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How do proteins unfold upon adsorption on nanoparticle surfaces?

Owing to their many outstanding features, such as small size, large surface area, and cell penetration ability, nanoparticles have been increasingly used in medicine and biomaterials as drug carriers and diagnostic or therapeutic agents. However, our understanding of the interactions of biological entities, especially proteins, with nanoparticles is far behind the explosive development of nanotechnology. In typical protein-nanoparticle interactions, two processes (i.e., surface binding and conformational changes in proteins) are intermingled with each other and have not yet been quantitatively described. Here, by using a stopped-flow fast mixing technique, we were able to shed light on the kinetics of the adsorption-induced protein unfolding on nanoparticle surfaces in detail. We observed a biphasic denaturation behavior of protein GB1 on latex nanoparticle surfaces. Such kinetics can be adequately described by a fast equilibrium adsorption followed by a slow reversible unfolding of GB1. On the basis of this model, we quantitatively measured all rate constants that are involved in this process, from which the free-energy profile is constructed. This allows us to evaluate the effects of environmental factors, such as pH and ionic strength, on both the adsorption and the conformational change in GB1 on the latex nanoparticle surface. These studies provide a general physical picture of the adsorption-induced unfolding of proteins on nanoparticle surfaces and a quantitative description of the energetics of each transition. We anticipate that it will greatly advance our current understanding of protein-nanoparticle interactions and will be helpful for the rational control of such interactions in biomedical applications.