A high resolution ¹H magic angle spinning NMR study of a high-M_r subunit of wheat glutenin

This work describes the application of ¹H magic angle spinning (MAS) nmr to the study of hydrated 1Dx5 wheat high-M_r subunit. 1Dx5 is a water-insoluble 88 kDa protein, associated with good baking performance, and whose structure in the solid and low-hydration states is not known. High-resolution MAS (HR-MAS) results in a threefold resolution improvement of the ¹H spectra of the hydrated wheat protein, compared to standard MAS. The spectral resolution achieved enables, for the first time, two-dimensional nmr methods to be employed for the study of hydrated IDx5 and the assignment of the spectrum to be carried out on the basis of total correlated spectroscopy and ¹³C/¹H correlation experiments. Considerable shifts are observed for some resonances, relative to the chemical shifts of amino acids in solution, indicating that specific interactions occur in the hydrated protein network. Two main environments are identified for glutamine residues, Q₁ and Q₂, and these were characterized in terms of possible conformation and relative dynamics, with the basis of comparison between the single 90° spectrum and the Carr-Purcel-Heiboom-Gill (CPMG) spectrum. The Q₁ residues are proposed to be situated in protein segments that adopt the β-sheet conformation and that remain relatively hindered, possibly by hydrogen bonds involving the glutamine amide groups. On the other hand, Q₂ residues are proposed to be situated in a more mobile environment, adopting a looser conformation, possibly a β-turn conformation. Based on the proximity of the Q₂ residues with glycine residues, as viewed by the nuclear Overhauser effect spectroscopy experiment, it is proposed that the protein segments that form the more mobile (or loop) sections of the network are rich in both glutamine and glycine residues.