Direct observation and characterization of DMPC/DHPC aggregates under conditions relevant for biological solution NMR

We have used cryo-transmission electron microscopy (cryo-TEM) for inspection of aggregates formed by dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholine (DHPC) in aqueous solution at total phospholipid concentrations cL < or = 5% and DMPC/DHPC ratios q < or = 4.0. In combination with ocular inspections, we are able to sketch out this part of phase-diagram at T = 14-80 degrees C. The temperature and the ratio q are the dominating variables for changing sample morphology, while cL to a lesser extent affects the aggregate structure. At q = 0.5, small, possibly disc-shaped, aggregates with a diameter of approximately 6 nm are formed. At higher q-values, distorted discoidal micelles that tend to short cylindrical micelles are observed. The more well-shaped discs have a diameter of around 20 nm. Upon increasing q or the temperature, long slightly flattened cylindrical micelles that eventually branch are formed. A holey lamellar phase finally appears upon further elevation of q or temperature. The implications for biological NMR work are two. First, discs prepared as membrane mimics are frequently much smaller than predicted by current "ideal bicelle" models. Second, the q approximately 3 preparations used for aligning water-soluble biomolecules in magnetic fields consist of perforated lamellar sheets. Furthermore, the discovered sequence of morphological transitions may have important implications for the development of bicelle-based membrane protein crystallization methods.