Laurdan Studies of Membrane Lipid-Nicotinic Acetylcholine Receptor Protein Interactions

The extrinsic fluorescent probe Laurdan (6-dodecanoyl-2-dimethylamino naphthalene) exhibits extreme sensitivity to the polarity and to the molecular dynamics of the dipoles in its environment. Dipolar relaxation processes are reflected as relatively large spectral shifts. Steady-state measurements of the so-called general polarization (GP) of Laurdan exploit the advantageous spectral properties of Laurdan. Since the main solvent dipoles surrounding Laurdan in biological membranes are water molecules, when no relaxation occurs GP values are high, indicating low water content in the hydrophilic/hydrophobic interface region. Laurdan fluorescence can also be used to obtain topographical information. A hitherto unexploited property of Laurdan, namely its ability to act as a F�rster-type resonance energy transfer (FRET) acceptor of tryptophan emission, was used to learn about the physical state of lipids within F�rster distance from donor tryptophan residues in integral membrane proteins. The application of this technique to the paradigm integral membrane protein, the nicotinic acetylcholine receptor, is described in this chapter.