SIMULTANEOUS DETERMINATIONS OF THE MOLECULAR-WEIGHT DISTRIBUTIONS OF AMYLOSES AND THE FINE-STRUCTURES OF AMYLOPECTINS OF NATIVE STARCHES

Native A (wheat and waxy rice), B (potato), and C (cassava and sweet potato) types of starches were each debranched with isoamylase, and separated into amylose and amylopectin fractions by HPLC on size exclusion columns coupled on-line to multi-angle-laser-light-scattering and differential refractometer detectors. The absolute molecular weights of amyloses and chain length distributions of amylopectins were determined simultaneously, and pre-isolation of the amylopectin was not necessary. The molecular weights of debranched amylose from starches that have not been fractionated to separate amylose and amylopectin are significantly higher than published values for the undebranched fractionated amylose. The polymodal profiles of the refractive index chromatograms showed the complexity of the amylopectin structure of starches. The chain length distribution of amylopectin depends critically on the method for analysing the broad chromatogram when determined by either noting the minima/inflections or deconvoluting the overlapping amylopectin fraction into numerous normal/Gaussian distributions. Although the results from the former (conventional) method of analysis were comparable with the literature values, they did not appear to be as sensitive a technique for detecting differences as the multiple Gaussian approach. Overall, the study suggested that the amylopectin chain units might be more complex than originally envisaged and that different degrees of chain packing for the molecules can be inferred from this multiple component analysis.