Role of divalent cations in plasmid DNA adsorption to natural organic matter-coated silica surface

The adsorption kinetics of supercoiled and linear plasmid DNA onto a natural organic matter (NOM)-coated silica surface are acquired using a quartz crystal microbalance with dissipation monitoring (QCM-D) in the presence of common divalent electrolytes CaCl2 and MgCl2. The adsorption kinetics of both DNA are noticeably higher in the presence of CaCl2 compared to MgCl2. We hypothesize that specific bridging between the DNA phosphate groups and NOM carboxyl functional groups in the presence of Ca2+ cations may lead to more efficient attachment than in the presence of Mg2+ cations, which are only likely to allow for charge neutralization. The influence of background Na+ cations on the adsorption kinetics in the presence of CaCl2 is found to be insignificant, while the presence of Na+ leads to slower attachment kinetics in MgCl2. Rinsing the DNA layer adsorbed in the presence of CaCl2 with a solution of low NaCl concentration followed by deionized water does not result in observable detachment, indicating irreversibility of DNA adsorption. Instead, softening of the DNA layer adsorbed in the presence of CaCl2 with background Na+ occurs with the rinses due to the increase in electrostatic repulsion between the phosphate functional groups along the DNA backbone. In the case of the DNA layer adsorbed in the presence of CaCl2 without background Na+, softening of the layer does not occur with the rinses.