Molecular dynamics simulations of model monolayers of diacylglycerolipids 18:0/18:1(n-9)cis, 18:0/18:2(n-6)cis, 18:0/18:3(n-3)cis have been carried out. The simulation cell, with periodic boundary conditions in the lateral directions, contained 24 molecules in each monolayer; 1 nanosecond simulations were performed. The systems were coupled to an external temperature bath of 295 K and pressure bath of 10 dyn/cm.

C-H order parameter profiles (SCH) with respect to the monolayer normal and the bond orientation distributions R(A) have been calculated; A is the angle between C-H bond and the normal. The relation of R(A) to the parameters SCH are analyzed in terms of geometric factors A_m (A_m corresponds to the maximum of R(A), R(A_m)=max) and factors of fluctuations DA (DA is width of the distribution R(A)).

The order parameter SCH profiles obtained from the simulations are in good agreement with experimental data on bilayers (for 18:1 and 18:2 acyls) in the liquid-crystal phase - the magnitudes of the order parameters of C-H bonds flanking the double bonds C=C significantly differ from one another. The profiles of 18:0 chains show a plateau which is also observed in experiments.

The fluctuation factors depend on both the segment chemical structure and location in the chain. The two Da values of C-H bonds flanking the cis double bond are much smaller than that obtained for adjacent CH₂-group. In most cases factors Am and DA establish which contribution to the order parameter SCH dominates. Defining these properties is a necessary step to gaining a more complete understanding of polyunsaturated lipid hydrocarbon chains significance.

This investigation was supported by Russian Foundation for Basic Researches (Grant N 97-04-48247).