Molecular differences in the formation and structure of fine-stranded and particulate beta-lactoglobulin gels

In order to reveal at a molecular level differences betweeen fine-stranded and particulate gels, we present an Fourier transform infrared spectroscopi c study of the thermal behavior of beta -lactoglobulin (beta -lg) in salt-f ree D2O solutions and low ionic strength at different pDs. Differences are found in the denaturation mechanism, in the unfolded state of the protein, in the aggregate formation, and in the strength of the intermolecular inter actions. For fine-stranded gels (pD 2.8 and 7.8), heating induces the disso ciation of the dimers into monomers. The protein undergoes extensile struct ural modifications before aggregation begins. Aggregation is characterized by the appearance of a new band attributed to intermolecular beta -sheets w hich is located in the 1613-1619 cm(-1) range. For particulate gels (pD 4.4 and 5.4), the protein structure is almost preserved up to 75-80 degreesC w ith no splitting of the dimers. The band characteristic of aggregation orig inates from rite component initially located at 1623 cm(-1), suggesting tha t at the beginning of aggregation, globular beta -lg in the dimeric form as sociate to constitute oligomers with higher molecular mass. Aggregation may result in the association of globular slightly denatured dimers, lending t o the formation of spherical particles rather than linear strands. The aggr egation band is always located in the 1620-1623 cm(-1) range for particulat e gels showing that hydrogen bonds are weaker for these aggregates than for fine-stranded ones. This has been related to a more extensive protein unfo lding for fine-stranded gels that allows a closer alignment of the polypept ide chains, and then to the formation of much stronger hydrogen bonds. Smal l differences are also found in protein organization and in intermolecular hydrogen bond strength vs pD within the same type of gel. Protein conformat ion and protein-protein interactions in the gel state may be responsible of the specific macroscopic properties of each gel network. A coarse represen tation of the different modes of gelation is described. (C) 2000 John Wiley & Sons, Inc.