Electron paramagnetic resonance study of water distribution in starch granules

An electron paramagnetic resonance (EPR) study was performed for potato and wheat starch containing Cu 2+ ions as a paramagnetic probe. Distribution of water in the starch granules as well as the interactions between the copper and starch matrix of different crystalline structures were determined. EPR spectra of the native starches consisted of two different centers of Cu 2+ . One of them, giving at 293 and 77 K an EPR signal of axial symmetry with a well‐resolved hyperfine structure (HFS), was assigned to the Cu 2+ –starch complex in which Cu 2+ ions strongly interacted with oxygen atoms of the starch matrix. Another Cu 2+ species, exhibiting an isotropic signal at 293 K and an axial signal with resolved HFS at 77 K, was attributed to a [Cu(H 2 O) 6 ] 2+ complex freely rotating at room temperature and immobilized at low temperatures. Interaction of Cu 2+ with the starch matrix and the relative number of the particular copper species depended on the crystallographic type of starch. Dehydration at 393 K resulted in elimination of the rotating complex signal and decrease of the total intensity of the EPR spectrum caused by clustering of the Cu 2+ ions. Freezing at 77 K and thawing led to restoring of the spectrum intensity and reappearing of the signal of the [Cu(H 2 O) 6 ] 2+ complex. This effect, related to liberation of water molecules from the granule semicrystalline growth rings on freezing/thawing, was especially visible for wheat starch, indicating differences in the water retention ability of starch granules of different crystallographic structure. © 2006 Wiley Periodicals, Inc. Biopolymers 82:549–557, 2006 This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com