Raman spectroscopic study of amidated food proteins

Various amounts of tryptophan were attached to three food protein products: soy protein isolates, spray-dried egg white and gluten, using a water-soluble carbodiimide method. The extent of amidation was determined by a spectrophotometric method. Raman spectra (600–2000 cm−1) of the modified proteins were obtained and analyzed. The phenyl stretching vibration at 1552 cm−1, directly attributed to the attached tryptophan, was used as a marker band, and increases in band intensity were observed in the modified protein samples. Calibration curves were constructed by plotting the intensity ratio of the 1552 cm−1 band to the 1003 cm−1 phenylalanine stretching band (used as an internal standard) against the amount of tryptophan attached. High correlation coefficients, (r) ⩾ 0.99, were obtained from these calibration curves. The Raman spectral data showed a transition from ordered conformation to random coil structures in the amidated food protein products.