| Supported planar bilayers have been shown to be ideal models of cell membranes. We recently observed a putative phase transition during the initial stages of bilayer formation for the cationic lipid, dimyristoyl-trimethylammoniumpropane (DMTAP), and the neutral lipid, dimyristoyl-phosphatidylcholine (DMPC). This transition, marked by an ∼1 nm variation in the height of the bilayer, was directly resolvable by in-situ TappingMode atomic force microscopy (AFM), both in the height and phase imaging modes. We were able to resolve the kinetics of this structural transition and stabilize the two structural forms through manipulation of the temperature and ionic strength of the buffer solution. To confirm that this transition is due to a change in tilt and/or packing of the lipid tails, in-situ fluorescence spectroscopy was applied. The simultaneous acquisition of the fluorescence spectra and atomic force microscopy data provide an excellent correlated approach to monitoring and characterizing these transient structural rearrangements. |
