Determination of lattice structure and calculation of molecular tilt in lipid monolayers on water using x-ray diffraction

Abstract

We report the first determination of lattice structure for classical (fatty acid and alcohol) monolayer systems on the surface of water, based on our observation of first- and second-order x-ray diffraction peaks. Monolayers of both heneicosanoic acid and heneicosanol in the highest-pressure phase pack into a distorted hexagonal lattice with orthorhombic cell dimensions of a\ensuremath{\simeq}7.5 t/rA\r{}, b\ensuremath{\simeq}5.0 i/rA\r{}. From the observed peak intensities, we have calculated the molecular tilt directions and magnitudes, using the assumption (reasonable given the dense packing) that the chains are parallel to each other and in the all-trans configuration. the temperature range 0--5 \ifmmode^\circ\else\textdegree\fi{}C, the molecules tilt towards the b axis, but at higher temperatures the tilt direction changes to the a axis.