Structures and Stabilities of Adsorbed Proteins

Structural perturbations and thermodynamic-stability changes in two similar-sized globular proteins, hen egg-white lysozyme and bovine milk α-lactalbumin, upon physical adsorption to either microspheres of a negatively charged polystyrene (PS -) latex or a dispersion of variably charged hematite (α-Fe 2O 3) are determined from differential scanning microcalorimetry (micro-DSC), isothermal titration microcalorimetry, and more conventional electrophoretic-mobility and adsorption-isotherm data. Heat of adsorption data at pH 7 for α-lactalbumin on PS - indicate that adsorption is driven by entropic forces. Differential heat capacity data indicate that sorbent and protein-surface dehydration provide a substantial entropic driving force for adsorption. Both proteins are largely denatured on the hydrophobic PS - surface. In contrast, lysozyme loses only a fraction of its ordered secondary structure when adsorbed to α-Fe 2O 3 while α-lactalbumin denatures almost completely upon adsorption to this hydrophilic surface. This latter difference in adsorbed-state structures is consistent with the significantly larger native-state structural stability of lysozyme as measured by micro-DSC. Kinetic analysis of adsorbed-protein micro-DSC data suggests that adsorbed lysozyme maintains a relatively high internal cohesion.