Characterization of Small Isotropic Bicelles with Various Compositions.

Structural studies of membrane proteins are of great importance and interest, with solution and solid state NMR spectroscopy being very promising tools for that task. However, such investigations are hindered by a number of obstacles, and in the first place by the fact that membrane proteins need an adequate environment that models the cell membrane. One of the most widely used and prospective membrane mimetics is isotropic bicelles. While large anisotropic bicelles are well-studied, the field of small bicelles contains a lot of "white spots". The present work reports the radii of particles and concentration of the detergents in the monomeric state in solutions of isotropic bicelles, formed by 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC), 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate (CHAPSO), and sodium cholate, as a function of lipid/detergent ratio and temperature. These parameters were measured using (1)H NMR diffusion spectroscopy for the bicelles composed of lipids with saturated fatty chains of different length and lipids, containing unsaturated fatty acid residue. The influence of a model transmembrane protein (membrane domain of rat TrkA) on the properties of bicelles and the effect of the bicelle size and composition on the properties of the transmembrane protein were investigated with heteronuclear NMR and nuclear Overhauser effect spectroscopy. We show that isotropic bicelles that are applicable for solution NMR spectroscopy behave as predicted by the theoretical models and are likely to be bicelles rather than mixed micelles. Using the obtained data, we propose a simple approach to control the size of bicelles at low concentrations. On the basis of our results, we compared different rim-forming agents and selected CHAPS as a detergent of choice for structural studies in bicelles, if the deuteration of the detergent is not required.