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 be longing to membrane-spanning domains of transmembrane proteins. Electr on transport across the 35-40-angstrom membrane dielectric takes place via suitably oriented electron-transfer groups associated with transm embrane domains of membrane-bound proteins. We propose here that the d esign of lipids bearing electron-transport groups oriented at differen t depths can provide the necessary supramolecular assembly in the form of a monolayer or a bilayer to carry out electron transfer. The desig n of these modified lipids is crucial to the success of such a molecul ar device. We report here the design and synthesis of three benzopheno ne-based phospholipids capable of orienting the benzophenone group at different depths in a bilayer. The orientation of the benzophenone gro up 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 o f cross-linking on the myristoyl chain was determined, and it was obse rved that a range of carbon atoms are functionalized. The range of car bon atoms functionalized was found to be centered around the position expected from the transverse location of the benzophenone-based phosph olipid in the bilayer. The data could be best interpreted in terms of zones of carbon atoms functionalized rather than any discreet site. Th is is in keeping with the current models of membranes which suggest th e presence of a fluid gradient as one goes down the fatty acyl chain i n the membrane. However, the range of carbon atoms functionalized was narrowed with probes reported here. The use of a hydrophobic tail atta ched to the benzophenone group assisted in directing the orientation o f the photoactive group at different depths. Besides providing an effe ctive design strategy for the orientation of electron-transfer groups at different depths in a bilayer, the high insertion yield and the dep th-dependent labeling observed in artificial membranes suggest that th e benzophenone-based phospholipids reported here could also prove usef ul for studying the structure of single and multiple spanning transmem brane proteins.