Abstract

The monolayer and bilayer behavior of 1,2-dipalmitoylgalloylglycerol (DPGG 1 ) is thought to be governed by strong intermolecular hydrogen bonding. To probe this hypothesis, monolayers of DPGG and its analogs ( 2-5 ) with variation in the position and numbers of hydroxyl groups on the aromatic headgroup, and 6 , in which all the hydroxyl groups of 5 were replaces by methoxyl groups in order to disrupt the hydrogen bonds, were studied experimentally and computationally. It was found that at 25°C, all phenolic lipids formed aggregates at the air/water interface even at very high molecular areas, which suggests strong interactions between the phenolic lipid headgroups. Moreover, isotherms of 2 showed a shift to larger molecular area compared to those of the other phenolic lipids. A para --OH group played an important role in the monolayer behavior of the phenolic lipids. To rationalize the differences in the surface properties of the phenolic lipids that are caused by changing the substitution pattern on the aromatic headgroup, the interactions between small model phenolic esters in the gas phase was studied with density-functional theory. The carbonyl group involved configuration had the lowest total energy for all the phenolic lipid headgroup dimers. Aggregation of the phenolic lipid headgroups was energetically favorable. The preferred conformation of the full DPGG molecule had two non-interacting side chains, and in order to change into the conformation for the solid state monolayer, in which the two side chains were aligned, an energy barrier of about 5.5 kcal mol -1 needed to be overcome.