Mechanistic Information from Analysis of Molecular Weight Distributions of Starch

Abstract

A methodology is developed for interpreting the molecular weight distributions of debranched amylopectin, based on techniques developed for quantitatively and qualitatively finding mechanistic information from the molecular weight distributions of synthetic polymers. If the only events occurring are random chain growth and stoppage (i.e., the rates are independent of degree of polymerization over the range in question), then the number of chains of degree of polymerization N, P(N), is linear in ln P(N) with a negative slope, where the slope gives the ratio of the stoppage and growth rates. This starting point suggests that mechanistic inferences can be made from a plot of lnP against N. Application to capillary electrophoresis data for the P(N) of debranched starch from across the major taxa, from bacteria (Escherichia coli), green algae (Chlamydomonas reinhardtii), mammals (Bos), and flowering plants (Oryza sativa, rice; Zea mays, maize; Triticum aestivum, wheat; Hordeum vulgare, barley; and Solanum tuberosum, potato), gives insights into the biosynthetic pathways, showing the differences and similarities of the alpha-1,4-glucans produced by the various species. Four characteristic regions for storage starch from the higher plants are revealed: (1) an initial increasing region corresponding to the formation of new branches, (2) a linear ln P region with negative slope, indicating random growth and stoppage, (3) a region corresponding to the formation of the crystalline lamellae and subsequent elongation of chains, and (4) a second linear ln P with negative slope region. Each region can be assigned to specific enzymatic processes in starch synthesis, including determining the ranges of degrees of polymerization which are subject to random and nonrandom processes.

Molecular structural differences between maize leaf and endosperm starches

“…3, because such plots bring out features which are often not apparent in the commoner ways of presenting as number or as difference distributions (Castro, Dumas, Chiou, Fitzgerald & Gilbert, 2005). The normalization of a CLD is arbitrary, and for convenience those in Fig.…”

“…3 are normalized to have the same maximum. It is essential to avoid "conclusions" which are in fact artifacts of the way one decides to normalize a distribution (Castro et al, 2005); such artifacts are unfortunately not uncommon in the literature when using simple difference plots. Now, the best way of making a comparison between different CLDs is by fitting to the biosynthesis-based model for this (Wu & Gilbert, 2010;Wu et al, 2013;Wu, Ral, Morell & Gilbert, 2014b), to find differences between fitting parameters for the biosynthesis processes and the underlying genetics.…”

The importance of amylose and amylopectin fine structure for textural properties of cooked rice grains

“…For a linear polymer (such as debranched starch), the number 156 distribution of chains (obtained by debranching), N de (X), is related to the corresponding SEC 157 weight distribution by (Castro, Dumas, Chiou, Fitzgerald & Gilbert, 2005): 158…”

Characterization Methods for Starch-Based Materials: State of the Art and Perspectives

“…[16] While the former is what is frequently used for presenting CLD data in the literature, the logarithmic representation of the same data brings out features at higher DPs which are not obvious in the conventional presentation (CLD of higher DPs is at least one to two orders of magnitude less than of the lowest DPs), as well as being a form based on mechanistic precepts. [1,[16][17][18] The conventional treatment of N de (X) is the difference plot, where one CLD is subtracted from another. However, difference plots require particular care not to introduce artefacts from incorrect normalization.…”

“…However, difference plots require particular care not to introduce artefacts from incorrect normalization. [16] A quantitative treatment of N de (X) has been shown to provide in-depth understanding of starch biosynthesis (see below).…”