Translational order in liquid-expanded lipid monolayers functionalized with nucleosides

The monolayer behavior of a lipid containing two unsaturated alkyl chains a nd a nucleoside derivative as polar headgroup has been investigated by the Langmuir technique. From the surface pressure Irs. molecular area isotherm, the monolayer appears as a pure liquid-expanded phase and should be then c onsidered as a two-dimensional liquid. However: grazing incidence X-ray dif fraction experiments evidence a translational order that does not exist whe n the lipid headgroup is a choline moiety. Since unsaturated chains are exp ected to induce a fluid state of the monolayer at the temperature considere d, this order is likely to originate from the natural tendency nucleosides have to establish among themselves pi-stacking interactions between the bas es. The collected X-ray data are consistent with the geometrical requiremen ts for bases stacking.