The Molecular Structure And Adhesion Mechanism Of Urea-Modified Soybean Protein

Soy protein-based adhesives are gaining increasing attention due to their biodegradability and renewability. Urea is a common chemical denaturant of proteins. The molecule of urea has carbonyl amide bond which is similar to the molecule of protein. The action mechanism of urea on protein has been sought by many studies but it is still an unsolved and important problem in protein chemistry, experimental data on specific protein-urea interactions are scarce. Urea is a common component in adhesive. So the molecular structure and adhesion mechanism of soy protein modified with different concentrations of urea were studied.Firstly, wettability and adhesive properties of the major soy protein components conglycinin (7S) and glycinin (11S) after urea modification were characterized. Modified 7S and 11S soy proteins were evaluated for gluing strength with pine, walnut, and cherry plywood and for wettability using a bubble shape analyzer. The molecular structure change was studied by chemical analysis, DSC and Rheometer. The results showed that different adhesives had varying degrees of wettability on the wood specimens. The 7S soy protein modified with urea had better wettability on cherry and walnut. The 11S soy protein modified with 1 mol/L urea had better wettability on pine. The 1 mol/L urea modification gave 11S soy protein the greatest bonding strength in all the wood specimens. The 3 mol/L urea modification gave 7S soy protein stronger adhesion on cherry and walnut than did 11S protein; but with pine, 11S soy protein had greater adhesion strength than 7S soy protein. Measurement of protein secondary structures indicated that the change of secondary structure after urea modification can affect the adhesion strength. DSC result and Rheology profile showed that urea modification unfolded the molecular structure of proteins.Secondly, the effects of different concentrations of urea modification on soy protein isolates (SPI) were investigated by chemical analysis, Fluorescence, SEC-HPLC and particle size distribution analysis. Chemical analysis showed urea can unfold the structure of protein and the unfold degree increased with urea concentration increasing. SEC-HPLC and particle size distribution analysis revealed that with the increasing of the urea concent tion, aggregates were produced. 1 mol/L urea modification gave SPI the biggest polydispersity index. Rheological analysis indicated that all samples were shear thinning systems. Orthogonal tests of adhesion strength showed that temperature and press time were the major factors that affect the adhesion strength. Pressing at 120℃and 2 MPa for 10 min gave urea-modified SPI better adhesion property, and in the urea concentration tested, 1 mol/L urea modification gave SPI the highest bonding strength under this conditions, the adhesion strengths of SPI pressed at 100℃and that at 120℃were not statistically different thereof.Thirdly, the physicochemical properties changes of SPI which was modified with different concentrations of urea and was heated at 100℃for 10 min were studied. Thermodynamic analysis showed free energy of SPI decreased as a result of urea modification and heating. Heating reduced the accessible surface area of urea-modified SPI and caused tangled molecular structure. 1 mol/L and 3 mol/L urea unfolded partially protein molecular structure and unfolded fraction increased after heating at 100℃. FTIR analysis confirmed that heating further changed the secondary structures. Whether heated or unheated, SPI modified by 1 mol/L urea exhibited the highest surface hydrophobicity, which may be beneficial to water resistance of adhesive. This was supported by the lowest delamination rate of SPI modified by 1 mol/L urea. SEC-HPLC and particle size distribution analysis revealed that soy protein modified with different concentrations of urea had varying degrees of aggregation and varying polydispersity. Lower urea concentration resulted to the higher polydispersity of soy protein after heating. Uniform molecular distribution was benefited for better adhesive strength.Fourthly, SPI modified with urea and heated at 100℃for 10min were freeze-drying and the inter-molecular interactions were investigated by kinetics study, the method of breaking special bonds between molecules using different buffers, and different electrophoresises. The kinetics study showed that 1 mol/L urea modified SPI after heating had the highest activating energy. SEC-HPLC of soy protein samples dissolved in 0.1 mol/L, pH7.0 phosphate buffer, 2%SDS solution and 2% SDS + 0.5%β-ME showed that aggregation existing in sample was benefit for the adhesion strength. The results of reducing-PAGE, non-reducing SDS-PAGE and SDS-PAGE analysis further confirmed the study of intermolecular forces, the results showed that: urea modification unfolded the protein molecule, intermolecular bonds mostly were hydrophobic interaction, and disulfide bonds existed between subunits.Finally, various concentrations of glutaraldehyde were used to crosslink and fix the structure of SPI modified with 1 mol/L urea. The results of DSC, TGA, SEC-HPLC, SDS-PAGE and adhesion strength test showed that 80 mmol/L glutaraldehyde had the best cross-linking performance and had the highest wet and dry adhesion strength. The release of glutaraldehyde from crosslinked adhesive was evaluated by GC and the release of glutaraldehyde was 0.0031 %/( g adhesive) at 80 mmol/L glutaraldehyde.