Reversibility of the Adsorption of Lysozyme on Silica

A central paradigm that underpins our understanding of the interaction of proteins with solid surfaces is that protein adsorption leads to changes in secondary structure. The bound proteins tend to denature, and these non-native, adsorbed structures are likely stabilized through the loss of alpha-helices with the concomitant formation of intermolecular beta-sheets. The goal of this work is to critically assess the impact this behavior has on protein desorption, where irreversible conformational changes might lead to protein aggregation or result in other forms of instability. The adsorption, desorption, and structural transitions of lysozyme are examined on fumed silica nanoparticles as a function of the amount of protein adsorbed. Surprisingly, the data indicate not only that adsorption is reversible but also that protein desorption is predictable in a coverage-dependent manner. Additionally, there is evidence of a two-state model which involves exchange between a native-like dissolved state and a highly perturbed adsorbed state. Since the in situ circular dichroism (CD) derived secondary structures of the adsorbed proteins are essentially unaffected by changes in surface coverage, these results are not consistent with previous claims that surface-induced denaturation is coverage dependent. Inspired by results from homopolymer adsorption experiments, we speculate that more local descriptors, such as the number of amino acids per chain that are physically adsorbed on the surface, likely control the desorption process.