Role of electrostatic interactions in two-dimensional self-assembly of tobacco mosaic viruses on cationic lipid monolayers

We explore two-dimensional self-assembly of tobacco mosaic viruses (TMVs) on a substrate-supported, fluid lipid monolayer by manipulating the electrostatic interactions, with specific focus on the effects of the cationic lipid concentration in the monolayer and the presence of Ca2+ ions in the surrounding bulk solution. The TMV assemblies were characterized by grazing-incidence X-ray scattering and atomic force microscopy, and the inter-particle interaction quantified through X-ray scattering data analysis. In the absence of Ca2+ ions, we found that higher charge densities on the lipid monolayer led to poorer in-plane order, which may be attributed to faster adsorption kinetics, due to the surface potential that increases with charge density. At the same time, higher lipid-charge densities also resulted in weaker repulsion between TMVs, due to partial screening of Coulomb repulsion by mobile cationic lipids in the monolayer. The lipid-charge dependence was diminished with increasing concentration of Ca2+ ions, which also led to tighter packing of TMVs. The results indicate that Ca2+ ions strengthen the screening of Coulomb repulsion between TMVs and consequently enhance the role of attractive forces. Control experiments involving Na+ ions suggest that the attractive inter-TMV interaction has contributions from both the van der Waals force and the counter-ion-induced attraction that depends on ion valence.

Graphical abstractEffects of the lipid-charge density and the concentration/valence of the bulk solution ions reveal convoluted roles of electrostatic interactions in forming ordered TMV assemblies on a supported lipid monolayer.Figure optionsDownload full-size imageDownload high-quality image (53 K)Download as PowerPoint slideHighlights► Electrostatic interactions play convoluted roles in forming 2D TMV assemblies. ► Mobile DOTAP in the monolayer partially screen repulsion between TMVs. ► Ca2+ ions screen electrostatic interactions and induce attraction between TMVs. ► Attraction between TMVs depends on the valence of the ions in the bulk solution. ► Slower kinetics generally results in better order.