomic scattering factors and van der Waals radii within the f ramework of the improved cube method [J. J. Muller (1983) Journal of A pplied Crystallography, Vol. 16, pp. 74-82]. All model data and the sc attering curves are stored for a fast information retrieval in the dat abase OBIOSCAT controlled by the ORACLE management system.In the conte xt of a mixture of different structures existing in an amylose solutio n orgel, the parallel-stranded left-handed B-form double helices (Imbe rty 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 antipara llel strands have structures that produce new scattering features. Up to now,such features have not been experimentally detected. Extended o r collapsed single helices, too, can be discriminated by their scatter ing 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. Rappen ecker 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 wa s simulated with atomic resolution. The scattering curves of independe ntly scattering nanocrystals hold information about crystallite shape, size, surrounding, and the structure factors of the asymmetric unit i n the unit cell, hence, they differ remarkably from the recently publi shed fiber-structure factors and provide this structure information at an early stage of crystallization. Experimental scattering data of wh eat amylose recorded during the gelation process can be explained by a remarkable amount of V-helices with 6-12 glucopyranosyl residues in s olution at 70 degrees C. Extended single helices probably exist also u nder these conditions. A mixture of independently scattering V- and B- form nanocrystallites is detectable in freshly cooled samples (40 degr ees C), and pure B-form nanocrystallites embedded in a matrix of an el ectron density comparable to that in the crysrallites exist together w ith amorphous material after five weeks of aging at 21 degrees C. From the scattering of the amylose sample during the gelation process it f ollows that Gidley's gelation model [M. J. Gidley (1989) Macromolecule s, Vol. 22, pp. 351-358], which assumes crystallization during the pha se separation, is preferred to the model proposed by Miles et al. [M. J. Miles, V. J. Mor I is, and S. G. Ring(1985) Carbohydrate Research, Vol. 135, pp. 257-269], where crystallization is preceded by phase sep aration. (C) 1995 John Wiley & Sons, Inc.