Liquid Crystalline Layering and Divalent Cations Cooperatively Enhance DNA Condensation

The layered liquid crystalline (LC) phases formed by DNA molecules which include rigid and flexible segments ( gapped DNA) enable the study of both end-to-end stacking and side-to-side lateral interactions that drive the condensation of DNA molecules. The resulting layer structure exhibits long-range inter-layer and intra-layer positional correlations. Using synchrotron small-angle x-ray scattering (SAXS) measurements, we investigate the impact of divalent Mg2+ cations on the stability of the inter- and intra-layer DNA ordering as a function of temperature between 5-65 {degrees}C and for different terminal base pairings at the blunt ends of the gapped DNA constructs, which mediate the strength of the attractive end-to-end interaction. We demonstrate that the stabilities at a fixed DNA concentration of both inter-layer and intra-layer order are significantly enhanced even at a few mM Mg2+ concentration. The stability continues to increase up to [~]30 mM Mg2+ concentration, but at higher ([~]100 mM) Mg2+ content repulsion between positive ions counteracts and reverses the increase. On the other hand, sufficiently strong base-stacking interactions promote intra-layer order even in the absence of multivalent cations, which demonstrates the impact of liquid crystal layering on the DNA condensation process. We discuss the implications of these results in terms cation-mediated DNA-DNA attraction.