Mechanism of montmorillonite catalysis in the formation of RNA oligomers.
The montmorillonite clay-catalyzed reactions of nucleotides generate oligomers as long as 50-mers. The extent of catalysis depends on the magnitude of the negative charge on the montmorillonite lattice and the number of cations associated with it. When cations in raw montmorillonites are replaced by sodium ions, the resulting Na(+)-montmorillonite does not catalyze oligomer formation because they saturate the interlayers between the platelets of montmorillonites, which blocks the binding of the activated monomers. Treating the montmorillonite with dilute hydrochloric acid replaces the cations on the raw montmorillonite with protons. The protonated montmorillonite, titrated to pH 6-7, serves as a catalyst for the formation of RNA oligomers. The titration does not add sufficient sodium ions to the interlayers of the montmorillonite platelets to prevent the activated monomer from entering. It was noted that noncatalytic montmorillonites have a higher negative charge on their platelets that is due mainly to the natural substitution of the tetravalent and trivalent elements in the montmorillonite lattice with trivalent and divalent metal ions, respectively. The larger negative charge on these montmorillonites was demonstrated by the almost 2-fold greater amounts of sodium hydroxide needed to titrate noncatalytic montmorillonites as compared to the catalytic montmorillonites. Adsorption isotherms established that the equilibrium binding is strongest for ImpA and weakest for ImpU. Of the 22 montmorillonites investigated, 12 were catalysts. This research provides insight into the mechanism of the catalytic process.
Joshi
Prakash C
PC
Department of Chemistry and Chemical Biology and the New York Center for Astrobiology and Rensselaer Polytechnic Institute, Troy, New York 12180, USA.
Aldersley
Michael F
MF
Delano
John W
JW
Ferris
James P
JP
D001546:Bentonite / Q000737:chemistry*; D002384:Catalysis; D006863:Hydrogen-Ion Concentration; D008672:Metals, Alkali / Q000737:chemistry; D012313:RNA / Q000737:chemistry* / Q000378:metabolism; D017186:Titrimetry
D008672:Metals, Alkali; D001546:Bentonite; D012313:RNA