Department of Chemistry and Chemical Biology and the New York Center for Astrobiology and Rensselaer Polytechnic Institute, Troy, New York 12180, USA.
J Am Chem Soc. 2009 Sep 23;131(37):13369-74. doi: 10.1021/ja9036516.
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.
蒙脱土粘土催化核苷酸反应生成长达 50 个核苷酸的寡聚物。催化程度取决于蒙脱土晶格上负电荷的大小以及与之相关的阳离子数量。当原始蒙脱土中的阳离子被钠离子取代时，得到的 Na(+)-蒙脱土不会催化寡聚物的形成，因为它们会使蒙脱土片层之间的层间饱和，从而阻止了活化单体的结合。用稀盐酸处理蒙脱土可以用质子取代原始蒙脱土上的阳离子。质子化的蒙脱土被滴定至 pH 6-7，可作为 RNA 寡聚物形成的催化剂。滴定不会向蒙脱土片层的层间添加足够的钠离子来阻止活化单体进入。需要注意的是，非催化蒙脱土在其片层上具有更高的负电荷，这主要是由于蒙脱土晶格中的四价和三价元素分别被三价和二价金属离子自然取代。这些蒙脱土的负电荷较大，表现在滴定非催化蒙脱土所需的氢氧化钠量几乎是催化蒙脱土的两倍。吸附等温线表明，ImpA 的平衡结合最强，ImpU 的平衡结合最弱。在所研究的 22 种蒙脱土中，有 12 种是催化剂。这项研究为催化过程的机制提供了深入的了解。
