[Intramolecular bond ordering of cis-octadecadienoic chains of natural lipids. Monte-Carlo simulation].

The Monte Carlo method is applied in the calculation of C-C and C-H bond orientation distributions rho(theta) with respect to the maximum molecule span axis (theta is the angle between the bond and the axis) and bond order parameters SCC and SCH of isolated unperturbed hydrocarbon chains of cis -C18;2 at T = 298 K. The molecule-fixed coordinate system is used. The relation of the bond orientation distributions rho(theta) to the order parameters S are analysed in terms of angles theta max (a "geometric" factor, rho(theta max) = max) and widths delta theta of the distributions (factor of "fluctuations"). The results indicate that the fluctuation factor depends on both the segment chemical structure and location in the chain. The two delta theta values of C-H-bonds flanking the double bond are smaller than that obtained for adjacent CH2-groups by a factor of 1.5-2. The C = C double bond parameters SCC are found to be higher than those of adjacent single C-C bonds. The order parameter profiles -SCH obtained from the simulations (at the portion which corresponds to the double bonds location) are in qualitative, agreement with experimental data on the bilayer in the liquid-crystal phase.