Monolayer structures of triple-chain phosphatidylcholines as substrates for phospholipases

The structural properties of monolayers formed by different triple-chain phosphatidylcholines (PC) have been investigated at the air–water interface. In order to elucidate the competitive interactions between the hydrophilic headgroup and the hydrophobic chain regions, a number of molecular properties (chain length, linkage to the glycerol, branching) were varied. Film-balance pressure–area isotherms and grazing incidence X-ray diffraction (GIXD) experiments were utilized in the present study. At higher lateral pressures, monolayer structures of all triple-chain compounds investigated show hexagonal packing, as determined by GIXD. Differences are observable at lower lateral pressures. Components substituted (branched) at the two-position result in monolayers consisting of rectangular unit cells with tilted chains. Branching at the four-position reduces the stiffness of the molecules, acting as a hydrophobic spacer. Therefore four-substituted branched components exhibit hexagonal packing of upright-oriented chains already at lower pressures. The cross-sectional area of the chains is rather large (ca. 20.8 Å2).