Binding of adenine and adenine-related compounds to the clay montmorillonite and the mineral hydroxylapatite.

The first living things may have consisted of no more than RNA or RNA-like molecules bound to the surfaces of mineral particles. A key aspect of this theory is that these mineral particles have binding sites for RNA and its prebiotic precursors. The object of this study is to explore the binding properties of two of the best studied minerals, montmorillonite and hydroxylapatite, for possible precursors of RNA. The list of compounds investigated includes purines, pyrimidines, nucleosides, nucleotides, nucleotide coenzymes, diaminomaleonitrile and aminoimidazole carboxamide. Affinities for hydroxylapatite are dominated by ionic interactions between negatively charged small molecules and positively charged sites in the mineral. Binding to montmorillonite presents a more complex picture. These clay particles have a high affinity for organic ring structures which is augmented if they are positively charged. This binding probably takes place on the negatively charged faces of these sheet-like clay particles. Additional binding sites on the edges of these sheets have a moderate affinity for negatively charged molecules. Small molecules that bind to these minerals sometimes bind independently to sites on the minerals and sometimes bind cooperatively with favorable interactions between the bound molecules.