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 compared in terms of their mechanisms and degree of adsorption on three clay minerals, kaolinite, montmorillonite, and illite. Based on adsorption isotherms 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 were at least 2-fold greater than the amounts adsorbed on kaolinite and montmorillonite, regardless of whether excess divalent Ca (5 mM) was present in the solution. Increasing the concentration of DNA (>25 mu g mL(-1)) increased the adsorption of linear DNA, whereas the adsorption of plasmid DNA. molecules decreased, probably as the result of self-aggregation in solution. Titration of acidic groups of DNA showed a narrow range of strong acidity for the plasmid form, whereas the pH of linear chromosomal acidic groups ranged from very low to neutral or slightly alkaline pK(a) values. The amount of 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 plasmid DNA adsorbed on clay minerals by low temperature scanning electron microscopy (LTSEM) indicated that these molecules could act as bridges between clay domains by the ends of the supercoiled molecule. The location and strength of the acidic groups of DNA determine the interaction between clay and DNA. Supercoiled plasmid DNA interacts by a low number of strongly acidic 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 observations (Paget, E.; et al. FEMS Microbiol Letters 1992, 97, 31), through acidic groups distributed along the DNA. molecules. Such differences in interactions between clay and DNA should influence the accessibility to nucleases and persistence of DNA in soil environments.