Molecular mechanisms of Fe2+-induced beta-lactoglobulin cold gelation

To get more insight into the mechanisms of cold gelation of beta-lactoglobulin (beta-lg), macroscopic and molecular structural changes during Fe(2+)-induced gelation of beta-lg were investigated using Fourier transform-infrared (FTIR) spectroscopy and rheological methods. The FTIR spectroscopy results show that, upon the preheating treatment (first step of gel process), native globular proteins are denatured and aggregated molecules are found in solution. The spectra are similar to those of gels obtained in the second step of the process upon incorporation of Fe, which suggests that aggregated molecules formed during the preheating treatment constitute the structural basis of the aggregation. However, the rheological data show that the aggregation is achieved via two molecular mechanisms, both of which are modulated by the iron concentration. At 30 mM of iron, gel formation is essentially controlled by van der Waals interactions, while at 10 mM of iron, hydrophobic interactions predominate. At the two concentrations, disulfide bonds contribute to gel consolidation, the effect being more pronounced at 10 mM of iron. These mechanisms lead to the formation of gels of different microstructures. At the highest iron concentration, a strong and rapid decrease in the repulsion forces is produced, resulting in random aggregation. At the lowest iron concentration, the iron diminishes the superficial charge of both molecules and aggregated molecules, facilitating the interaction among hydrophobic regions and leading to the growth of the aggregation in the preferential direction and to filamentous gel formation. This study provides a comprehensive view of the different modes of gelation.