Hydrodynamic characterisation of the molar mass and gross conformation of corn cob heteroxylan AGX

The water-soluble fraction of arabinoglucuronoxylan AGX, extracted from corn cob has been studied previously using a combination of static light scattering and viscometry (A. Ebringerova et al. (1992). Carbohydr. Polym. 19, 99-105). The non-linearity of the resulting Mark-Houwink relation was interpreted as being due to possible self-association/aggregation phenomena. The aim of the present study is to investigate this possibility further, together with the size and gross conformation of xylan, using analytical ultracentrifugation and viscometry. Studies were conducted in a phosphate-chloride buffer (pH=6.8, I=0.10). Sedimentation equilibrium in the analytical ultracentrifuge gave absolute weight- and z-average molar masses of (1.50 +/- 0.15) x 10(5)g/mol and (1.91 +/- 0.20) x 10(5)g/mol using, respectively, Rayleigh and Schlieren optical systems to record equilibrium solute distributions. A value for the second thermodynamic virial coefficient, B of (2.28 +/- 0.05) x 10(-4)ml.mol/g2, from the Rayleigh distribution and a similar value from the Schlieren optical records (approximately 2.6 x 10(-4) ml.mol/g2) were obtained. Capillary viscometry yielded an intrinsic viscosity [eta] = (41.6 +/- 2.2)ml/g and a Huggins constant K eta of approximately(3.6 +/- 0.6). Sedimentation velocity using a novel application of the Beckman Optima XL-A ultracentrifuge gave an infinite dilution sedimentation coefficient s degree 20,W = (0.99 +/- 0.20)S. No direct evidence could be seen of any 'simple' self-association phenomena from neither the concentration dependence of the apparent molar masses, Mw,app, Mz,app, from the concentration dependence of S20,w, nor from the shape of the sedimenting boundaries. Analysis of the conformation by the II-function and the frictional ratio (in terms of the equivalent hydrodynamic ellipsoid) suggested that this macromolecule adopts a highly expanded conformation in solution consistent with an asymmetric coiled structure. Further comment is made on the low values for [eta] in this and a previous study, in terms of the presence of compact supramolecular aggregates.