Adsorption kinetics and reversibility of linear plasmid DNA on silica surfaces: influence of alkaline earth and transition metal ions.

A quartz crystal microbalance with dissipation monitoring is used to study the adsorption of linear plasmid DNA on silica surfaces and silica surfaces coated with poly-L-lysine (PLL) in solutions containing either alkaline earth (calcium and magnesium) or transition (cobalt, copper, and zinc) metals. Our results show that electrostatic attraction alone does not fully explain the significantly higher adsorption rate of DNA on the positively charged PLL layer in Cu(2+) solution compared to solutions containing Ca(2+), Mg(2+), Co(2+), or Zn(2+). Diffusion coefficients measured by dynamic light scattering reveal that the compactness of plasmid DNA molecules is greater in solutions containing Cu(2+) compared to that of DNA in other divalent electrolyte solutions. When the adsorption rate of plasmid DNA on silica is normalized to the corresponding adsorption rate on PLL-coated surfaces at the same solution condition, the attachment (adsorption) efficiencies are about 0.01 for Ca(2+) or Mg(2), but close to unity for Co(2+), Cu(2+), or Zn(2+). Results from viscoelastic modeling of adsorbed DNA layers suggest that the DNA layer formed in Cu(2+) solutions is thicker and more viscous compared to that formed in Co(2+) solutions. This study demonstrates that plasmid DNA has a strong affinity to Cu(2+), which results in a more compact conformation of DNA molecules compared to the case with the other divalent cations investigated.