Microcalorimetry of the adsorption of lysozyme onto uncharged substrates.

Abstract

Microcalorimetric data for the adsorption of lysozyme by three uncharged polymeric substrates of varying interfacial energies have been obtained in the monolayer and submonolayer regions. The work was undertaken in an effort to evaluate the suitability of implant/blood interfacial energy as a predictor of biocompatibility. The conformation of lysozyme upon adsorption by each substrate was of particular interest, because the extent of conformational change upon adsorption may be related to clotting rates. Polymer/water interfacial energies were estimated by assuming a harmonic mean relationship of polar and dispersion forces at the interface. Polymer surface energies at room temperature were extrapolated from measurements using the pendant drop method. The work of adhesion at the interface was estimated by using two methods, first, a harmonic mean relationship and contact angles of two liquids on each polymer, and second, the contact angle of methylene iodide on each surface, a geometric mean relationship, and pendant drop results. The time dependent nature of sample polymer surface characteristics as a function of contact time with water was verified by a Wilhelmy plate study. Integral heats of adsorption of lysozyme for each polymer substrate were measured using a Tian-Calvet microcalorimeter. The amount of lysozyme adsorbed at each point was calculated from the change in solution concentration. Step patterns were found in each adsorption isotherm. Endothermic heats appeared at rises between plateaus in the cases of polystyrene and poly(co-styrene-butyl methacrylate) substrates, but were largely absent in the case of poly(co-styrene-allyl alcohol). These results suggest the presence of multiple conformational states of lysozyme on each surface. Endothermic heats are probably latent heats associated with transition between conformational states. The relative magnitudes of endothermic heats for the three substrates suggests that conformation upon adsorption is substrate dependent. Infrared results of amide I and II bands for lysozyme on each substrate suggest the possibility of greater conformational change upon adsorption by polystyrene and styrene/butyl methacrylate than by styrene/allyl alcohol. Conclusions regarding the effect of interfacial energy on conformation were not possible due to the difficulty of accurately ranking materials with similar interfacial properties.