MODELING OF COMPLEX BIOLOGICAL-SYSTEMS .1. MOLECULAR-DYNAMICS STUDIESOF DIGLYCERIDE MONOLAYERS

Molecular dynamics simulations of diglyceride monolayers at the air-wa ter interface have been performed to study the dynamical behavior of t hese Langmuir layers at surface densities varying from 36.2 to 40.5 an gstrom2/molecule. The monolayers are treated in full atomic detail, wi th the exception of methyl and methylene groups that are considered to be ''anisotropic united atoms.'' The presence of phase transitions ar e explored and correlated to the changes in the microstructure of the layer. Good agreement is found between the surface pressure-surface ar ea (pi-A) isotherms calculated in the simulations and determined in mo nolayer technique experiments. The diglyceride film shows a complex dy namical behavior during expansion. On expansion from a closely packed, highly ordered state, the film undergoes two phase transitions. The f irst transition, which occurs at 38.3 angstrom2/molecule, involves a ' 'seesaw'' mechanism of the sn-1 and sn-2 carboxyester groups. The latt er forms the backbone of the diglyceride molecule at high surface pres sure. This mechanism is governed by hydrophobic/hydrophilic forces. It is stabilized by a balance between inter- and intramolecular interact ions. On expansion, the intermolecular interaction decreases, and at t he first transition, the intramolecular interaction between the two ch ains are strong enough to cause an increase of the cross sectional are a of the molecules. This swelling of the molecules is accompanied with an increase of intermolecular contributions. The second transition. w hich occurs at 39.8 angstrom2/molecule, is caused by tilting over near est neighbors.