Control of the Depth of Molecules within Membranes by Polar Groups: Determination of the Location of Anthracene-Labeled Probes in Model Membranes by Parallax Analysis of Nitroxide-Labeled Phospholipid Induced Fluorescence Quenching

The location of anthracene-labeled molecules incorporated into model membranes was measured by fluorescence quenching. The depth of the anthracene group was calculated from the degree of quenching by lipids carrying a nitroxide at different depths, using the parallax analysis (Chattopadhyay & London (1987) Biochemistry 26, 39-45). A series of anthracene derivatives was examined in order to determine what polar functional groups would anchor at the membrane surface, and at what depth anchoring would occur. An anthracene with only a methyl group was not anchored at the membrane surface, but derivatives with polar or charged groups did anchor near the membrane surface as demonstrated by a shallower anthracene depth. Based on anthracene depths, protonated primary amine, secondary amine, and hydroxyl groups appear to be located 15-16 Å from the center of the membrane. A quaternary amino locates more shallowly, at 18 Å from the bilayer center. A protonated carboxyl group is slightly deeper, at 14 A from the center of the bilayer. Ester groups are found to be weakly anchoring, having a location dependent on the structure of the molecule to which they are attached. In methyl 9-anthracenepropionate, the ester group is located about 13 Å from the bilayer center. Anthracene esters attached to cholesterol or cholesterol esters showed various depths. An anthracene ester attached to the tail of cholesterol was located 1-6 Å from the center of the bilayer for a cholesterol derivative, but at 12 Å from the bilayer center for a cholesterol oleate derivative. These studies show that a single polar group is sufficient to anchor molecules at the membrane surface and that small changes in chemical structure can greatly influence the depth of a molecule in membranes. Overall, there was a close correlation between the location of those polar groups which are found in lipids and the same polar groups when linked to anthracene. This approach may make it possible to predict the depth of molecules in membranes from their chemical structure.