Improvement of heat-induced fibril assembly of soy β-conglycinin (7S Globulins) at pH 2.0 through electrostatic screening

Protein fibril assembly at acidic conditions has recently attracted increasing interests in the fields of biochemical and food science, however, the obtained knowledge about the assembly of the proteins with complex structure (e.g. plant 7S/11S globulins) is still limited. This study investigated the influence of NaCl addition (0-300 mM) on the fibril assembly of soy 7S globulins ((beta-conglycinin), induced by heating at 80 C and pH 2.0, with the aim of confirming the improvement of their fibril assembly by electrostatic screening. The formed fibril formation was investigated by using Th T fluorescence and atomic force microscopy (AFM) techniques. Heat-induced structure changes were traced by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and circular dichroism spectroscopy (CD). The results indicated that the whole fibril assembly experienced protein dissociation/hydrolysis and subsequent fibrillization process, and increasing NaCl concentration progressively increased the amount of formed fibrils, and even fibril length. AFM analyses confirmed that the increase in NaCl concentration shortened the time at which the short fibrils were visually observed, and the morphology of formed fibrils, including height of fibrils, width at half-height, and coil periodicity, slightly varied with the added NaCl concentration. The CD analyses indicated that increasing NaCl concentration greatly favored formation of beta-type secondary structure, as well as extensive disruption of tertiary and/or quaternary conformations. These results confirmed that the electrostatic screening greatly improved the heat-induced fibril assembly process of soy p-conglycinin at pH 2.0. The improvement of fibril formation was largely attributed to increased extent of conformational changes at higher ionic strength. These findings would provide important information about the fibril assembly of plant oligomeric globulins at acidic pH that have important implications for the development of protein protein-based fibrillar gels. (C) 2011 Elsevier Ltd. All rights reserved.