Magic angle spinning NMR study of the hydration of the wheat seed storage protein omega-gliadins

The hydration of the wheat protein omega-gliadins was investigated by carbon and proton magic angle spinning (MAS) NMR spectroscopy. The changes observed in the protein carbon spectrum with increasing hydration in the range 0-50% show a general mobility increase but, even at 50% hydration, a number of glutamine side-chain carbons remain relatively immobilized. The results suggest that a conformational change occurs at about 35% hydration, giving a looser conformation. Carbon T₁ relaxation times reflect the general mobility increase, in the MHz frequency range, by showing an order of magnitude decrease upon hydration. No distinction between T₁ of the backbone and glutamine side-chain carbonyls is observed. This confirms the relative rigidity of these side-chains even at high hydration. MAS at high spinning rates has been used previously to resolve the proton spectra of hydrated omega-gliadins. Resolution was further improved by using a new high-resolution MAS probe. Interpretation of the resulting protein spectrum showed that some phenylalanine residues are considerably motionally hindered. Moreover, evidence shows that some glutamine side-chain amino groups are inaccessible to solvent. A structural model for hydrated omega-gliadins is advanced involving the formation of hydrophobic pockets held by stable intermolecular and/or intramolecular hydrogen bonding between glutamine residues. The high-resolution spectra obtained using the new probe design permitted the use of high-resolution 2D experiments for assignments and to investigate conformational properties. In an attempt to use proton relaxation parameters to characterize the protein system further, it was found that, under MAS conditions, proton T₁ relaxation times are strongly dependent on spinning rate. The results indicate that great care is required when interpreting proton relaxation times recorded under MAS conditions.