Fourier transform IR spectroscopic study of hydration-induced structure changes in the solid state of ω-gliadins

The hydration of ω-gliadins and partly deamidated and esterified ω-gliadins has been studied by Fourier transform IR spectroscopy. The secondary structure of the fully hydrated proteins was a mixture of β-turns and extended chains, with a small amount of intermolecular β-sheets. The absorption of the glutamine side chain amide groups contributed considerably to the amide I band with two well-defined peaks at 1658 and 1610 cm^-1. The amide I band of the dry native sample could not be resolved into single component bands. There the backbone structure seemed to be distorted by extensive hydrogen bonding involving glutamine side chains. With increasing water content, these hydrogen bonds were broken successively by water molecules, resulting in an increase in extended, hydrated structures, which gave rise to the formation of intermolecular β-sheet structures. Above 35% (w/w) water the β-sheet content fell sharply and was replaced by extensively hydrated extended structures. An amide I band similar to dissolved poly-L-proline proved that parts of the polymer were in a solution-like state. The replacement of many glutamine side chains in the esterified protein produced more resolved secondary structures even in the dry sample. The β-sheet content of the dry sample was higher than in the native ω-gliadins, but hydration generally caused very similar changes. At all hydration levels the spectra indicated a more ordered structure than in the native sample. Overall, the modification caused changes that go beyond the simple presence or absence of glutamine bands.