Solid-state NMR studies on the structure of starch granules

Resonances expected for carbon atoms in the amorphous regions of moistened starch granules are not visible in 13C CP/MAS NMR spectra. These resonances could be observed in spectra obtained by using a single-pulse, Bloch-decay sequence, with magic-angle spinning (SP/MAS). This implies that the amorphous regions in moistened starch granules are mobile and have properties similar to a rubberlike polymer. For moistened wheat starch granules, proton rotating frame relaxation times for resonances assigned to the amylose-lysophospholipid inclusion complexes were shorter than those of the crystalline starch. Relaxation differences were used to generate subspectra of the crystalline starch and the amylose-lysophospholipid inclusion complex. Our results suggest that the inclusion complexes occur in distinct regions of the starch granule. There are thus three distinct components making up the wheat starch granule: (i) highly crystalline regions formed from double-helical starch chains, (ii) solid-like regions formed from lipid inclusion complexes of starch, and (iii) completely amorphous regions, associated with the branching regions of the amylopectin component of starch and possibly the lipid-free amylose. All these components can be observed separately using different solid-state NMR techniques and would be expected to contribute to the physical properties of a wheat starch granules.