Magnetic resonance spectroscopy of wheat proteins: a magic-angle-spinning 13C nuclear magnetic resonance and an electron spin resonance spin label study

Abstract

The freeze-dried wheat protein fractions, glutenin and gliadin, mixtures of high molecular weight (HMW) subunits of glutenin from flour, and gliadin-enriched and glutenin-enriched fractions from a gluten ball, were investigated by cross-polarization magic-angle-spinning 13 C nuclear magnetic resonance spectroscopy. No characteristic differences in the molecular structure of these systems could be recognised in the spectra obtained. Sharp resonances in the aliphatic and aromatic spectral regions in the glutenin-enriched fraction, which have been described by other workers, were not observed: these may be artefacts arising from impurities present in the materials used. Isolated HMW subunits 2, 3, 10 and 11 were studied in solution using electron spin resonance after alkylation of cysteine residues with a maleimide spin label. Spectra obtained in 3 m urea were identical for all HMW subunits, with similar rotational correlation times (τ c ) of 0·22 ± 0·02 ns. Spectra obtained in a less denaturing solvent (10 mM acetic acid) indicated that τ c increased for all HMW subunits ranging from 0·38 to 0·52 ns and that τ c for HMW subunit 10 was significantly lower than τ c for subunit 11. It is suggested that a high spin label mobility in 10 m m acetic acid is related to a better crosslinking function of subunit 10 in forming the extended, disulphide-bonded network of glutenin.