Formation of thermally induced aggregates of the soya globulin β-conglycinin

The effect of ionic strength (T) on the formation of thermally induced aggregates by the 7S globular storage protein of soya, beta -conglycinin, has been studied using atomic force microscopy. Aggregates were only apparent when I greater than or equal to 0.1, and had a fibrous appearance, with a height (diameter) of 8-11 nm. At high ionic strength (I = 1.0) the aggregates appeared to associate into clumps. When aggregate formation was studied at I = 0.2, it was clear that aggregation only began at temperatures above the main thermal transition for the protein at 75 degreesC, as determined by differential scanning calorimetry. This coincided with a small change in secondary structure, as indicated by circular dichroism spectroscopy, suggesting that a degree of unfolding was necessary for aggregation to proceed. Despite prolonged heating the size of the aggregates did not increase indefinitely, suggesting that certain beta -conglycinin isoforms were able to act as chain terminators. At higher protein concentrations (1% W/V) the linear aggregates appeared to form large macroaggregates, which may be the precursors of protein gel formation. The ability of beta -conglycinin to form such distinctive aggregates is discussed in relation to the presence of acidic inserts in certain of the beta -conglycinin subunits, which may play an important role in limiting aggregate length. (C) 2001 Elsevier Science B.V. All rights reserved.