Study of high molecular weight wheat glutenin subunit 1Dx5 by ¹³C and ¹H solid-state NMR spectroscopy. I. Role of covalent crosslinking

This work describes a carbon and proton solid-state NMR study of the hydration of a high molecular weight wheat glutenin subunit, 1Dx5. The effect of the presence of disulfide bonds on the hydration behavior of the subunit is investigated by a comparison of the unalkylated and alkylated forms of the protein. Hydration induces partial plasticization of the protein so that some segments become more mobile than others. The ¹³C cross-polarization and magic-angle spinning (MAS) spectra of the samples in the dry state and at two hydration levels (∼40 and ∼65% D₂O) were used to monitor the protein fraction resisting plasticization (trains). Conversely, ¹³C single pulse excitation and ¹H-MAS experiments were used to gain information on the more plasticized segments (loops). The molecular motion of the two protein dynamic populations was further characterized by ¹³C T₁ and ¹H T_1ρ, T₂, and T₁ relaxation times. The results suggest that hydration leads to the formation of a network held by a cooperative action of hydrogen bonded glutamines and some hydrophobic interactions. The looser protein segments are suggested to be glycine- and glutamine-rich segments. The primary structure is therefore expected to significantly determine the proportion of trains and loops in the network. The presence of disulfide bonds was observed to promote easier plasticization of the protein and the formation of a more mobile network, probably involving a higher number of loops and/or larger loops.