Dynamics of GlobularProteins when Interacting withZwitterionic Silica Nanoparticles by Nuclear Magnetic Resonance SpinRelaxation

The many emerging applications of nanoparticles in diverse fields in chemistry and biology require the characterization of interactions between nanoparticles and surrounding biomolecules such as proteins. Nuclear magnetic resonance spin relaxation of proteins, which is highly sensitive to interactions with nanoparticles, contains rich information about protein mobility and binding kinetics. The interactions of globular proteins with silica nanoparticles differ markedly from those with liposome nanoparticles, although both are driven by electrostatic forces. For unmodified silica nanoparticles, their interactions with an internally rigid protein like ubiquitin uniformly increase the backbone amide ¹⁵N transverse <i>R</i>₂ relaxation for most residues. In contrast, for ubiquitin-POPG liposome interactions, their characteristic transverse <i>R</i>₂ profiles indicate that ubiquitin undergoes diffusive rotational motions on the liposome surface. Here, we show that coating silica nanoparticles with sulfobetaine siloxane zwitterionic molecules profoundly alters their interactions with proteins in a manner that closely resembles the mode of interaction observed with liposomes. ¹⁵N-<i>R</i>₂ relaxation reveals that ubiquitin and the Ras-binding domain of B-Raf both exhibit axial reorientational motions about an axis perpendicular to the nanoparticle surface in the bound state, where the interactions involve predominantly positively charged surface regions. These findings point toward a global dynamics mechanism of proteins when interacting with organic or inorganic nanoparticles with densely charged soft surfaces. This information may help tailor the coatings of nanoparticles to adopt specific modes of interaction with proteins that can be used to control their function in vivo and in vitro.