ENDOCYCLIC AND EXOCYCLIC CLEAVAGE OF PHOSPHORANE MONOANION - A DETAILED MECHANISM OF THE RNASE-A TRANSPHOSPHORYLATION STEP

The free energy profile for (HOC2H4PO4CH3)- --> (C2H4PO4)- + MeOH, modelling the transphosphorylation step in RNase A catalysis, is explored using ab initio molecular orbital calculations. Geometry optimizations are carried out at the Hartree-Fock level with the 3-21+G* basis set. The correlation energy is estimated with second-order Moller-Plesset theory and the 6-31+G* basis using fully optimized 3-21+G* geometries. The gas-phase reaction, (HOC2H4PO4CH3)- --> (C2H4PO4)- + MeOH, is computed to proceed via a stepwise mechanism involving a monoanionic pentacovalent intermediate which has to rotate about an equatorial P-O(H) bond to yield a phosphorane monoanion in a conformation activated for exocyclic cleavage. The stereochemistry of the gas-phase reaction is in accord with an in-line mechanism. The pentacovalent intermediates and transition states have a distorted trigonal-bipyramidal geometry, and the conformation of the transition states are activated for endocyclic and exocyclic cleavage. A detailed mechanism of the RNase A-catalyzed transphosphorylation step is presented and compared to other proposed mechanisms as well as to the nonenzymatic reaction in solution.