| Soybean has a long history in the food industry owing to its high nutritional value and processability. However, soybean also contains a large fraction of proteins (up to 40%) that has attracted research interests for the development of environment-friendly protein materials with potential excellent physical properties. Soy protein isolate (SPI), more than 90% protein content, contains two major components:glycinin (11S, approximately 52% of the total protein content) and (3-conglycinin (7S, approximately 35% of the total protein content). These two components are responsible for the nutritional, physicochemical and physiological properties of soy proteins. In order to take full advantage of the green feature of soy protein, a fundamental understanding of the relationship between structure and properties of soy protein will be very valuable for the preparation and application of natural protein materials in the future, as well as the more direct application to soy-based foodstuffs and environmentally friendly structural polymers.Soy protein has complex composition and structure and there are relatively strong interactions in and between the subunits, which prevents soy protein from dissolving in water. Therefore, the preparation of uniform and steady aqueous solution of soy protein is important for further investigation. Herein, two biochemical agents, guandine hydrocholoride (GuIICl) and dithiothreitol (DTT), were used to destroy the hydrogen bonds and disulfide bonds in soy protein, which were removed by dialysis against water to obtain soy protein solution with initial concentration of 3.0 wt% and without obvious degradation. More concentrated soy protein solutions were obtained by reverse dialysis against PEG solution. It is essential to know the properties of soy protein solution for fully understanding and improving the structure and properties of soy protein. So rheological measurements were applied to study the properties of soy protein solution with different concentrations. When the concentration was equal to or lesser than 7.2 wt%, the modulus were independent on the concentrations of soy protein solution and the storage modulus were higher than the loss modulus in low frequency, observations revealed the existence of some microgel structure. However, there was a significant change when the concentration was equal to or higher than 9.0 wt%, the dynamic modulus and the frequency conformed to the following relations:G′(ω)～G″(ω)～ωn, fitting in with Winter's self-similar relaxation line model, that is, when the concentration is equar to the critical concentration 11.3 wt% it can form an metastable critical gel; otherwise, when the concentration is lower than 11.3 wt% it changed to solution while weak gel could form when the concentration is higher than 11.3 wt%.The functions of soy protein are closely related to its conformation and aggregation structure, so the thermally-induced conformational transitions of soy protein films were studied detailedly and corresponding mechanism of conformational changes was proposed. In the heating process, the heat energy made the soy protein molecular chain segment move with partial adjustment or rearrangement, some volatile random coil structure destructed and formed a relatively stable P-turn conformation. We also suggested that the peak at 1500 cm-1 was assigned to the characteristic of p-turn with the homologous at the peak of 1700 cm-1 according to FTIR and 2D-IR. The rate of change in amide I was faster than at the peak of 1550 cm-1 because of the combination mode of amide II and Tyrosine. The mechanism of conformational transitions of soybean protein films under isothermal conditions was the same as in the heating process. Moreover, the observations in heated-cooled-heated experiment revealed that about 30%～40 %conformation could be reversible, however, the peak at 1700 cm-1 had little change which showed that the heating-inducedβ-turn conformation was very stable.Generally speaking, properties such as water pick-up and mechanical properties of protein materials are often seen to change significantly over a period of minutes or hours after preparation, even under moderate laboratory conditions around ambient temperature. So we tried to study the effect of water on the thermally-induced conformational transitions of soy protein films. By comparing the kinetics of protein-water interactions as a function of temperature using time resolved FTIR, TGA, and DTMA measurements on soybean protein films, we found that simply evaporating water from the film (TGA) is insufficient to explain the rate of conformational changes (FTIR and DMTA). The onset of mobility in the water molecules allows the amide groups to reconfigure and form stronger amide-amide hydrogen bonds. Thus, simple loss of water (evaporation) is insufficient by itself to allow the protein chains to reconfigure, which requires an activation step for the water to become mobile. We suggest that an elastic instability condition of denaturation or glass transition events in water-amide interactions is the governing mechanism for conformational changes that allows the evolution of disordered structures into more ordered secondary structures, thereby controlling the changes in physical properties such as stiffness and water sensitivity.Owing to its sustainability, abundance, low cost and functionality, soy protein has attracted great research interests for the development of environment-friendly protein materials with potentially good properties, such as regeneration, biocompatibility and biodegradability, etc. To date, numerous soy protein-based materials have been studied which can be divided into plastics, gels, films, and additives or coatings. However, there have not been any studies on electroactive protein hydrogels made of pristine soy protein. A natural electroactive protein hydrogel was prepared from soy protein isolate (SPI) solution by crosslinking with epichlorohydrin. Under electrical stimulus, such SPI hydrogel quickly bends towards one electrode, showing a good electroactivity. Because of its amphoteric nature, the SPI hydrogel bends either toward the anode (pH< 6) or cathode (pH> 6), depending on the pH of the electrolyte solution. Other factors, such as electric field strength and ionic strength also influence the electromechanical behavior of the SPI hydrogels. Moreover, this SPI hydrogel exhibits a good electroactive behavior under strong acidic (pH=2-4) or basic (pH=10-12) solutions, which is a significant improvement over two other kinds of natural electroactive hydrogels, i.e., chitosan/carboxymethylcellulose and chitosan/carboxymethylchitosan hydrogel, which we reported previously. The wide pH range and good electroactivity of this natural protein hydrogel suggests its great potential for microsensor and actuator applications, especially in the biomedical field, and also to increase the scope of natural polymer-based electroactive hydrogels.In conclusion, we first obtained high concentrated soy protein solution without obvious degradation and also studied the properties of this solution by rheological measurements. Then, we studied the thermally-induced conformational transitions of soy protein films detailedly by FTIR and thermodynamic analysis and suggested the mechanism of conformational changes. At last, we successfully prepared a natural electroactive protein hydrogel from soy protein solution by crosslinking with epichlorohydrin. Thus, in this thesis, we systematically studied the soy protein in both the basic research and its practical application, so as to learn more about the soy protein from the perspective of bipolymer and finally expand its applications.
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