Structure and organization within films of arabinoxylans extracted from wheat flour as revealed by various NMR spectroscopic methods

Abstract

Arabinoxylans (AXs) extracted from wheat flour were characterized by using three different techniques of NMR spectroscopy. Liquid-state (1)H NMR and solid-state (13)C NMR allowed the investigation of the fine structure of the three specific fractions of AXs representative of the structural heterogeneity of AX in wheat tissues. Three pure AX fractions exhibiting an arabinose to xylose ratio of 0.33, 0.53, and 0.73 were compared relative to their substitution feature and also to their assembly into thin films. Measurements of M(2), i.e. the second moment of proton dipolar interactions between the polysaccharide chains, were achieved using time-domain (TD) (1)H NMR at different water contents and temperatures. Transitions of the M(2) values were observed at a certain temperature close to the glass transition temperature T(g) values of AXs in films. Comparison of the different AX films containing various water contents pointed out stronger dipolar interactions for lowly substituted AX. This indicated that, in films, contiguous unsubstituted xylan chains can interact together through hydrogen bonding resulting in a compact structure with small nanopores because of the lower chain motions and the shorter average distances between the lowly substituted AX chains.