RELATIONSHIP BETWEEN ADSORPTION AND DESORPTION OF ADENINE BY MONTMORILLONITE

抄録

Clay minerals are believed to play an important role in the origin of life. Bio-molecules initially formed in the shallow ocean could be adsorbed by clay minerals (e.g. montmorillonite) and/or intercalated between their interlayers, thereby protected from damage by cosmic ray and UV irradiation. The mineral particles could sink to the ocean floor and release bio-molecules under basic pH condition, such as near a highly basic hydrothermal vent. The mineral particles also serve as the location and catalyst for the polymerization of these bio-molecules, which is a crucial step in chemical evolution. As a model system, we experimentally investigated the adsorption and desorption of adenine on montmorillonite. Adsorption at pH 2.5 was found to be much larger than at pH 8.5. Since adenine is positively charged in the acidic condition, it is readily adsorbed by montmorillonite and/or intercalated into its interlayers. It was difficult to remove adenine from the interlayer, as it does not desorb in neutral or acidic (HCl) solutions. In a basic solution adenine is charge neutral, and the amount of its adsorption by montmorillonite at pH 8.5 is very small. Meanwhile, at pH 8.5, adenine in the montmorillonite interlayers is readily replaced by Na⁺ in the solution, and therefore desorbs easily. pH value clearly affects the adsorption/desorption behavior of adenine on/from montmorillonite.