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.The deuterium transverserelaxation was multiple exponential, with the slowest-relaxing component corre-sponding to bulk D2O.The thermal treatment caused a decrease in the populationof this slowly relaxing component as bulk D2O was adsorbed into the glutenmatrix.Such behavior is characteristic of hydrophilic biopolymers and contrastswith hydrophobic polymers like the protein elastin, which contracts and dispelswater.The data for the lipid-free and whole gluten were very similar, whichsuggests that the lipid has little effect on the gluten chain dynamics.This is con-sistent with the observation that the proton transverse relaxation in whole and31lipid-free gluten samples is also very similar.Indeed, P NMR spectroscopy(36), in conjunction with freeze-fracture electron microscopy of wheat gluten,shows that the lipids are organized in small vesicles in which polar lipids exhibita lamellar liquid crystalline phase.Taken together, the results suggest that wheatprotein lipid associations are not significant in determining the elasticity ofgluten.The HMW subunits of wheat glutenin are a relatively minor group of pro-teins, accounting for about 10% of the prolamins of wheat, but they are function-ally very important.It has been shown that allelic variation (i.e., genetic variationsresulting from mutation) resulting in changes in the number and properties ofHMW subunits is also associated with variation in breadmaking quality and maybe responsible for the elastic properties of doughs (37).Proton transverse relax-ation measurements have been undertaken on the dry and D2O-hydrated subunitand, like the gluten data, suggest the coexistence of rigid and mobile domainswithin the protein (38, 39).These results, in conjunction with FTIR studies, whichshow increases in ²-sheet in HMW subunits on hydration, have given rise to a loop and train theory (39) that may partially explain the origins of elasticityin wheat flour doughs.The NMR solid-state spectra of the total gliadin fraction of wheat (40, 41)and, more recently, of the É-gliadin fraction (42) have been reported.Sidechainmotions (methyl and amino group rotation, proline ring puckering) were foundto act as relaxation sinks for the proton longitudinal relaxation, and this wasunaffected by the glass transition, indicating that these motions persist in theglassy state.Magic-angle spinning experiments have been used to observe linenarrowing in the proton and carbon cross-polarization spectra.In the proton spec-tra, at high hydration levels, backbone and sidechain NH groups are observed,Copyright 2003 by Marcel Dekker, Inc.All Rights Reserved. indicating that whole segments of the protein chain are in the mobile regime.Atthe same hydration level, the carbon spectra are characterized by a loss of theproline C´ signal, showing that this is motionally mobile and involved in thehydration process.A model was proposed involving the formation of mobileloops together with more rigid regions of strong interchain interaction.2.NMR Studies of Barley ProteinsBarley is widely used as an animal feedstock and also, of course, in the brewingindustry.The main barley storage protein, C-hordein, is structurally homologousto the É-gliadins in what.This, together with the fact that it is relatively easy topurify in a homogenous form makes it an ideal model for studying É-glidian-type proteins.The repetitive primary structure of C-hordein results in a simplesolution-state NMR spectrum, which allows the assignment of the majority ofthe resonances to five residues (43).These residues, with the exception of thearomatic signals, are also present in the solid-state spectrum, and in both statesthe proline residues are found in the trans configuration, suggesting a ²-turn-richstructure.The effects of hydration on the domain dynamics in C-hordein have been13investigated with solid-state NMR (44).In particular, a comparison of the CCPMAS and DD-MAS spectra of dry and hydrated C-hordein (45) suggests agel-type molecular structure in which the more rigid part of the system involvesintermolecular hydrogen-bonded Gln sidechains as well as some hydrophobic pockets involving Pro and Phe residues.The liquidlike domain is character-ized by considerable backbone and sidechain motion as well as rapid ring-puck-ering motion in Pro residues.Hydration results in swelling and disappearance ofthe Phe residue signals as they acquire flip-flop motion.3.NMR Studies of Sorgum ProteinsThe kaffirins, which are the main prolamins of sorghum, are characterized by amore regular structure than wheat proteins and have a high degree of hydropho-bicity, which no doubt accounts for their much lower level of digestibility, bothin the gut and in in vitro enzymatic studies, compared to other cereal proteins.Proton transverse relaxation measurements on D2O-hydrated total kaffirin showthat mobility increases with increasing hydration (46).However, a high percent-age of the relaxation can be described by a fast-decaying Gaussion component,even at high levels of hydration.This is significantly different to the behaviorof the highly hydrated HMW subunits in wheat, where the transverse relaxationwas mostly exponential in nature, and suggests that, compared to the HMW sub-units, kaffirins retain a high degree of structure
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