Raman spectroscopic study of deamidated food proteins

Three food protein products, soy and whey protein isolates and spray-dried egg white powder, were deamidated to various levels by 0.04 N hydrochloric acid. The extents of deamidation were determined using an ammonia electrode. Raman spectra of the modified proteins were obtained and analyzed. A new CO stretching vibration band was observed at 1780 cm−1, attributed to γ-carboxyl groups of aspartic and glutamic acids. Calibration curves were constructed by plotting the intensity ratio of the 1780 cm−1 band to the 1003 cm−1 phenylalanine stretching band (as an internal standard) against the extent of deamidation. Linear fits were obtained with high correlation coefficients r > 0.987. Effect of deamidation on the conformation of these proteins was also studied by monitoring changes in Raman spectral characteristics, including a transition from ordered to disordered structures, exposure of tryptophan residues from a buried, hydrophobic microenvironment, and probable conformational changes of the aliphatic residues.