The regiospecific formation of oligomers from unblocked monomers in aqueous solution is one of the central tenets in research on the origins of life on earth. Direct experimental support for this hypothesis has been obtained in studies of the condensation of the 5'-phosphorimidazolide of adenosine (ImpA) with itself and with P1,P2-diadenosine-5',5'-pyrophosphate (AppA) in water in the presence of a montmorillonite clay. Oligomers of up to ten nucleotides in length are formed. Analysis of the trimers, tetramers, and pentamers formed from a 9:1 ImpA:AppA mixture has shown that 85% of the bonds formed are 3',5'-linked and that any 2',5'-linkages present are at the phosphodiester bond next to the 3'-terminus of the oligomers.
