Functional assays for membrane protein on polymer-grafted nanoporous supports

There is a growing need for functionalization strategies aimed to improve performance of platforms for pharmaceutical screening and biophysical assays of membrane proteins, as more than 60% of current drugs target these proteins. However, the compatibility of nanostructured supports with bilayer lipid membranes where membrane proteins are embedded is a major drawback, and limits the long-term immobilization of these proteins in a functional state. In addition, to selectively probe the functionality of membrane ion channels suspended in nanopores, it would be desirable to have screening platforms with controlled ion permeation. Polymer brushes synthesized by surface-initiated atom transfer radical polymerization (SI-ATRP) provide a robust and reproducible platform for surface modification. Here, zwitterionic polymer brushes are used to improve formation and stability of supported bilayer lipid membranes, whereas control of ion permeation is achieved with pH-responsive layers of poly(methacrylic acid) (PMAA) grafted inside the nanopores. Results show that responsive PMAA brushes grafted via SI-ATRP exhibit rapid and reversible switching between conformations, which makes them ideal candidates for miniaturized devices with fast response times. Furthermore, the effective pKa of the polymer inside the brushes is shifted to larger values, due to ion confinement effects. This is especially relevant for membrane protein assays as switching can be triggered in a pH range closer to physiological conditions. The assembly and stability of bilayer lipid membranes on the functionalized supports are probed by various surface sensitive techniques. The precise control of surface properties using defined polymer brush architectures can be successfully related to conditions of lipid membrane formation from fusion of phospholipid vesicles. This research is funded by the European Commission through the FP7 project ASMENA.