Oligomerization of uridine phosphorimidazolides on montmorillonite: A model for the prebiotic synthesis of RNA on minerals

The 5'-phosphorimidazolide of uridine reacts on Na+-montmorillonite 22A in aqueous solution to give oligomers as long as 7 mers. The maximum chain length increases to 9 mers and the overall oligomer yield increases when 9.1 ImpU, A(5') PPA mixtures react under the sane conditions. The oligomer yield and maximum chain length decreases with the structure of the added pyrophosphate in the order A(5')ppA>A(5')ppU>U-5' ppU. Structure analysis of individual oligomer fractions was performed by selective enzymatic hydrolyses followed by HPLC analysis of the products. The regioselectivity for 3',5'-bond formation is 80-90% in the 9:1 ImpU, A(5') ppA reaction, a percentage comparable to that observed in the 9:1 ImpA, A(5') ppA reaction. Oligomerization of ImpU is inhibited by addition of dA(5') ppdA, and MeppA. No oligomers containing A(5') ppU were products of the 9:1 ImpU, A(5') ppA reaction, a finding consistent with the simple addition of the ImpU to the A(5') ppA and not the rearrangement of an ImpU-A(5') ppA adduct. Concentrations of lysine or arginine which were close to that of the ImpU did not inhibit oligomer formation. Treatment of Naf-montmorillonite with 1 M arginine yielded arginine-montmorillonite, an amino acid-mineral adduct which did not catalyze ImpU oligomerization. Neither the 4-9 mers formed in the 9:1 ImpU, A(5') ppA reaction nor the 4-9 mers formed by the base hydrolysis of poly(U) served as templates for the formation of oligo(A)s.