Differences between linear chromosomal and supercoiled plasmid DNA in their mechanisms and extent of adsorption on clay minerals

Plasmid (mainly as the supercoiled form) and linear chromosomal DNA were co mpared in terms of their mechanisms and degree of adsorption on three clay minerals, kaolinite, montmorillonite, and illite. Based on adsorption isoth erms on Ca-clays, adsorption was complete for both plasmid and linear DNA. at low concentrations of DNA. Amounts of DNA adsorbed on illite in water we re at least 2-fold greater than the amounts adsorbed on kaolinite and montm orillonite, regardless of whether excess divalent Ca (5 mM) was present in the solution. Increasing the concentration of DNA (>25 mu g mL(-1)) increas ed the adsorption of linear DNA, whereas the adsorption of plasmid DNA. mol ecules decreased, probably as the result of self-aggregation in solution. T itration of acidic groups of DNA showed a narrow range of strong acidity fo r the plasmid form, whereas the pH of linear chromosomal acidic groups rang ed from very low to neutral or slightly alkaline pK(a) values. The amount o f acidic groups per gram of DNA was higher in linear DNA (13.4 cmol g(-1)) than in supercoiled plasmid DNA (1.8 cmol g(-1)). Direct observations of pl asmid DNA adsorbed on clay minerals by low temperature scanning electron mi croscopy (LTSEM) indicated that these molecules could act as bridges betwee n clay domains by the ends of the supercoiled molecule. The location and st rength of the acidic groups of DNA determine the interaction between clay a nd DNA. Supercoiled plasmid DNA interacts by a low number of strongly acidi c groups, presumably located at the maximum of bending of the double strand where a high charge density exists. Linear chromosomal molecules appear to attach on the clay surface and edges, as demonstrated by previous observat ions (Paget, E.; et al. FEMS Microbiol Letters 1992, 97, 31), through acidi c groups distributed along the DNA. molecules. Such differences in interact ions between clay and DNA should influence the accessibility to nucleases a nd persistence of DNA in soil environments.