Combining sedimentation velocity with SEC-MALLS to probe the molecular structure of heterogeneous macromolecular systems that cannot be preparatively separated: application to three rice starches

Abstract

A combined approach using sedimentation velocity in the Beckman Optima XL-I ultracentrifuge together with high performance size exclusion chromatography coupled to multiangle laser light scattering (SEC-MALLS) was used to assess the molecular structure of rice amylose from three different rice starches without pre-separation by preparative means. The amylose content of the rice starches was analysed by the iodo-colorimetric method. We found for the three species of amylose starch studied (Thaibonnet, Cypress and Dixiebell): (1) the weight average molecular weights, M _w, (from SEC-MALLS) for the amyloses were similar (4.9, 4.8, 5.1 × 10^su5 g/mol); (2) the sedimentation coefficients, s _20w ^° (from apparent distribution of sedimentation coefficient analysis) were also similar (5.7, 5.3, 7.1 S), despite the fact that the amylose content (relative to amylopectin) for the three rice starches was quite different. The study also shows that attempts to obtain molecular weight by sedimentation velocity (s-D and s- k _s on the unseparated materials are not useful: SEC-MALLs is the method of choice for these substances. For heterogeneous materials where preparative separation is not possible, the utility of using SEC-MALLs to characterize component molecular weights and velocity sedimentation to characterize sedimentation coefficients is thus indicated.