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Handbook of Size Exclusion Chromatography and Related ...

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. B-type:doublehelicescrystallizedinthehexagonalspacegroupP6;less<br />

compactly packed, higher water-content [36 H 2O molecules with<br />

12 anhydroglucose units (AGU); 6AGU per helix turn, 1.04 nm height<br />

for each turn].<br />

. C-type: amix <strong>of</strong> A- <strong>and</strong> B-type; however, listed as adistinct type.<br />

. V-type: formed by 6AGU in ahelical structure with aheight <strong>of</strong> 0.8 nm<br />

per helix turn.<br />

By means <strong>of</strong> enzymatically supported fragmentation analysis, A-type starch<br />

glucans can be seen to also differ from B-type glucans in their branching pattern,<br />

in particular in their ratio <strong>of</strong> terminal (A-chains) <strong>and</strong> internal (B-chains) glucan<br />

segments (37–40).<br />

Water definitely needs to be considered as afundamental structural feature<br />

in theformation <strong>of</strong> starch granules <strong>and</strong> isnot just another bulk material. All kinds<br />

<strong>of</strong> crystallinity represent more or less ordered structures on a more or less<br />

dominant amorphous background (41,42) (Fig. 3). 13 CCP/MAS spectra support<br />

the idea <strong>of</strong> amorphous single-chain <strong>and</strong> ordered double-helix glucans (43).<br />

Thermal stress on B-type glucans results in loss <strong>of</strong> water <strong>and</strong> transformation into<br />

A-type; swelling <strong>of</strong> A-type in aqueous media <strong>and</strong> destruction <strong>of</strong> crystalline<br />

structure yields B-type when recrystallizing.<br />

Whereas scb-glucans (amylopectin) are assumed to form crystalline<br />

lamellae through parallel double helices with branching positions in amorphous<br />

regions, nb- <strong>and</strong> lcb-glucans (amylose) are preferably located in the amorphous<br />

layers (44) <strong>and</strong> are subject to complex formation with lipids. Additionally,<br />

limited cocrystallization <strong>of</strong> scb- <strong>and</strong> lcb-glucans forming small ( 25 nm) <strong>and</strong><br />

large (80–120 nm) blocks was observed by scanning electron microscopy (SEM)<br />

<strong>and</strong> atomic force microscopy (AFM) (45–48).<br />

Based on differences in lcb/scb-glucan ratio, there are reasonable<br />

suggestions for preferred localizations <strong>of</strong> scb- (amylopectin) <strong>and</strong> lcb-glucans<br />

(amylose) within starch granules:<br />

. Waxy maize starch granules represent an arrangement <strong>of</strong> more or less<br />

100% scb-glucans (amylopectin) <strong>and</strong> 0% lcb-glucans (amylose). The<br />

scb-glucans are closely packed in concentric layers in dry waxy maize<br />

granules <strong>and</strong> exp<strong>and</strong> on swelling in aqueous media. These concentric<br />

layers <strong>of</strong> scb-glucans represent the framework for the majority <strong>of</strong> starch<br />

granules.<br />

. Potato starch granules are a mix <strong>of</strong> a major fraction <strong>of</strong> 80% scbglucans<br />

(amylopectin) <strong>and</strong> a minor fraction <strong>of</strong> 20% lcb-glucans<br />

(amylose). The lcb-glucans are localized in distinct concentric layers<br />

alternating with scb-glucan layers. On hydration, these granules swell<br />

due to the exp<strong>and</strong>ing layers <strong>and</strong> simultaneously reduce the volume for<br />

© 2004 by Marcel Dekker, Inc.

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