Ordering in Langmuir monolayers of branched chain phospholipids

The ordering of branched chain glycerophosphoethanolamines (PE) with different side chain length (n=1, 2, 5 and 14) is systematically studied and compared with that of the corresponding double chain dipalmitoyl-phosphatidylethanolamine (DPPE) monolayers. The PEs have a 2-branched (methyl, ethyl, pentyl and tetradecyl) hexadecanoyl chain at the sn-1 position of the glycerol backbone and an unbranched hexadecyl residue at the sn-2 position. Thermodynamic data obtained by π–A isotherm measurements at different temperatures are coupled with two-dimensional (2D) texture and 2D structure information obtained by Brewster angle microscopy (BAM) and X-ray diffraction at grazing incidence (GIXD), respectively. With increasing side chain length up to n=5, the ordering of the long alkyl chains is increasingly disturbed so that n-16PE monolayers are highly disordered and cannot form a condensed phase. Already the ethyl side chain reduces the ordering of 2-16PE monolayer so strongly that not only the polar tilt of the alkyl chains increases, but also in a stretched lattice a varying tilt azimuth of the alkyl chains is indicated. A side chain elongation nearly to the length of the two main chains 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 direction occur, whereas at long side chain the domains show no differences in the inner texture.