Effect of Pulsed Electric Field on Microstructure of Some Amino Acid Group of Soy Protein Isolates

The effect of a pulsed electric field (PEF) on the microstructure of some amino acids was studied. Raman spectrum was used to determine the effect of PEF on tyrosine, tryptophan, proline residues, histidine, arginine, aliphatic amino acid, disulfide bond, and polypeptide backbone in soy protein isolates (SPI). Results suggested that increasing the intensity of PEF gradually to 50 kV cm-1led to a reduction in gauche C-S conformationof CCSSCC dihedral angles. The increase of the PEF intensity caused an increase in the gauche-gauche-gauche conformation of the disulfide bond accompanying a decrease in alpha-helix and beta-sheet and an increase in antiparallel beta-sheet and disorder structure. A critical pulse intensity of 30 kV cm-1 was observed for unfolding and reassembling of SPI, which was verified in our previous study (Liu et al., Eur Food Res Technol 233:841-50). When the pulse intensity gradually increased to around 30 kV cm-1, the exposure of tyrosine and tryptophan, the vibration of CH2 wagging in proline and CH2 in the midazole ring of histidine, the vibration of C-H bending and C-N stretching inside a charged arginine, and asymmetric H-C-H bending deformation vibration in CH2 and CH3 groups in aromatic and aliphatic amino acids gradually increased, suggesting an unfolding of protein molecules. When the pulse intensity continually increased from30 to 50 kVcm-1, the microstructure of all above amino acids decreased due to the reassembly of unfolding proteins.