Chiral discrimination in a monolayer of a triple-chain phosphatidylcholine.

Monolayers of an enantiomeric and a racemic triple-chain phosphatidylcholine (PC) at the air/water interface are studied by film balance measurements and x-ray diffraction. Although the area per three tails exceeds that per head, we observe tail ordering dependent on headgroup chirality and chain tilt. This indicates lateral headgroup interactions. The influence of the chiral carbon is suppressed at higher lateral pressures, and a centered-rectangular unit cell with tails tilted into the nearest neighbor (NN) direction is observed for both the enantiomer and the racemate. The distortion of the lattice changes at medium pressures from NN to NNN (next-nearest neighbor direction) with decreasing temperature. The phase behavior of the racemate at 15 degrees C is compared with that of a triple-chain PC with a branched chain of reduced length. Whereas the PC with the longer branched chain exhibits only a NN tilted phase at all pressures, the PC with the shorter branched chain has a rich polymorphism (NNN-NN-upright hexagonal packing) under increased lateral pressure.