We report evidence for long-range molecular orientational order in monolayer solid phase domains of dipalmitoylphosphatidylcholine (DPPC) at the air-water interface. Monolayers of DPPC in the solid-fluid coexistence region of the pressure-area diagram were studied by using fluorescence excitation. A low concentration of a fluorescent lipid probe that dissolves in both the solid and fluid domains was included in the monolayer. When fluorescence is excited by a laser beam incident on a monolayer at glancing angles to the surface, the fluorescence emission depends on the orientation of the domains and the direction of the electric vector of illuminating light. The observed anisotropy of the fluorescence emission can be interpreted in terms of long-range molecular orientational order in the solid domains, an order that is doubtless associated with the tilt of the hydrocarbon chains. In monolayers composed of a single enantiomer of DPPC (R-DPPC), the solid domains have spiral, chiral shapes, and the molecular orientational order has a curvature that parallels the arms of these spirals.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry