NMR and biomolecules on interfaces
NMR investigations of the interactions of biomolecules with interfaces
The transport of small molecules like drugs through cell membranes requires the crossing of several hydrophilic and lipophilic barriers. For this reason a characterization of the partitioning of small molecules into membrane bilayers is of paramount importance in e.g. drug development. Liquid-state NMR spectroscopy is an effective tool for providing high-resolution information on the structure and dynamics of biological compounds under near native conditions. In order to model the native membrane condition, membranes can be reconstituted into synthetic bilayers composed of phospholipids. Liposomes of aqueous dispersions of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) e.g. can be utilized to model the bilayer: If the size of the liposome is small enough and a rapid molecular reorientation occurs, narrow and isotropic NMR resonances are obtained. Both homonuclear and heteronuclear Overhauser enhancement spectroscopy (NOESY and HOESY, resp.) can then be used to establish the location and the dynamics of small molecules in the phospholipid bilayer. In combination with measurements of the spin-spin relaxation rates as well as the diffusion behavior (Figure 1) of membrane molecules, a complete description of the dynamics of the membrane components is accessible. The study of the location and dynamics of water in the lipid requires 1H magic-angle spinning (MAS) NMR spectra: Two dimensional NMR exchange spectra under MAS then serve to differentiate between slowly exchanging interlamellar and bulk water, thus revealing new molecular-level information about model biological membranes. Another strategy to elucidate the interaction of biomolecules with interfaces exploits the inherent anisotropies of the system. While under conditions of rapid molecular reorientation, the dipolar couplings are averaged out to zero, they are partially retained as residual dipolar couplings (RDC) in systems comprising immobilized compounds like peptides located in membranes or bound to nanoparticles. The RDCs are measurable as additive anisotropic contributions to the isotropic spin-spin couplings of the peptides and serve to distinguish and quantify bound (immobilized) from unbound (freely reorienting) peptides. This strategy can be helpful to optimize affinity chromatographic separations like e.g. IMAC.