History dependence of protein adsorption kinetics

The behavior of proteins at biological and synthetic interfaces is often ch aracterized by a strong history dependence caused by long relaxation times or irreversible transitions. In this work, we introduce the rate of adsorpt ion as a means to systematically quantify the extent, and identify the unde rlying causes, of history dependence. We use multistep kinetic experiments in which the Pth step is an exposure of a Si(Ti)O-2 surface to a flowing fi bronectin or cytochrome c solution of concentration c(i) for a time t(i) (c (i) = 0 corresponds to a rinse) and measure the protein adsorption by optic al waveguide light mode spectroscopy. The rate of adsorption is sensitive t o the structure of the adsorbed layer, and we observe it to greatly increas e, for a given adsorbed density, when going from a first to a subsequent ad sorption step. This increase is most pronounced when the duration of the in itial adsorption step is long. We attribute these observations to the gradu al and irreversible formation of protein clusters or locally ordered struct ures and use them to explain previous underestimates of kinetic data by mes oscopic model descriptions. A thorough understanding of these complex posta dsorption events, and their impact on history dependence, is essential for many physiological and biotechnological processes. Optical waveguide lightm ode spectroscopy is a promising technique for their macroscopic quantificat ion.