Conformational studies of a synthetic peptide corresponding to the repeat motif of C hordein

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Abstract
C hordein, a storage protein from barley grains, has an Mr of about 53,000 and consists predominantly of repeated octapeptides with a consensus sequence of Pro-Gln-Gln-Pro-Phe-Pro-Gln-Gln. Previously reported hydrodynamic and c.d. studies indicate the presence of .beta.-turns, the repetitive nature of which may lead to the formation of a loose spiral. In order to study these turns we have compared the structures of a synthetic peptide corresponding to the consensus repeat motif and total C hordein by using c.d. and Fourier-transform i.r. spectroscopy. The synthetic peptide exhibited spectra typical of .beta.I/III reverse turns when dissolved in trifluoroethanol at 22.degree.C and in water at 70.degree.C, but ''random-coil''-like spectra in water at 22.degree.C. The whole protein also showed increases in .beta.I/III reverse turns when dissolved in increasing concentrations of trifluoroethanol (50-100%, v/v) or heated in ethanol/water (7:3, v/v). Two cryogenic solvent systems were used to determine the c.d. spectra of the peptide and protein at temperatures down to -100.degree.C. Methanol/glycerol (9:1, v/v) and ethanediol/water (2:1, v/v) were selected as analogues of trifluoroethanol/water and water respectively. The peptide exhibited .beta.I/III-reverse-turn and ''random-coil''-like spectra in methanol/glycerol and ethanediol/water respectively at 22.degree.C, but a spectrum similar to that of a poly-L-proline II helix in both solvents at -100.degree.C. Similarly the proportion of this spectral type also increased when the whole protein was cooled in both solvents. These results indicate that a poly-L-proline II conformation at low temperatures is in equilibrium with a .beta.I/III-turn-rich conformation at higher temperatures. The latter conformation is also favoured in solvents of low dielectric constant such as trifluoroethanol. The ''random-coil''-like spectra exhibited by the protein and peptide in high-dielectric-constant solvents at room temperature may result from a mixture of the two conformations rather than from the random-coil state.