【作者】 张忠慧；

【导师】 华欲飞；

【作者基本信息】 江南大学 ， 粮食、油脂及植物蛋白工程， 2008， 博士

【摘要】 大豆蛋白胶粘剂由于具有良好的可再生性、可降解性、来源丰富等优点而受到人们的关注从而再次成为研究的热点。尿素是常用的蛋白质变性剂,在胶粘剂研究中起着重要的作用。本课题主要是通过研究尿素变性大豆蛋白的分子结构的变化来揭示其产生胶粘的机理。论文首先系统研究了大豆蛋白中的两个主要球蛋白:7S和11S分别经过尿素变性后分子结构的变化,在软、中、硬三种不同硬度的木块上的湿润能力的变化及胶粘强度的变化。特性粘度及流变实验结果表明,不同浓度尿素变性导致了7S、11S分子结构的不同变化,差示扫描量热结果证明尿素变性部分展开了7S、11S的分子结构。液滴形状分析结果表明不同的胶粘剂在不同的木块上有不同的湿润性能,7S蛋白经过尿素变性后在胡桃木和樱桃木上有较好的湿润性能,11S蛋白经过1 mol/L尿素变性后在松木上有好的湿润性能。1 mol/L尿素变性使得11S蛋白在三种木块上的胶粘强度在所有胶粘剂中为最大;3 mol/L尿素变性使得7S蛋白在胡桃木和樱桃木上的胶粘强度比11S的大,但在松木上的胶粘强度则是11S/3M高于7S/3M。结合傅立叶红外光谱分析胶粘剂的二级结构结果表明,胶粘强度与尿素变性前后蛋白质的二级结构的变化有关。通过测定不同浓度尿素变性后的大豆蛋白的内源性荧光光谱,二级结构,特性粘度,巯基含量,表面疏水性,相对分子质量分布和粒径分布等表征了尿素变性蛋白质的结构变化,并通过正交试验确定了尿素变性大豆蛋白胶粘榉木的实验条件。内源性荧光光谱实验结果表明,尿素展开了大豆蛋白分子,尿素浓度不同展开的程度不同,展开的蛋白质会进一步聚集,二级结构,特性粘度及巯基含量测定结果进一步证实了这一点。1 mol/L尿素变性后的蛋白质的表面疏水性最大,相对分子质量分布和粒径分布实验结果表明,尿素变性后蛋白质中都有聚集体出现,1 mol/L尿素变性蛋白质分子中组分分布最不均匀。流变测定结果表明,所有样品都是剪切变稀体系,当8 mol/L尿素变性蛋白质时则几乎为牛顿流体。通过正交试验表明,对于胶粘榉木而言,温度和时间是影响胶粘强度的两个主要因素,热压压强是次要因素。1 mol/L和3 mol/L尿素变性提高了大豆蛋白在榉木上的胶粘强度,其中1 mol/L尿素变性蛋白具有最大的胶粘强度,此时,当热压温度为100℃和120℃时,胶粘强度没有显著性差异。将尿素变性大豆蛋白经过100℃加热处理10 min来模拟实际热压时的反应条件,通过测定样品的分子结构的变化来研究高温加热蛋白质对其结构的进一步影响。热力学实验结果表明,尿素变性和加热处理都降低了蛋白质的变性自由能,加热减少了蛋白质分子可接触表面面积,变性展开了蛋白质分子,同时分子之间互相缠绕成了网状结构,1 mol/L和3mol/L尿素部分展开了蛋白质结构,加热后蛋白质展开的比例增加。傅立叶红外光谱分析二级结构结果表明,与加热前的样品相比较,加热进一步改变了蛋白质的二级结构。当尿素浓度达到8 mol/L时,蛋白质已经完全变性了。样品经过加热处理后,随着尿素浓度的增加,蛋白质的表面疏水性降低。在尿素变性后的样品中,1 mol/L尿素变性导致蛋白质具有最大的表面疏水性,抗水性实验证明了这一点。相对分子质量分布及粒径分布实验结果表明,尿素变性蛋白质在加热后会不同程度地形成聚集体,同时,由于尿素展开蛋白质结构的程度不同从而导致各个样品中组分的分散性不同,低浓度尿素变性蛋白加热后多分散性较高,均一的分布有利于胶粘。通过测定不同浓度尿素变性大豆蛋白加热固化以后蛋白质样品的溶出活化能,使用不同溶剂溶解及使用不同电泳方法研究了尿素变性大豆蛋白在100℃加热处理10 min固化后的分子间相互作用。活化能实验结果表明,固化后的SPI/1M/100℃的活化能最高,使用不同溶剂溶解样品后测定溶出物的相对分子质量分布,分析结果表明,尿素变性大豆蛋白经过加热固化后通过二硫键和范德华引力形成聚集体,聚集体之间通过疏水相互作用和非共价键结合会有利于胶粘强度。使用还原原态电泳、非还原SDS-PAGE电泳和SDS-PAGE电泳来进一步对蛋白质样品的分子间作用力进行研究,结果表明:尿素变性导致蛋白质分子展开,分子形状变大,蛋白质分子主要以疏水相互作用结合,亚基之间存在二硫键。使用戊二醛对1 mol/L尿素变性后的大豆蛋白进行了结构的交联与固定,通过使用差示扫描量热仪、热重分析仪、凝胶色谱、SDS-PAGE凝胶电泳测定不同浓度戊二醛交联后的样品的性质,结果表明,80 mmol/L戊二醛交联效果最好,胶粘后的榉木的干、湿胶粘强度都证实了这一结果,气相色谱分析结果表明,此时胶粘剂中残留的戊二醛浓度为0.0031%/(g胶粘剂)。更多还原

【Abstract】 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.更多还原