Protein adsorption orientation in the light of fluorescent probes: mapping of the interaction between site-directly labeled human carbonic anhydrase II and silica nanoparticles

Little is known about the direction and specificity of protein adsorption to solid surfaces, a knowledge that is of great importance in many biotechnological applications. To resolve the direction in which a protein with known structure and surface potentials binds to negatively charged silica nanoparticles, fluorescent probes were attached to different areas on the surface of the protein human carbonic anhydrase II. By this approach it was clearly demonstrated that the adsorption of the native protein is specific to limited regions at the surface of the N-terminal domain of the protein. Furthermore, the adsorption direction is strongly pH-dependent. At pH 6.3, a histidine-rich area around position 10 is the dominating adsorption region. At higher pH values, when the histidines in this area are deprotonated, the protein is also adsorbed by a region close to position 37, which contains several lysines and arginines. Clearly the adsorption is directed by positively charged areas on the protein surface toward the negatively charged silica surface at conditions when specific binding occurs.