A ¹³C-NMR study on the conformational and dynamical properties of a cereal seed storage protein, C-hordein, and its model peptides

We have recorded the ¹³C CP-MAS and DD-MAS nmr spectra of dry and hydrated barley storage protein, C-hordein, as a model for wheat S-poor prolamins, together with those of model synthetic peptides (Pro)₂(Gln)₆ (1) and (Pro-Gln-Gln-Pro-Phe-Pro-Gln-Gln)₃ (II) under dry or hydrated conditions. The spectral features of C-hordein as well as these peptides were appreciably different from each other depending on the extent of hydration, reflecting different domains that adopt different types of conformations as well as dynamics. In particular, considerable proportions of the peak intensities were lost in the CP-MAS spectra, and well-resolved ¹³C-nmr signals emerged in DD-MAS nmr spectra owing to acquisition of molecular motions by swelling. It was shown that local β-turn or (Pro)_n type II conformation is more preferable for individual Pro residues and β-sheet type conformation is dominant for individual Gln residues in the dry and hydrated systems. In addition, two types of Gln environments are originated in C-hordein that differ in their mobility. Further, ¹³C spin-lattice relaxation times (T₁'s) of C-hordein and peptide II were reduced by more than one order of magnitude by hydration, reflecting the presence of well-swollen molecular chains. In contrast, the T₁ values of peptide I upon hydration remained one third of those in the dry state. Carbon-resolved proton spin-lattice relaxation times in the rotating frame (T₁p's) were also decreased by about 50% upon hydration, although these parameters were less sensitive as compared to T₁ values. In addition, the ¹³C-nmr signals of the aromatic side chain of Phe residues disappeared on hydration owing to interference between the frequency of the acquired flip-flop motion and the proton decoupling frequency. This information gives a new insight into establishing the structural properties of the studied protein system. A model may be put forward for a gel-type structure in which the more rigid part of the system involves intermodular hydrogen-bonded Gin side chains as well as some hydrophobic "pockets" involving Pro and Phe residues. The liquid-like domain is characterized by considerable backbone and side-chain motion as well as rapid ring-puckering motion in Pro residues.