Proton magnetic resonance studies of barley and wheat thionins: structural homology with crambin.

The thionins comprise a group of very basic proteins of Mr approximately 5000 found in the seeds of Gramineae. They each contain 45 amino acid residues arranged along a single polypeptide chain that is constrained by four disulfide bridges. Five thionins of known sequence, from barley and wheat, have been investigated and compared by 1H NMR spectroscopy at 600 MHz. From their spectral characteristics it is concluded that the five proteins have very similar, nonrandom conformations in 2H2O solution. Moreover, on the basis of selective nuclear Overhauser experiments at 300 MHz, features of their secondary and tertiary structures are shown to be similar to those of crambin, a related, hydrophobic protein extracted from seeds of the crucifer Crambe abyssinica. The strong compositional homology of the thionins facilitates the assignment of methyl and aromatic resonances, as only a few residues are replaced and these are at known sites. The substitution of leucine for an isoleucine does not affect significantly the local magnetic environment, suggesting that those isomeric side chains easily accommodate the same spatial constraints. A fast hydrogen-deuterium exchange is observed at pH 6.25, 25 degrees C. This indicates that, although of folded conformation, the thionins are structurally flexible polypeptides that efficiently expose all amides to the solvent.