Figure 5.

A note on amphiphile concentrations. It is difficult to quantitatively compare the effects of amphiphiles on membrane protein function in different experimental setups. Amphiphiles adsorb to all hydrophobic surfaces and the aqueous concentration, generally, will be lower than the nominal concentration. First, adsorption to the walls of experimental containers (e.g., pipettes) may be considerable, and increasing with the surface area/volume ratio. For LPC, with a molecular area ∼0.7 nm², 80% of the molecules in 5 μl of a 10 μM solution would be needed to cover the surface of a hydrophobic cube with that volume. Second, the adsorption to lipid bilayers may be considerable. A recent study, measuring the effects of free fatty acids on lipid bilayer stiffness, found that in the absence of a lipid bilayer 30% of a fatty acid adsorbed to the Teflon experimental chamber; when fatty acid was added to a chamber containing a bilayer, however, the aqueous concentration was about two orders of magnitude lower than the nominal concentration! (Bruno et al., 2007). Because the lipid/water partition coefficient for amphiphiles is high, the bilayer amphiphile concentration will be affected by the ratio between lipid volume and total volume in the experimental setup. In binding experiments using membrane fragments, the membrane volume (of which the majority is lipids) may constitute 2% of the total volume (Søgaard et al., 2006). In bilayer stiffness measurements the bilayer-forming lipid solution constitutes only ∼0.1% of the total volume. Moreover, in whole cell voltage clamp experiments using continuously flowing experimental solution, the lipid volume may be considered as infinitely small compared with the total volume. As a consequence, a given nominal amphiphile concentration will result in bilayer concentrations that are related as: whole cell voltage clamp > bilayer stiffness measurements > binding experiments.