Effects Of Sulfhydryl Modification And Heat Treatment On Properties Of Soybean 11S Glycinin

Soy protein has superior application value in the food industry because of its high nutritional value and potential functional properties.As a major component of soybean protein,11S has highly reactive surface thiol groups,and it is of great significance to improve the functional properties of 11S by sulfhydryl modification.Therefore,in this dissertation,the effect of green oxidant hydrogen peroxide(H2O2)on the quaternary structure of 11S was investigated,and the effect of substituent groups in N-subtituted maleimide derivatives on thermal aggregation of 11S was also investigated.These studies will provide experimental guidelines for improving functional properties of 11S by sulfhydryl modification.The main results are as follows:In the first part,using analytical ultracentrifugation,the storage stability of the 11S under nonreducing conditions,the effect of H2O2 concnetration on the conversion of 11S to 15S,and the effect of 2-mercaptoethanol(2-ME)on 15S formed during storage and H2O2-induced 15S were investigated.First,when 2-ME was removed from the buffer solution by size exclusion chromatography,the 11S with a high surface sulfhydryl group content was obtained.Then,the 11S was gradually transformed into 15S and 21S during storage in solution;after 82 days,the weight percentage of 15S and 21S was 22%and 4%,respectively.Next,after treating 11S with 1 mmol/L H2O2,the weight percentage of 15S was highest as～20%.At last,the 15S and 21S formed during storage and H2O2-induced 15S could be reduced to 11S by 10 mmol/L 2-ME.In the second part,using dynamic light scattering and sodium dodecyl sulphate-polyacrylamide gel electrophoresis,the effects of the combined treatment of different N-substituted maleimide derivatives and heating on the size distribution and the subunit composition of 11S were investigated.Four N-substituted maleimide derivatives were used,including N-ethyl maleimide(NEM),N-(3-carboxypropyl)maleimide(Mal-COOH),N-[2-(2-hydroxyethoxy)ethyl]-maleimide(Mal-DEG-OH)and methoxy poly(ethylene glycol)-maleimide(1000 Da)[mPEG-Mal(1000 Da)].11S modified with a relatively high concentration(4.3 mmol/L)of Mal-COOH after heat treatment at 95℃ for 15 min had a smaller size,compared to that modified with NEM.The result indicates that increasing the negative charges on the 11S surface suppressed the thermal aggregation of the 11S.Compared to that modified with Mal-DEG-OH,11S modified with mPEG-Mal(1000 Da)after heat treatment had a smaller size but a greater degree of dissociation of AB subunits.The result indicates that the longer the PEG chain in the maleimide derivative,the easier it is to suppress the thermal aggregation of 11S after being attached to 11S,but the lower the degree of reaction with the sulfhydryl group of 11S.Therefore,changing the charges of the substituent group and the PEG chain length in the maleimide derivatives are effective methods for controlling the thermal aggregation of 11S.