Abstract
Second harmonic generation (SHG) microscopy is employed to study changes in crystalline organization due to altered gene expression and hydration in barley starch granules. SHG intensity and susceptibility ratio values (R'SHG ) are obtained using reduced Stokes-Mueller polarimetric microscopy. The maximum R'SHG values occur at moderate moisture indicating the narrowest orientation distribution of nonlinear dipoles from the cylindrical axis of glucan helices. The maximum SHG intensity occurs at the highest moisture and amylopectin content. These results support the hypothesis that SHG is caused by ordered hydrogen and hydroxyl bond networks which increase with hydration of starch granules.
Highlights
Second harmonic generation (SHG) microscopy is an effective analytical tool for structural investigations of ordered, non-centrosymmetric biological samples
These results support the hypothesis that SHG is caused by ordered hydrogen and hydroxyl bond networks which increase with hydration of starch granules
The structural origin of SHG in starch granules from WX, WT and AO barley lines was investigated at 3 hydration conditions: hydrated, air-dried, and ultra-dried
Summary
Second harmonic generation (SHG) microscopy is an effective analytical tool for structural investigations of ordered, non-centrosymmetric biological samples. A starch granule can be considered as a model biological structure for SHG microscopy. Amylose is a mixture of unbranched and poorly branched glucans covering a wide range of degree of polymerization, but has a smaller size than amylopectin. The latter is the quantitatively dominant constituent of the granule possessing approximately 95% α-1,4 interglucose linkages and 5% of branching points, i.e. α-1,6 bonds. Amorphous regions are enriched in branching points while crystalline areas are formed by highly ordered double glucan helices, the organization of which leads to the A- or B-type allomorph of the entire starch granules [5]. The nanoscale ultrastructure of starch structures is highly variable and affected by chemical parameters, such as chain length distribution, frequency of branching points as well as physical factors, such as temperature and the degree of hydration [6]
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