Ordering in Langmuir monolayers of branched chain phospholipids

The ordering of branched chain glycerophosphoethanolamines (PE) with differ ent side chain length (n = 1, 2, 5 and 14) is systematically studied and co mpared with that of the corresponding double chain dipalmitoyl-phosphatidyl ethanolamine (DPPE) monolayers. The PEs have a 2-branched (methyl, ethyl, p entyl and tetradecyl) hexadecanoyl chain at the sn-1 position of the glycer ol backbone and an unbranched hexadecyl residue at the sn-2 position. Therm odynamic data obtained by pi-A isotherm measurements at different temperatu res are coupled with two-dimensional (2D) texture and 2D structure informat ion obtained by Brewster angle microscopy (BAM) and X-ray diffraction at gr azing incidence (GIXD), respectively. With increasing side chain length up to n = 5, the ordering of the long alkyl chains is increasingly disturbed s o that n-16PE monolayers are highly disordered and cannot form a condensed phase. Already the ethyl side chain reduces the ordering of 2-16PE monolaye r so strongly that not only the polar tilt of the alkyl chains increases, b ut also in a stretched lattice a varying tilt azimuth of the alkyl chains i s indicated. A side chain elongation nearly to the length of the two main c hains enhances the ordering to that of a triple chain phospholipid with the alkyl chains oriented perpendicularly in a hexagonal lattice. Texture and shape of the dendritic or fractal-like condensed phase domains are similar to that known from DPPE monolayers. However, the individual differences in the inner textures found for the different side chain lengths, corroborate the GIXD results. At short side chains, domain regions of different tilt di rection occur, whereas at long side chain the domains show no differences i n the inner texture. (C) 1999 Elsevier Science S.A. All rights reserved.