DNA condensation by cobalt hexaammine(III) in alcohol–water mixtures: Dielectric constant and other solvent effects

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Abstract
DNA molecules condense into compact structures in the presence of a critical concentration of multivalent cations. To probe the contribution ofelectrostatic forces to condensation, we used mixtures of water with methanol (MeOH), ethanol (EtOH), and isopropanol (iPrOH) to vary the dielectric constant ϵ from 80 to 50. The condensation of pUC18 plasmids by hexaammine cobalt (III), Co(NH3), was monitored by total intensity and dynamic light scattering, electron microscopy, andCD. The total scattering intensity increased as ϵ went from 80 to 70, and then decreased as ϵ decreased further. Ultraviolet spectrophotometry confirmed that the loss of intensity at low ϵ was not due to the particles' settling out of solution. The rate as well as the extent of condensation increased asϵ was lowered from 80 to 70, and also depended on the species of alcohol (MeOH < EtOH < iPrOH). The hydrodynamic radii RH of the particles, however, remained roughly the same at 300–350 A and was independent of the species of alcohol. RH increased below ϵ = 70. The critical concentration of Co(NH3) required to induce DNA condensation decreased from 21 μM to about 16 μM as the dielectric constant decreased from 80 to 70, and decreased moderately with the nonpolarity of the alcohol. The fraction of DNA charge neutralized at the onset of DNA condensation was calculated by a modification of Manning's two‐variable counterion condensation theory to be 0.90 ± 0.01, independent of ϵ. By electron microscopy we observed that the condensed particles changed from about 93% toroids at ϵ = 80 to 89% rods at ϵ = 70 and 98% rods at ϵ = 65. At epsi; lower than 65, DNA collapsed into a network of multistranded fibers. The morphology of condensed DNA particles, whether toroids, rods, or fibers, was independent of the alcohol species. CD spectra in ethanol–water mixtures indicated that both closed circular and linearized plasmids were in the B conformation when condensed with Co(NH3)3+6 at ϵ≥ 70, although the closed circular molecules exhibited a weak Ψ‐DNA spectrum. A transition from the B to A formtook place between ϵ = 70 and 60, well above the normal dielectric constant of ϵ = 40 for this transition, indicating that ethanol and Co(NH3) synergistically promote the B–A transition.We interpret these results to mean that alcohols have both electrostatic and structural effects on DNA, leading to three regimes of condensation. At the lowest alcohol concentrations the B conformation is stableand condensation is relatively slow, allowing time for the packing adjustments necessary to form toroids. At intermediate alcohol concentrations condensation is faster, and the combined effects of solvent and Co(NH3) locally destabilize the double helix, permitting DNA foldbacks that lead to rodlike condensates. Rods become shorter as wellas more numerous as ϵ decreases from 80 to 65–60, indicatingincreasing destabilization as alcohol increases. At the lowest dielectric constants, alcohol and Co(NH3) produce A‐DNA, which strongly self‐adheres and rapidly aggregates intofibrous networks, not allowing time for more compact condensates to form. © 1995 John Wiley & Sons, Inc.