Modeling of complex biological systems. I. Molecular dynamics studies of diglyceride monolayers

Molecular dynamics simulations of diglyceride monolayers at the air-water interface have been performed to study the dynamical behavior of these Langmuir layers at surface densities varying from 36.2 to 40.5 Å2/molecule. The monolayers are treated in full atomic detail, with the exception of methyl and methylene groups that are considered to be “anisotropic united atoms.” The presence of phase transitions are explored and correlated to the changes in the microstructure of the layer. Good agreement is found between the surface pressure-surface area (π-A) isotherms calculated in the simulations and determined in monolayer technique experiments. The diglyceride film shows a complex dynamical behavior during expansion. On expansion from a closely packed, highly ordered state, the film undergoes two phase transitions. The first transition, which occurs at 38.3 Å2/molecule, involves a “seesaw” mechanism of the sn-1 and sn-2 carboxyester groups. The latter forms the backbone of the diglyceride molecule at high surface pressure. This mechanism is governed by hydrophobic/hydrophilic forces. It is stabilized by a balance between inter- and intramolecular interactions. On expansion, the intermolecular interaction decreases, and at the first transition, the intramolecular interaction between the two chains are strong enough to cause an increase of the cross sectional area of the molecules. This swelling of the molecules is accompanied with an increase of intermolecular contributions. The second transition, which occurs at 39.8 Å2/molecule, is caused by tilting over nearest neighbors.