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Infrared and laser‐Raman spectroscopy have been used to follow secondary structure changes during the heat‐set gelation of a number of aqueous (D2O) globular protein solutions. Measurements of the infrared Amide I' absorption band around 1650 cm‐1, for BSA gels of varying clarity and texture, have shown that the very considerable variations in network structure underlying these materials are not reflected in obvious differences in secondary structure. In all cases aggregation is accompanied by development of beta‐sheet of a kind common in fibrous protein systems, but for BSA at least this does not appear to vary significantly in amount from one gel type to another. Infrared studies of gels formed from other protein systems have confirmed this tendency for beta‐sheet to develop during aggregation, and the tendency is further substantiated by laser‐Raman evidence which provides the extra information that in most of the examples studied alpha‐helix content simultaneously falls. From these, and other observations, some generalisations are made about the thermally‐induced sol‐to‐gel transformations of globular proteins.

Thin sections of heat‐set protein gels formed from bovine serum albumin, insulin, lysozyme, ribonuclease, and alpha‐chymotrypsin, have been studied by transmission electron microscopy. Micrographs have been interpreted as showing protein networks with strands between one and two times as thick as the native protein diameters. Considerable differences in the persistence characteristics, and frequencies of cross‐linking, of the strands are observed, and there are variations in network homogeneity over long distances which correlate well with changes in gel opacity caused by alterations in pH and ionic strength. Evidence that arte facts are unlikely to have influenced these interpretations has been obtained in the BSA case in particular, by studying the aggregation process in solution, using alternative microscope approaches such as heavy‐metal shadowing and negative staining. Assuming that artefacts are absent, gel section micrographs have been simulated by a computer procedure, and the results suggest that, in most cases, the simplest interpretation of the microscope data is in terms of a “string of beads” model for the aggregation process, involving only moderately unfolded, and still globular, protein molecules. Other structural interpretations cannot be ruled out, however, as the degree of protein unfolding, and the exact mode of incorporation of the monomers into the network filaments, cannot be established by the microscope technique alone.

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