Polyion Adsorption onto Catanionic Surfaces. A Monte Carlo Study R. S. Dias,* ,² A. A. C. C. Pais, ‡ P. Linse, ² M. G. Miguel, ‡ and B. Lindman ² Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund UniVersity, P.O. Box 124, S-221 00 Lund, Sweden, and Departamento de Quı ´mica, UniVersidade de Coimbra, 3004-535 Coimbra, Portugal ReceiVed: January 10, 2005; In Final Form: April 7, 2005 The adsorption of a single and negatively charged polyion with varying flexibility onto a surface carrying both negative and positive charges representing a charged membrane surface has been investigated by using a simple model employing Monte Carlo simulations. The polyion was represented by a sequence of negatively charged hard spheres connected with harmonic bonds. The charged surface groups were also represented by charged hard spheres, and they were positioned on a hard surface slightly protruding into the solution. The surface charges were either frozen in a liquidlike structure or laterally mobile. With a large excess of positive surface charges, the classical picture of a strongly adsorbed polyion with an extended and flat configuration emerged. However, adsorption also appeared at a net neutral surface or at a weakly negatively charged surface, and at these conditions the adsorption was stronger with a flexible polyion as compared to a semiflexible one, two features not appearing in simpler models containing homogeneously charged surfaces. The presence of charged surface patches (frozen surface charges) and the ability of polarization of the surface charges (mobile surface charges) are the main reasons for the enhanced adsorption. The stronger adsorption with the flexible chain is caused by its greater ability to spatially correlate with the surface charges. 1. Introduction Polymer and protein adsorption onto lipid monolayers and bilayers is of fundamental importance in biology as well as in a large range of technological processes, such as pharmacology. For example, the adsorption of macromolecules onto surfaces of substrates is an intermediate step in fabrication of drug and gene delivery vehicles. One of the most studied systems of nonviral gene therapy consists of the so-called lipoplexes, complexes formed between DNA molecules and liposomes (vesicular structures formed typically by a mixture of a neutral and a cationic lipid). 1,2 The formation of such complexes starts with the adsorption of DNA onto the positively charged membrane. These systems have been extensively studied, and even though the mechanism of forma- tion is still far from understood, 3,4 the structure of the complexes is believed to be a short-ranged lamellar structure composed of amphiphilic bilayers with DNA molecules ordered and packed between the lipid stacks. This type of structure has been observed for systems with different lipid components. 5-8 Moreover, DNA with its unique structure can also act as a good candidate for future nanodevices such as templates, biosensors, and semiconducting molecules. There are a significant number of experimental studies on the adsorption of polyelectrolytes, especially DNA, on liposomes composed of neutral and positively charged lipids (for reviews, see refs 2 and 9-11). Ellipsometry studies have shown that a thick layer of a few DNA molecules adsorbed on a hydrophobic surface undergoes strong condensation into a thin and denser layer by the addition of a cationic surfactant. 12,13 DNA adsorp- tion on cationic lipid bilayers was also studied by atomic force microscopy, and the large DNA molecules were shown to destabilize the membrane. 14,15 The adsorption has also been followed by surface plasmon spectroscopy 16 and fluorescence microscopy, and it was found that the molecules, when confined in two dimensions, adsorb in an extended conformation. 17,18 Specific studies on the adsorption of polyelectrolytes on vesicles formed by mixtures of positively and negatively charged surfactants are more scarce. 8,19-21 It was observed that DNA, in general, destroys the vesicles, 8 whereas more flexible macromolecules induce changes in the shapes of the vesicles. 19,21,22 However, for minute concentrations of DNA, most of the vesicles were intact, and the interaction caused a certain degree of compaction in the polymer backbone. 23 Figure 1 illustrates the experimental observation of a single DNA chain adsorbing upon a catanionic vesicle. There is some indication that the polymer chain has a tendency to reside at the surface of the vesicle, but the underlying adsorption mechanism or the disruption caused to the surface calls for rationalization. When a lamellar phase is in its fluid state, the lipids possess relatively fast lateral diffusion, which are, in principle, respon- sive when a charged object approaches. In fact, this demixing of the lipids and formation of domains in mixed lipid membranes have been observed recently by fluorescence microscopy experiments upon the adsorption of DNA on cationic mem- branes 18 or of peptides on giant unilamellar vesicles. 24 In biological membranes, the domains formed are often denoted as “rafts”. The rapid lateral movement of lipid molecules, or membrane fluidity, is believed to be responsible for the proper functioning of several membrane properties. This mobility of charges in the plane can lead to interesting properties. Indeed, calculations have shown that the membrane fluidity has a considerable influence on DNA adsorption, 25,26 even leading to the counterintuitive phenomenon of a negatively charged polymer adsorbing on an overall negatively charged surface. 26 However, atomic force experiments on the DNA interaction with * Corresponding author. ² Lund University. ‡ Universidade de Coimbra. 11781 J. Phys. Chem. B 2005, 109, 11781-11788 10.1021/jp050158b CCC: $30.25 © 2005 American Chemical Society Published on Web 05/20/2005