Preliminary results

Three different thio analogs of the natural RNA substrate have been studied experimentally: in the first one (pro-RpS), the pro-R non-bridging oxygen of the phosphate group is replaced by sulfur; in the second one (pro-RpS, pro-SpS) both the pro-R and pro-S non-bridging oxygen atoms are replaced by sulfur; in the last one the bridging O5' oxygen corresponding to the leaving group is replaced by sulfur. The pro-RpS analog is cleaved at a lower rate but the catalytic activity is partially restored by substituting the normal metal cofactors (rescue effect), Mg²⁺, by more thiophilic metal ions such as Mn²⁺ or Cd²⁺ [#!Scott2!#]. On the other hand, the dithioate analog (pro-RpS, pro-SpS) is still cleaved in presence of Mg²⁺ though at a lower rate.

The studies have been carried out on a small model representing a minimum active site of the hammerhead ribozyme. The RNA model used in the study includes a ribose moiety, a methyl-phosphate group and two solvated metal ions (Mg²⁺). Different metal configurations involving inner or outer-sphere coordinations have been explored. We have identified two configurations favorable to the in-line attack: the coordinations of the two metal cations were inferred from experimental data on the ``rescue effect'', and from ab initio calculations at the Hartree-Fock level (HF/3-21+G*).