Biological membranes are essential and complex structures in every living cell consisting of a fluid lipid bilayer sheet and membrane proteins. Its significance makes biological membranes not only interesting for medical research, but also has made it a target for toxins in the course of evolution. Today, we know more than ever
before about the properties of biological membranes. Advanced biophysical techniques and sophisticated membrane models allow us to answer specific questions about the structure of the components within membranes and their interactions. However, many detailed structural mechanisms of membrane compounds, including compounds associated with membranes, are still unknown due to the challenges that arise when probing the hydrophobic nature of the membrane's interior.
For integral membrane proteins that span through the entire membrane, the amphiphilic environment is essential to retain their native structure. This creates a challenge for studying the true structures of such proteins. Here, we present an approach via the immobilization of the transmembrane leucine transporter protein (LeuT) to a functionalized surface. Moreover, we created a native-like lipid environment post the surface-immobilization of LeuT by exchanging the detergent with natural phosphatidylcholine (PC) lipids. Various surface sensitive techniques, including neutron reflectometry (NR), are employed and finally enabled us to confirm
the gross structure of LeuT in a lipid environment as predicted by molecular dynamic simulations.
In a second study, the co-localization of three toxic plant-derived diterpene resin acids (RAs) within DPPC membranes was investigated. These compounds are reported to disrupt the membrane and increase its
fluidity. The RAs used in this study vary in their toxicity while they are structurally closely related. Therefore,
the RA-induced structural alterations within phospholipid bilayers were determined and correlated with the co-localization of the RAs within dipalmitoylphosphatidylcholine (DPPC) utilizing NR and polarization transfer solid state nuclear magnetic resoncance. In particular, we observed the formation of elongated tubular vesicles
under the impact of the more toxic RAs that were located within the acyl chain region of DPPC. In contrast, the least toxic RA, which was suggested to be located in the head group region, did not cause structural alterations on membranes. The physical chemical properties of these RAs in terms of polarity and flexibility determined
their co-localization within lipid bilayer, and in turn their toxicity. Moreover, further experiments using a giant unilamellar vesicle model system combined with fluorophores and confocal fluorescence microscopy demonstrated that the medically used diterpene forskolin also interacts with phospholipid membranes while the membrane's integrity was not visibly affected by this method.