Unfolding and Intermolecular Association in Globular Proteins Adsorbed at Interfaces

The conformational transitions that occur on heating solutions of globular proteins, unfolding and aggregation, were compared with the analogous transitions undergone by proteins adsorbed at interfaces. Fourier transform infrared spectrometry in solution and in the attenuated total reflection geometry revealed, for the globular proteins hen egg lysozyme and bovine serum albumen, both qualitative and quantitative differences between the transitions as they occur in bulk and adsorbed at an interface. In the bulk, unfolding is a sharp transition, followed sequentially on further heating by the relatively sharp onset of the intermolecular association associated with heat set gelation. In contrast, for adsorbed proteins, we found that both processes occur simultaneously over a wide range of temperatures. Proteins were more structurally stable adsorbed at a relatively hydrophilic, solid surface than at a liquid, hydrophobic surface; in the latter case, onset temperatures for both unfolding and intermolecular association were substantially lower than for bulk solutions.