Interaction Between Soy Protein Isolate Composites By Two-Dimensional Correlation Infrared Spectroscopy

Recently,biopolymer composites have aroused more attention as a promising replacement of traditional petroleum-based plastics owing to their good biocompatibility,biodegradability and low cost.Taking the high protein content and nutrition value into account,soy protein isolate(SPI)is specially focused.The properties of SPI composites are significantly improved with the addition of active constituents or blending with functional polymers.Fourier transform infrared(FTIR)spectroscopy is a powerful tool not only to study the specific structures of compound,but also to explore the dynamics and the interaction of different groups.Due to the serious overlap of the amide I bands of protein secondary structures,it is difficult to resolve them by one-dimensional IR spectroscopy.Using two-dimensional correlation spectroscopy(2DCOS),the spectral resolution is obviously enhanced by spreading the specific band over a second dimension.Furthermore,2DCOS provides useful information on the sequential order of specific bands along the external perturbation,which is essential to the analyses of the changes of structures and the interaction of different groups.In this paper,a series of glycerol-plasticized SPI films were prepared at different SPI/glycerol ratio.Attenuated total reflection Fourier transform infrared(ATR/FTIR)spectroscopy coupled with two-dimensional correlation spectroscopy(2DCOS)and perturbation-correlation moving-window two-dimensional(PCMW2D)analyses were applied to investigate the effect of H-bonding on the secondary structures of glycerol-plasticized SPI films and thus on the plasticization.An“S”profile of the H-bonding between SPI and glycerol with an abrupt jump in the glycerol range 10-30%were obtained from ATR/FTIR results.On the basis of PCMW2D results,the experimental concentration range were split up in two parts for 2DCOS analysis.The conformation change sequences under the effect of H-bonding were revealed in 2DCOS.When glycerol was involved,it entered into ?-sheet,and the H-bonds of SPI peptide backbone(C=O…H-N-)were replaced by the new H-bonds between SPI and glycerol(C=O…H-O-).The transformations of parallel(3-sheet to ?-turn in the range 0-20%and anti-parallel ?-sheet to(3-turn in the range 20-35%were obtained.In the concentration range 35-60%?-sheet was first changed to a transition state conformation,then together with(3-turn,to random coil.The 2DCOS results clearly suggest that the conformations of SPI gradually change from the ordered to the less-and dis-ordered,which significantly improve the plasticity of SPI film.ATR/FTIR coupled with 2DCOS and PCMW2D analyses were also applied to study the interaction between SPI/chitosan(CHT)films.Firstly,the homogeneity of SPI/CHT films was studied by ATR/FTIR and scanning electron microscope(SEM).The ATR/FTIR spectra series of SPI/CHT films showed that the absorbance of amide I band increased with the increase of CHT concentration,suggesting the existence of an interaction between SPI and CHT.Secondly,the secondary structures changed at varied CHT concentration and the experimental concentration range were split up in two parts for 2DCOS analysis based on the PCMW2D results.The conformation change sequences under the effect of interaction were thus revealed.The transformation of antiparallel ?-sheet to ?-turn is observed in the range 50-70%though this transformation is weak.In the concentration range 70-95%less-ordered P-turn was changed to ordered antiparallel ?-sheet,which increase the ordered structure.The reason for these transformation is that,in the concentration range 70-95%,CHT is essentially the major component of the blend and an ordered cross network structure between the linear macromolecular chains of CHT and the peptide structure of SPI can be formed,which significantly increase the ordered secondary structure of SPI.