Joachim O. Rädler

Cationic liposomes complexed with DNA (CL-DNA) are promising synthetically based nonviral carriers of DNA vectors for gene therapy. The solution structure of CL-DNA complexes was probed on length scales from subnanometer to micrometer by synchrotron x-ray diffraction and optical microscopy. The addition of either linear lambda-phage or plasmid DNA to CLs resulted in an unexpected topological transition from liposomes to optically birefringent liquid-crystalline condensed globules. X-ray diffraction of the globules revealed a novel multilamellar structure with alternating lipid bilayer and DNA monolayers. The lambda-DNA chains form a one-dimensional lattice with distinct interhelical packing regimes. Remarkably, in the isoelectric point regime, the lambda-DNA interaxial spacing expands between 24.5 and 57.1 angstroms upon lipid dilution and is indicative of a long-range electrostatic-induced repulsion that is possibly enhanced by chain undulations.

A two-dimensional columnar phase in mixtures of DNA complexed with cationic liposomes has been found in the lipid composition regime known to be significantly more efficient at transfecting mammalian cells in culture compared to the lamellar (LalphaC) structure of cationic liposome-DNA complexes. The structure, derived from synchrotron x-ray diffraction, consists of DNA coated by cationic lipid monolayers and arranged on a two-dimensional hexagonal lattice (HIIC). Two membrane-altering pathways induce the LalphaC --> HIIC transition: one where the spontaneous curvature of the lipid monolayer is driven negative, and another where the membrane bending rigidity is lowered with a new class of helper-lipids. Optical microscopy revealed that the LalphaC complexes bind stably to anionic vesicles (models of cellular membranes), whereas the more transfectant HIIC complexes are unstable and rapidly fuse and release DNA upon adhering to anionic vesicles.

A general strategy is described which allows for transferring hydrophobically capped nanocrystals from organic to aqueous solution by wrapping an amphiphilic polymer around the particles. In particular, high quality CoPt3, Au, CdSe/ZnS, and Fe2O3 nanocrystals have been water-solubilized in this way. Analysis with transmission electron microscopy, gel electrophoresis, and fluorescence correlation spectroscopy demonstrates that monodispersity of the particles is conserved upon phase transfer to aqueous solution.

Protein adsorption to nanoparticles (NPs) is a key prerequisite to understand NP-cell interactions. While the layer thickness of the protein corona has been well characterized in many cases, the absolute number of bound proteins and their exchange dynamics in body fluids is difficult to assess. Here we measure the number of molecules adsorbed to sulfonate (PSOSO(3)H) and carboxyl-(PSCOOH) polystyrene NPs using fluorescence correlation spectroscopy. We find that the fraction of molecules bound to NPs falls onto a single, universal adsorption curve, if plotted as a function of molar protein-to-NP ratio. The adsorption curve shows the build-up of a strongly bound monolayer up to the point of monolayer saturation (at a geometrically defined protein-to-NP ratio), beyond which a secondary, weakly bound layer is formed. While the first layer is irreversibly bound (hard corona), the secondary layer (soft corona) exhibits dynamic exchange, if competing unlabeled is added. In the presence of plasma proteins, the hard corona is stable, while the soft corona is almost completely removed. The existence of two distinct time scales in the protein off-kinetics, for both NP types studied here, indicates the possibility of an exposure memory effect in the NP corona.

The conformation of DNA molecules electrostatically bound to fluid cationic lipid bilayers is investigated by fluorescence microscopy. The DNA diffuses freely in the plane and follows Rouse dynamics, $D\ensuremath{\sim}1/N$, with an increasing number of base pairs $N$. The chain extension scales as $〈{R}^{2}〉\ensuremath{\sim}{N}^{2\ensuremath{\nu}}$, with $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.79\ifmmode\pm\else\textpm\fi{}0.04$ in good agreement with the exact exponent $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3/4$ for a self-avoiding random walk in two dimensions. The structure factor of dilute and semidilute DNA solutions shows fractal scaling behavior, $S(k)\ensuremath{\sim}{k}^{\ensuremath{-}1/\ensuremath{\nu}}$. In highly concentrated two-dimensional DNA solutions, the chains were found to segregate.