STRUCTURES AND STABILITIES OF ADSORBED PROTEINS

Structural perturbations and thermodynamic-stability changes in two si milar-sized globular proteins, hen egg-white lysozyme and bovine milk alpha-lactalbumin, upon physical adsorption to either microspheres of a negatively charged polystyrene (PS-) latex or a dispersion of variab ly charged hematite (alpha-Fe2O3) are determined from differential sca nning microcalorimetry (micro-DSC), isothermal titration microcalorime try, and more conventional electrophoretic-mobility and adsorption-iso therm data. Heat of adsorption data at pH 7 for alpha-lactalbumin on P S- indicate that adsorption is driven by entropic forces. Differential heat capacity data indicate that sorbent and protein-surface dehydrat ion provide a substantial entropic driving force for adsorption. Both proteins are largely denatured on the hydrophobic PS- surface. In cont rast, lysozyme loses only a fraction of its ordered secondary structur e when adsorbed to alpha-Fe2O3 while alpha-lactalbumin denatures almos t completely upon adsorption to this hydrophilic surface. This latter difference in adsorbed-state structures is consistent with the signifi cantly larger native-state structural stability of lysozyme as measure d by micro-DSC. Kinetic analysis of adsorbed-protein micro-DSC data su ggests that adsorbed lysozyme maintains a relatively high internal coh esion. (C) 1995 Academic Press, Inc.