Effects of bound water on FTIR spectra of glycinin

Soy protein is a major food component; but the secondary structure of its major proteins is not well established, especially after heat treatment in the presence of moisture. To study secondary structure changes that take place in glycinin upon hydration using infrared spectroscopy, the effect of bound water must be removed from the protein spectra. This study examined the effects of added water on glycinin infrared spectra. Samples hydrated from 2.6 to 95% water, but not heated, showed significant broadening in the amide I region and changes in the amide I to amide II maximum absorbance ratio as water content increased. A spectral ratio method to derive coefficients for multiplying and subtracting spectra was used to obtain the bound water component spectrum and protein component spectra. The spectrum of dry glycinin could then be regenerated by subtracting the bound water component from glycinin that had added water. Curve-fitting of deconvoluted spectra gave the same secondary structure at all levels of added water after bound water had been subtracted from the spectra (30% beta, 24% helical, 35% turns, and 11% unordered). Spectra of glycinin in aqueous buffer were also determined. Side-chain contributions were removed, and the resulting secondary structure was found to be 33% beta, 25% helical, 31% turns, and 12% unordered. This compares to 32% beta, 21% helical, 34% turns, and 14% unordered before side-chain contributions were subtracted. The data indicate that glycinin has the same structure in solution and in hydrated solids.