Soft Report - Dipartimento di Fisica - Sapienza

Scientific Report – Elastic and inelastic scattering of neutrons and X-raysDynamics in Model Membranes, Membrane-Protein andMembrane-DNA InteractionsThe dynamics of different model membranes wasstudied in detail. A first study concerned thedynamics of sugar-based anphiphilic molecules(gangliosides) in bilayer domains. Gangliosides areglycosphingolipids abundant in neuronal plasmamembranes, which are believed to be involved inprocesses like protein binding, cell recognition andsignal transduction, while being embedded inmembrane microdomains. These sphingolipidenricheddomains, SED, are lipid-driven, whichmeans that their mechanical properties andbiological functions is strongly determined by thebehaviour of the embedded lipids. As a first step, weinvestigated the influence on both in-plane and outof-planelipid dynamics brought about by thepresence of glycosphingolipids. By spreadingmixtures of GM1-phospholipid solutions onmonocrystalline Si wafers, we were able to obtainoriented mutilayers and thus to in-plane and out-ofplanemotions. Elastic temperature scans wereperformed from cryogenic temperatures, where lipidmotions are essentially harmonic, to the gel-liquidphase transition of the phospholipid, occuring around300 K. No anisotropy in the MSD and no GM1-dependence is observed in the low-T region. Above310 K an abrupt decrease of the elastic intensity isobserved together with a strong anisotropic effect,linked to the presence of the GM1, since it is notdetected in the pure-phospholipid system. SED aremulticomponent systems much more complex thanjust one sphingolipid plus one ganglioside. The futureaim is to progressively approach the situation of areal membrane, both from the lipid and the proteinside.The interaction of lipid bilayers with otherbiomolecular species (proteins and nucleic acids) wasalso investigated. Highly oriented DMPA phospholipidmultilayers with physiological amounts of animportant membrane protein, myelin basic protein(MBP), were prepared with a technique analogous tothat previously described. MBP is the second majorprotein of myelin, a discontinuous multi-bilayermembrane sheath wrapped around the nerve axon.The integrity of the myelin sheath is fundamental tooptimise the action potential conduction along theaxon. Stacking abnormalities occur in the presenceof severe demyelinating diseases like multiplesclerosis. Therefore, it is of outermost importance tounderstand the nature of the interactions thatmaintain structural and dynamical integrity. Presenceof MBP induces a significant increase of MSD,particularly in the out-of-plane direction. In otherwords, the presence of the protein seems to increasethe diffusion volume accessible to protons in theinterfacial hydrophilic region, therefore allowingprotons to increase their displacement in the out-ofplanedirection. This could be due to inter-digitationof part of the protein into the lipid headgroups, or tothe penetration of some hydrophobic segments intothe acyl-chains, thus decreasing the energy barriersbetween the hydrophobic and hydrophilic regions ofthe bilayer itself. This interpretation is suggested bythe fact that both inter-digitation, and monolayerintercalation have been observed in the fluid phase.Interestingly, NMR studies suggested that the lipidacyl chains are more mobile in the presence of MBPpossibly as a result of protein-induced reorganizationof lipid headgroups.Highly oriented lamellar DOTAP-DOPC modelmembranes have also been studied as a function ofthe DOPC/(DOPC + DOTAP) molar weight ratio,and of the cationic lipid/DNA molar weight ratio.The main result of the elastic scattering experimentsis that a minimum amount of DNA phosphate groupsis not sufficient to induce modifications in membranedynamics. On the other hand, at the isoelectric point,the balance of the total net charge inside thecomplex, together with the displacement of boundwater molecules into solution provides new degreesof freedom to the lipoplex, enhancing the apolarregion fluidity and resulting in a very large increaseof the out-of-plane lipid motions. (Å 2 )0.60.40.20.00.60.40.20.00.60.40.2b)0.0c)0 50 100 150 200 250 300T (K)Fig. 1. Normalised mean square displacements ofthe CLs-DNA mixed multilayers vs T, obtained in theframework of the Gaussian model. Panel a): Lipids;panel b): = 4; panel c): iso = 2.2. Opensquares: out-of-plane direction; filled circles: inplanedirection.AuthorsA. Gliozzi, R. Rolandi, Università di Genova, S.Magazù, U. Wanderlingh, Università di Messina, M.Corti, L. Cantù, Univ. Milano, R. Cordone, A.Cupane,Università di Palermo, A. Deriu Università di Parma,G. Onori, A. Paciaroni Università di Perugia, CRS-SOFT, A. Congiu Castellano Università di Roma LaSapienza, A. Filabozzi, G. Paradossi Università diRoma Tor Vergata.a)SOFT Scientific Report 2004-06106