Soft Report - Dipartimento di Fisica - Sapienza

Scientific Report – Self Assembly, Clustering, Structural arrestPolyelectrolyte-Charged Liposome Complexes: Evidence ofEquilibrium Multi-Compartment AggregatesAggregation of colloidal particles induced byoppositely charged polyions has been extensivelyinvestigated both theoretically and experimentally,owing to its importance in many field of physicalchemistry. Recently, these phenomena attractedrenewed interest because the properties of theresulting aggregates, combined with thecharacteristics of self-organi-zation of the process,possess high potential for applications in materialresearch and bio-nanotechnologies. Among thesestructures, cationic lipid-DNA complexes havewitnessed of an increasing acceptance as preferentialDNA delivery vehicles in gene therapy [1], since theirFig. 1. Top: The average radius of the aggregatesfrom DLS measurements (o) reaches its maximumat the point of charged inversion, as measured by ζ-potential (o), i.e. at ζ=0.Bottom: a): TEM image of a typical pd-liposomeaggregate. Contrast is obtained mixing liposomesprepared loading their acqueous core with twodifferent concentrations of CsCl. “Heavily” and“lightly” loaded globules are clearly distinguished.b): Cs elemental map for the same frame, obtainedby filtering the transmitted electrons in an energywindow characte-ristic of the Cs-M 4,5 edge. c): theoriginal image a) and the map b) with a threshold at≈ 50% of the peak value are superimposed.enhanced permeability through the membranebilayer favors the delivery of DNA into the cells.“Columnar” (stacks of alternate planar layers of lipidand polymer) or “hexagonal” (lipids in the hexagonalphase H II with a DNA chain filling each honey-combcell) structures that form in these systems close tothe isoelectric point (where the stoichiometric chargeof DNA counterbalances the stoichiometric charge ofthe lipid phase) have been described [2].However, by means of the combined use of DynamicLight Scattering (DLS) and Transmission ElectronMicroscopy (TEM) measurements, we pro-vide astrong and direct evidence for the existence, in thepolyion-induced liposome aggregation, of anequilibrium cluster phase where liposomes maintaintheir integrity, with the ability of preserving theaqueous content of their core from the externalmedium. Oppositly charged polyelectrolytes stick tothe liposome surface and, with the increasingpolymer/lipid charge concentration ratio, ξ, theoverall electric charge of these “polyelectrolytedecoratedliposomes” (pd-liposomes) decreases,passing through the zero (isoelectric point) andreaching large negative values when polyions are inexcess (“overcharging” or charge inversion).Maximum aggregation occurs close to the point ofcharge inversion. As we have recently shown, theseclusters are equilibrium aggregates [3,4].Aggregation is due to a balance of long range electrostaticrepulsion and of short-range attraction,arising from the correlated adsorption of polyions atthe liposome surface ("charge patch" attraction).This cluster phase shows very interesting features inview of possible biomedical applications since thestructure of the aggregates, with liposomesreversibly "glued together" by a non-uniformdistribution of adsorbed polyelectrolytes, could findinteresting applications as "multi-compartmentliposomal aggregates" for controlled-release drugdelivery.References[1] D. Ferber, Science, 294, 1638 (2001).[2] I. Koltover, T. Salditt, J. O. Rädler, C. R. Safnya,Science, 281, 78 (1998).[3] F. Bordi, C. Cametti, M. Diociaiuti, S. Sennato,Phys. Rev. E, 71, 050401(Rd) (2005).[4] F.Bordi, C.Cametti, M.Diociaiuti, S.Sennato,Biophys. J. 2006, 91,1513-1520AuthorsF. Bordi § , C. Cametti § , S. Sennato § , M. Diociaiuti #§Dipartimento di Fisica, Università di Roma ″LaSapienza″, Roma and INFM CRS-SOFT#Dipartimento di Tecnologie e Salute, IstitutoSuperiore di Sanità, Roma.SOFT Scientific Report 2004-0686