Determination of the depth of bromine atoms in bilayers formed from bromolipid probes

X-ray diffraction analysis has been performed on a series of 1-palmitoyl-2-dibromostearoyl-phosphatidylcholines (BRPCs) with bromine atoms at the 6,7-, the 11,12-, or the 15,16-positions on the sn-2 acyl chains. The diffraction patterns indicate that, when hydrated, each of these lipids formed liquid-crystalline bilayers at 200C. For each lipid, electron density profiles and continuous Fourier transforms were calculated by the use of swelling experiments. In the electron density profiles, high-density peaks, due to the bromine atoms, are observed. The separation between these bromine peaks in the profile decreases as the bromine atoms are moved toward the terminal methyl of the acyl chain. For the 6,7- and 11,12-bromolipids, experimental Fourier transforms can be approximated by the sum of the transform of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and the transform of two symmetrically placed peaks of electron density (the bromines). For the case of the 15,16-bromolipids, a better fit is obtained for the transforms of a model bilayer where the thickness of the methylene chain region of the bilayer is 3 A greater than that of POPC