Structural effects in phospholipids monolayers in presence of poly-D-lysines

Langmuir monolayers of saturated phospholipids exhibit phase transition from "liquid expanded" (LE) to "liquid condensed" (LC) state with change of lateral pressure. In turn, changes in lateral ordering of serine-based lipids can be initiated by adsorption of polypeptide molecules on the lipid layer. We present the investigation of influence of adsorption of poly-D-lysines [H₁₂C₆N₂O]_n×HBr on the structure of Langmuir monolayer of dimyristoyl-phosphatidylserine (DMPS) on water with the use of synchrotron X-ray reflectometry (XRR) and Volta potential diagrams. XRR measurements have been carried out at beamline ID31, ESRF (E ≈ 17 keV, λ ≈ 0.18 Å). Surface pressure and Volta potential has been measured on a setup MicroTrough XS V4.0 (Kibron). Detailed description of the beamline, setup and reflectometry methodics as well as some preliminary results was presented in [3]. DMPS monolayer has been formed by droplet method with calibrated amount of 0.5 mg/ml solution of C₃₄H₆₅NO₁₀PNa in chloroform, corresponding to area-per-molecule 72 Ų (LE-phase) and 36 Å2 (LC-phase) respectively, on a buffer solution of 10mM KCl in triple-distilled water. Two variants of poylysine with chain length n=12 (M_r ~ 5000 Da) and n=200 (M_r ~ 70000 Da) in concentrations ~0.27 uM have been used for the buffer dopation, according to the expected maximal adsorption level on the interface. According to the obtained data, surface pressure and Volta potential diagrams exhibit significant difference between states of monolayer with and without poly-D-lysines in the buffer substrate. In particular, presence of polymer leads to more frequent detection of surface clusters of lipid in LE-state. Estimations of effective dipole momentum μ yields 0.48 and 0.38 Debye respectively. Depth-graded distributions of electron concentration obtained from XRR data show significant decrease in the integral density of lipid layers, which is evident to the change of inclination angle for [CH2] chains. In our opinion, combined results can be explained by the increase in stiffness of monolayer in both phase states, corresponding to more intense formation of lipid clusters. Presence of polymer macromolecules changes the equilibrium at the substrate/monolayer interface, which fundamentally changes the pattern of surface filling within the lipid membrane. (author)