Thermal aggregation and ion-induced cold-gelation of bovine serum albumin.

Protein cold-gelation has recently received particular attention for its relevance in bio and food technology. In this work, we report a study on bovine serum albumin cold-gelation induced by copper or zinc ions. Metal-induced cold-gelation of proteins requires two steps: during the first one, the heat treatment causes protein partial unfolding and aggregation; then, after cooling the solution to room temperature, gels are formed upon the addition of metal ions. The thermally induced behaviour has been mainly investigated through different techniques: Fourier transform infrared (FTIR) spectroscopy, circular dichroism, dynamic light scattering (DLS) and rheology. Data have shown that the aggregation process is mainly due to protein conformational changes--alpha-helices into beta-aggregates-forming small aggregated structures with a mean diameter of about 20 nm a few minutes after heating. After metal ion addition, the viscoelastic properties of the gels have been investigated by rheological measurements. The behaviour of the elastic and viscous moduli as a function of time is discussed in terms of ion concentration and type. Our results show that: (1) the elastic behaviour depends on ion concentration and (2) at a given ion concentration, gels obtained in the presence of zinc exhibit an elastic value larger than that observed in the Cu(2+) case. Data suggest that cold-gelation is the result of different mechanisms: the ion-mediated protein-protein interaction and the bridging effect due to the presence of divalent ions in solution.