Adsorption-Desorption, Nuclease Degradation And Microcalorimetric Characteristics Of DNA On Red Soil Active Particles

Red soil sampled from Xianning, Hubei province was divided into four types of clays: 0.2-2 μm organic clay, 0.2-2 μm inorganic clay, < 0.2 μm organic clay and < 0.2 μm inorganic clay. The adsorption-desorption, nuclease degradation and microcalorimetric characterics of DNA on soil colloids and minerals were investigated in this study. The main results were listed as following:1. Fine clays and kaolinite dominate the amount of DNA adsorption and organic matter also contributes to DNA adsorption in variable-charge soil. However, the contribution of goethite to DNA adsorption seems to be small. DNA was adsorbed more tightly on coarse clays, inorganic clays and kaolinite than on fine clays, organic clays and goethite.2. Environmental conditions have significant effects on DNA adsorption in soils. A marked decrease in the adsorption of DNA on soil colloids and minerals was observed with the increase of pH from 2.0 to 5.0, while a slow decrease was found with increasing pH from 5.0 to 7.0. Magnesium ion was more efficient than sodium ion in promoting DNA adsorption.3. To determine the percentage of various forces involved in DNA adsorption, DNA molecules adsorbed by soil colloids and minerals were desorbed by sequential washing with 10 mM NaAc, 100 mM NaCl and 100 raM phosphate. Only 10-20% DNA was adsorbed by electrostatic forces on soil colloids and minerals. DNA was adsorbed predominantly by dehydration effects and hydrogen bonding on organic clays and kaolinite and 43.3-65.2% of DNA was adsorbed on inorganic clays and goethite through ligand exchange.4. The extent of nuclease degradation of DNA adsorbed by soil colloids andminerals has no relation with the binding affinity and is mainly determined by the activity of enzyme adsorbed. Organic matter and particle size do not significantly affect the resistance of DNA by nuclease degradation in variable-charge soil.5. To our knowledge, this is the first paper providing the thermodynamic data of DNA adsorption on soil colloids and minerals. DNA adsorption on soil colloids and minerals is driven by an increase in entropy. The endothermic reaction of DNA on organic clays and minerals was probably due to the common action of hydrogen bonding, electrostatic repulsion and dehydration effects. However, the adsorption of DNA on inorganic clays was exothermic which was attributed to electrostatic attraction and ligand exchange. The adsorption enthalpies decreased with an increment of MgGb concentrations or a decrement of pH values. The positive values of enthalpy chang indicated that dehydration effects seem to dominate DNA adsorption on kaolinite and goethite at lower MgCI2 concentrations and higher pH values. However, the negative values of enthalpy change at lower pH or higher MgCk concentrations suggested that DNA adsorption on minerals was mainly controlled by electrostatic attractive forces.