HIGH FIELD NMR SPECTROSCOPY ANALYSIS OF BETA-LIMIT DEXTRINS FROM STARCH-LIKE POLYSACCHARIDES: AN APPROACH TO THE UNDERSTANDING OF AMYLOPECTINE CRYSTALLINITY

International audienceAmongst all branched a-1, 4 glucans, amylopectin occupies a very special place. Itsspecific, well-ordered a- 1, 6 branching pattern, originating from a complex enzymaticcontrol, permits an efficient packing of short linear chains within the currently acceptedcluster model and allows amylopectin to grow into exceptionally large macromolecules.This peculiar feature leads to unique physicochemical properties including reversibleswelling, crystallinity of high water content complexes as well as several others. It isessential to uncover this branching organisation to understand the unique architecture ofamylopectin and its relations to some of starch's macroscopic properties. In order to studythe branching sequences we have used the beta-amylase degradation on structurally relatedglucans : glycogen, phytoglycogen, waxy maize amylopectin as well as native wheat andpotato starches. The beta amylase degradation trims off all branches by removing maltoseresidues from the non-reducing ends until it reaches a branching-off point, leaving a shortstem of only a few glucose residues. The digestion removes half the mass of the startingglucan. The limit beta-dextrins so obtained are depleted in short chains and proportionally'enriched' of branched points. The polysaccharides were digested in a frontal stirredultrafiltration cell equipped with a 1000 MWCO cellulose membrane. The dextrins werewashed, concentrated, lyophilised then -OH protons were replaced with deuterium byexchanging twice in D20. NMR spectra were recorded in a deuterated methyl sulphoxide/D20 mixture at 500 MHz. On this poster we will show the changes observed in the protonand 13C carbon resonance frequencies corresponding to the transformation of the nativepolysaccharides into the beta-limit dextrins. The structural implications will then bediscussed