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 trans itions undergone by proteins adsorbed at interfaces. Fourier transform infr ared spectrometry in solution and in the attenuated total reflection geomet ry revealed, for the globular proteins hen egg lysozyme and bovine serum al bumen, both qualitative and quantitative differences between the transition s as they occur in bulk and adsorbed at an interface. In the bulk, unfoldin g is a sharp transition, followed sequentially on further heating by the re latively sharp onset of the intermolecular association associated with heat set gelation. In contrast, for adsorbed proteins, we found that both proce sses occur simultaneously over a wide range of temperatures. Proteins were more structurally stable adsorbed at a relatively hydrophilic, solid surfac e than at a liquid, hydrophobic surface; in the latter case, onset temperat ures for both unfolding and intermolecular association were substantially l ower than for bulk solutions.