Bioresponsive interfaces composed of calmodulin and poly(ethylene glycol): Toggling the interfacial film thickness by protein-ligand binding.

Responsive interfaces are often realized by polymer films that change their structure and properties upon changing the pH-value, ionic strength or temperature. Here, we present a bioresponsive interfacial structure that is based on a protein, calmodulin (CaM), which undergoes a huge conformational change upon ligand binding. At first, we characterize the conformational functionality of a double Cys mutant of CaM by small-angle X-ray scattering (SAXS) and Fourier transform infrared (FTIR) spectroscopy. The CaM mutant is then used to cross-link poly(ethylene glycol) (PEG) chains, which are bound covalently to a supporting planar Si surface. These films are characterized by X-ray reflectometry (XR) in a humidity chamber providing full hydration. It is well known that Ca2+ -saturated holo-CaM binds trifluoperazine (TFP) and changes its conformation from an open, dumbbell-shaped to a closed, globular one in solution. At the interface, we observe an increase of the PEG-CaM film thickness, when TFP is binding and inducing the closed conformation, whereas the removal of Ca2+ -ions and a concomitant release of TFP is associated with a decrease of the film thickness. This toggling of the film thickness is largely reversible. In this way, a structural change of the interface is achieved via protein functionality which has the advantage of being selective for ligand molecules without changing the environmental conditions in a harsh way via physico-chemical parameters.