ORIENTATION OF THE BENZOPHENONE GROUP AT VARIOUS DEPTHS IN BILAYERS

The hydrophobic core of biological membranes is primarily composed of fatty acyl chains of lipids and side chains of nonpolar amino acids belonging to membrane-spanning domains of transmembrane proteins. Electron transport across the 35-40-angstrom membrane dielectric takes place via suitably oriented electron-transfer groups associated with transmembrane domains of membrane-bound proteins. We propose here that the design of lipids bearing electron-transport groups oriented at different depths can provide the necessary supramolecular assembly in the form of a monolayer or a bilayer to carry out electron transfer. The design of these modified lipids is crucial to the success of such a molecular device. We report here the design and synthesis of three benzophenone-based phospholipids capable of orienting the benzophenone group at different depths in a bilayer. The orientation of the benzophenone group was determined by photochemical cross-linking of these lipids with dimyristoylphosphatidylcholine in single bilayer vesicles followed by mass spectral analyses of the cross-linked products. The actual site of cross-linking on the myristoyl chain was determined, and it was observed that a range of carbon atoms are functionalized. The range of carbon atoms functionalized was found to be centered around the position expected from the transverse location of the benzophenone-based phospholipid in the bilayer. The data could be best interpreted in terms of zones of carbon atoms functionalized rather than any discreet site. This is in keeping with the current models of membranes which suggest the presence of a fluid gradient as one goes down the fatty acyl chain in the membrane. However, the range of carbon atoms functionalized was narrowed with probes reported here. The use of a hydrophobic tail attached to the benzophenone group assisted in directing the orientation of the photoactive group at different depths. Besides providing an effective design strategy for the orientation of electron-transfer groups at different depths in a bilayer, the high insertion yield and the depth-dependent labeling observed in artificial membranes suggest that the benzophenone-based phospholipids reported here could also prove useful for studying the structure of single and multiple spanning transmembrane proteins