Abstract
Thermal aggregation and gelation of oat globulin were studied by FT-NIR Raman spectroscopy. The buffer-soluble aggregates exhibited a Raman spectrum similar to that of the unheated control, whereas the insoluble aggregates showed intensity increases in the tryptophan, C–H bending and C–H stretching bands, and a decrease in the tyrosine doublet (I850/I830), suggesting protein denaturation. However, analysis of the amide I region using Raman Spectral Analysis Package (RASP) program revealed marked decreases inα-helical and increases inβ-sheet structure in both soluble and insoluble aggregates. Similar conformational changes were also observed in the heat-induced oat globulin gels, and may be attributed to realignment of molecular segments and formation of intermolecularβ-sheet structures. Thermal gelation under the influence of different chaotropic salts showed some shifts in band positions and changes in band intensity, following the lyotropic series of anions. Several protein structure perturbants, including sodium dodecyl sulfate, dithiothreitol, urea and sodium laurate, were found to affect the Raman spectral characteristics of oat globulin gels. The data suggest that changes in gelling properties of oat globulin by these chemicals may be related to conformational changes of the protein during gelation.
Highlights
Thermal aggregation and gelation are important functional properties of proteins affecting their uses in food systems [1]
Previous study [9] showed that freeze-dried protein samples exhibited Raman spectra identical to those in dispersions or wet pellets indicating that freeze drying did not affect the conformation of oat globulin
The locations of the amide I and III peaks and Raman Spectral Analysis Package (RASP) analysis (Table 2) show that β-types and random coils are the major secondary structures in oat globulin. This is in agreement with circular dichroism (CD) data which indicate that oat globulin, similar to most plant globulins, has a relatively
Summary
Thermal aggregation and gelation are important functional properties of proteins affecting their uses in food systems [1]. Since aggregation and gelation generally occur at high protein concentrations with the formation of opaque coagulum/gel, the structural changes cannot be studies by techniques such as NMR or circular dichroism (CD) spectroscopy. Vibrational spectroscopic techniques such as Raman and FT-IR have the advantage of being adaptable to a wide range of samples including liquids, powders, semi-solids and films [6]. Fluorescence is a major problem with Raman spectroscopy and has limited its use in plant proteins. A significant advance in solving the fluorescence problem was the development of Fourier transform (FT) Raman system with the use of a diode-laser pumped Nd:YAG laser radiation [7,8]
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