Transitions between distinct compaction regimes in complexes of multivalent cationic lipids and DNA

We use X-ray scattering and molecular simulations to investigate the structural properties of complexes of multivalent cationic lipids and DNA molecules. At low mole fraction of neutral lipids (NLs), $\Phi_{\rm NL}$, the complexes show dramatic DNA compaction down to essentially close packed DNA arrays with a DNA interaxial spacing $d_{\rm DNA}=25\AA$. A gradual increase in $\Phi_{\rm NL}$ does not lead to a continuous increase in $d_{\rm DNA}$ as observed for DNA complexes of monovalent cationic lipids (CLs). Instead, distinct spacing regimes exist, with sharp transitions between them. Three packing states have been identified: (i) close packed, (ii) condensed, but not close packed, with $d_{\rm DNA}=27-28\AA$, and (iii) an expanded state, where $d_{\rm DNA}$ increases gradually with $\Phi_{\rm NL}$. Based on our experimental and computational results, we conclude that the DNA condensation is mediated by the multivalent cationic lipids, which assemble between the negatively charged DNA rods. Quite remarkably, the computational results show that the less tightly packed structure in regime (ii) is thermodynamically more stable than the close packed structure in regime (i). Accordingly, the constant DNA spacing observed in regime (ii) is attributed to lateral phase coexistence between this stable CL-DNA complex and neutral membranes. This finding may explain the reduced transfection efficiency measured for such complexes: Transfection involves endosomal escape and disassembly of the complex, and these processes are inhibited by the high thermodynamic stability. Our results, which demonstrate the existence of an inverse correlation between the stability and transfection activity of lamellar CL-DNA complexes are, therefore, consistent with a recently proposed model of cellular entry.