Humidity-Responsive Polymer Cushion-Supported Biomimetic Membrane: A Model System for X‑ray Studies

An effort aimed at replacing the conventional water column by a relative humidity (RH) environment for structural investigation of a soft polymer cushion-supported model phospholipid membrane has been reported. An RH-responsive well-hydrated polymer cushion layer capable of approximately 2-fold swellability under RH 96% has been employed for phospholipid model membrane fabrication. To validate the proposed method, supported lipid bilayers (SLBs) of phosphocholine and phosphoethanolamine were deposited and structurally characterized at molecular level by the X-ray scattering method. In addition, the molecular interaction of the porphyrin-based hemin molecule, having a drug-like structure, with the supported membrane has been studied for further validation. The swelling behavior of the polymer cushion has been studied at a range of RH values prior to the bilayer deposition. The RH environment, in comparison to the conventional water column, enhanced the dynamic range approximately by 100-fold and the structural resolution by 2-fold. Thus, the bilayer structural features can be assessed without being overwhelmed by the background signals from the traditional water column. This facilitates in extracting reliable layer parameters and exogenous molecule-induced minute changes from the model fit. The proposed method will have far-reaching implications in biosensor engineering, protein–lipid, and drug–lipid interaction studies, X-ray microscopy, imaging, and photon correlation spectroscopy studies from SLBs where acquiring sufficient scattered intensity is still a challenge. This study also predicts that lab-based rotating-anode X-ray instruments can potentially be an alternative to the hard-access synchrotron experiments on biomimetic membranes, keeping the dynamic range and structural resolution uncompromised.