Use of the Extended Fujita method for representing the molecular weight and molecular weight distributions of native and processed oat beta-glucans

Abstract

Beta 1-3, 1-4 glucans ("beta-glucans") are one of the key components of the cell wall of cereals, complementing the main structural component cellulose. Beta-glucans are also an important source of soluble fibre in foods containing oats with claims of other beneficial nutritional properties such as plasma cholesterol lowering in humans. Key to the function of beta-glucans is their molecular weight and because of their high polydispersity - molecular weight distribution. Analytical ultracentrifugation provides a matrix-free approach (not requiring separation columns or media) to polymer molecular weight distribution determination. The sedimentation coefficient distribution is converted to a molecular weight distribution via a power law relation using an established procedure known as the Extended Fujita approach. We establish and apply the power law relation and Extended Fujita method for the first time to a series of native and processed oat beta-glucans. The application of this approach to beta-glucans from other sources is considered.

Figure 1

Part of the 1–3, 1–4 β-D-glucan molecule. https://commons.wikimedia.org/wiki/File:Beta-1,3-1,4-glucan.png.

Figure 2

Analytical ultracentrifugation of oat beta-glucan BG90. (a) Sedimentation coefficient distribution plots g(s) vs s in phosphate-chloride buffer pH = 6.8, I = 0.10 M, at 3 serial dilutions from 1.0 mg/ml. A rotor speed of 40000 rpm was used. (b) Reciprocal plot of s versus concentration, fitted to (1/s) = (1/s°).(1 + k_sc) where k_s is the concentration dependence or ‘Gralén’ coefficient (Gralén, 1944; Harding & Johnson, 1985). From the fit a value of s° = (4.82 ± 0.10)S and k_s = (420 ± 40) ml/g are obtained.

Figure 3

SEC-MALS of oat beta-glucan BG90. (a) Elution profile with the beta-glucan peak limits selected in grey. Blue line: refractrometric (concentration) signal. Red line (light scattering signal recorded at a scattering angle of 90°). Both profiles normalized to a maximum of 1.0. (b) Mark Houwink-Kuhn Sakurada (MHKS) plot of intrinsic viscosity [η](V_e) values versus molecular weight M_w(V_e)  corresponding to elution volume values V_e within the marked limits of (a). Fit parameters shown in the inset.

Figure 4

Molecular weight distribution f(M) vs M for oat beta-glucan BG90. After transformation from the g(s) vs s distribution for c = 0.125 mg/ml (Fig. 2a), with coefficients b = 0.455 and κ_s = 0.01908.

Figure 5

Apparent molecular weight distributions for oat beta-glucan BG90. (a) obtained at 0.125 mg/ml, 0.25 mg/ml, 0.5 mg/ml and 1 mg/ml; (b) as (a) but normalized so the maximum value for f(M) = 1.0.

Figure 6

Molecular weight distribution f(M) vs M for beta-glucans in two roller milled and stored (3 years at 21 °C) oat samples (Matilda variety). Loading concentration c = 0.2 mg/ml. b = 0.455, κ_s = 0.01037. Inset: corresponding sedimentation coefficient distribution. Black line and squares: 200 μm aperture sieve used in the processing. Red line and circles: 710 μm.