| Surimi is one of the most popular seafood in our life. But the traditional source for surimi process rapidly decays. Jumbo squid becomes the promising candidate to fill this gap due to its abundance with low price. However, poor gel-formation induced by high moisture content and proteolytic enzymes, hamper the application of jumbo squid in surimi industry. Therefore, how to improve the performance of myofibrillar protein gel is a key issue in surimi industry. Former studies have shown that the protein conformation plays a key role in influencing the product effectiveness. Thus, our study is via the investigation of solubility under various controlled conditions to probe the molecular conformation change, because the solubility could directly reflect the intermolecular interactions among protein molecules. Then, those results would compare with jumbo squid gelation process in order to probe the effect of molecular conformation change on protein gelation process.In this paper, squid myofibril protein was used as a system to study the effect of pH and salt on the protein composition and molecular conformation in solutions. The aim of this study is to construct a further relationship between the protein conformation and the gel performance and to provide a theoretical basis of its application.The influence of dilution on protein solutions was studied by ultraviolet spectrum, fluorescence spectrum, differential scanning calorimeter(DSC), light scattering and other technical means. The results showed that there was a supramolecular structure in the squid myofibril protein solution. Via the exclusion chromatography with laser light scattering detection, one elution peak of the molecular weight proved the existence of super-molecular. With the same techniques, this type of structure consistently existed in various salt solutions, such as NaCl, NH4C1 and CH3COONa.This observation illustrated that the supermolecular structure in solution was not formed under certain conditions but universally existed.Further more studies with UV spectrophotometer and SDS electrophoresis showed that no matter whether in monovalent salt solutions, such as KCl, NaCl, CH3COONa, NH4Cl or bivalent ones, such as CaCl2, MgCl2, MgSO4 protein solubility always presented the same trend that with salt concentrated, solubility increased at first and followed by the reduction with the maximum value at 0.5 M. When pH close to the isoelectric point, the protein solubility was the lowest; while the pH deviating caused the solubility enhancement. The influence of different kinds of salt on the protein followed hofmeister sequences, NaCl> NH4Cl> CH3COONa; CaCl2> MgCl2> MgSO4. SDS electrophoresis displaced that protein composition changed with various salt species, especially for myosin heavy chain and actin content.The effects of different salts and pH on protein conformation were studied by lights cattering, circular dichroism(CD) ultraviolet spectrometer and other techniques. Through the static-dynamic laser light scattering technology, the decay time, the second virial coefficient (A2), the mean square radius of turn (Rg) and the hydraulic radius (Rh) of the protein clusters in solutions have been assessed. The data showed that different salts and pH values produced distinguished effects on the interaction between the protein molecules. With the salt concentration increased, the absolute value of A2 (A2<0)first decrease and then increased, indicating that attraction forces decreased at first and then increased with salt concentrated coincided with the solubility assessment. And the corresponding ratio of Rg and Rh increased and then decreased, which suggested that with the increase of salt concentration, protein molecules clusters became compacter. Furthermore, the secondary structure distribution of protein molecules depended on the salt species when concentration was 0.5 M and pH=6.5. For example, in CH3COONa solutions, the content of a-helix was the least, while in NaCl and MgCl2 solutions the content of P-sheet was relatively high. The UV spectrums also depended on the salt species. Compared with other salt solutions, the wavelengths of the maximum absorption have been red shift in KCl, CH3COONa and MgCl2 solutions. And the fractal dimension of protein clusters varied with salt species as well, this number has been significantly different in different salt solutions. This observation further supported our conclusions that salt species was able to modify the structure of protein clusters in solutions. |
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