MONTMORILLONITE CATALYSIS OF RNA OLIGOMER FORMATION IN AQUEOUS-SOLUTION - A MODEL FOR THE PREBIOTIC FORMATION OF RNA

Oligomers of adenylic acid of up to the 11-mer in length are formed by the reaction of the phosphorimidazolide of adenosine (ImpA) in pH 8 a queous solution at room temperature in the presence of Na+-montmorillo nite. These oligomers are joined by phosphodiester bonds in which the 3',5'-linkage predominates over the 2',5'-linkage by a 2:1 ratio. Reac tion of a 9:1 mixture of ImpA, A5'ppA results in the formation of olig omers with a 3:1 ratio of 3',5'- to 2',5'-linked phosphodiester bonds. A high proportion of these oligomers contain the A5'ppA grouping. A5' ppA reacts much more rapidly with ImpA than does 5'-ADP (ppA) or 5'-AT P (pppA). The exchangeable cation associated with the montmorillonite effects the observed catalysis with Li+, Na+, NH4+, and Ca2+ being the more effective while Mg2+ and Al3+ are almost ineffective catalysts. 2',5'-Linked oligomers, up to the tetramer in length, are formed using UO22+-montmorillonite. The structure analysis of individual oligomer fractions was performed by selective enzymatic and KOH hydrolytic stud ies followed by HPLC analysis of the reaction products. It is conclude d from the composition of the oligomers that the rate of addition ImpA to a 3'-terminus containing a 2',5'-linkage is slower than the additi on to a nucleoside joined by a 3',5'-linked phosphodiester bond. The p otential importance of mineral catalysis of the formation of RNA and o ther oligomers on primitive Earth is discussed.