The cleavage site of the Neurospora VS RNA ribozyme is located in a separate hairpin domain containing a hexanucleotide internal loop with an A-C mismatch and two adjacent G-A mismatches. The solution structure of the internal loop and helix la of the ribozyme substrate hairpin has been determined by nuclear magnetic resonance (NMR) spectroscopy. The 2 nt in the internal loop, flanking the cleavage site, a guanine and adenine, are involved in two sheared G.A base pairs similar to the magnesium ion-binding site of the hammerhead ribozyme. Adjacent to the tandem G.A base pairs, the adenine and cytidine, which are important for cleavage, form a noncanonical wobble A+-C base pair. The dynamic properties of the internal loop and details of the high-resolution structure support the view that the hairpin structure represents a ground state, which has to undergo a conformational change prior to cleavage. Results of chemical modification and mutagenesis data of the Neurospora VS RNA ribozyme can be explained in context with the present three-dimensional structure.

New cationic, square-planar, ethene complexes [(Rbpa)RhI(C2H4)]+ [2a]--[2c]+ (Rbpa = N-alkyl-N,N-di(2-pyridylmethyl)amine; [2a]+: alkyl =R=Me; [2b]+: R = Bu; [2c]+: R = Bz) have been selectively oxygenated in acetonitrile by aqueous hydrogen peroxide to 2-rhoda(III)oxetanes with a labile acetonitrile ligand, [(Rbpa)RhIII(kappa2-C,O-CH2CH2O-)(MeCN)]+, [3a]+-[3c]+. The rate of elimination of acetaldehyde from [(Rbpa)RhIII(kappa2-C,O-CH2CH2O-)(MeCN)]+ increases in the order R = Me< R = Bu< R = Bz. Elimination of acetaldehyde from [(Bzbpa)RhIII(kappa2-C,O-CH2CH2O)(MeCN)]+ [3c]+, in the presence of ethene results in regeneration of ethene complex [(Bzbpa)RhI(C2H4)]+ [2c]+, and closes a catalytic cycle. In the presence of Z,Z-1,5-cyclooctadiene (cod) the corresponding cod complex [(Bzbpa)RhI(cod)]+ [6c]+ is formed. Further oxidation of [3c]+ by H2O2 results in the transient formylmethyl-hydroxy complex [(Bzbpa)RhIII(OH)[kappa1-C-CH2C(O)H]]+ [5c]+.

New cationic, square-planar, ethene complexes [(Rbpa)Rh I (C 2 H 4 )] [2 a] ± [2 c] (Rbpa N-alkyl-N,Ndi(2-pyridylmethyl)amine; [2 a] : alkyl R Me; [2 b] : R Bu; [2 c] : R Bz) have been selectively oxygenated in acetonitrile by aqueous hydrogen peroxide to 2-rhoda(iii)oxetanes with a labile acetonitrile ligand, [in the presence of ethene results in re-generation of ethene complex [(Bzbpa)-Rh I (C 2 H 4 )] [2 c] , and closes a catalytic cycle. In the presence of Z,Z-1,5-cyclooctadiene (cod) the corresponding cod complex [(Bzbpa)Rh I (cod)] [6c] is formed. Further oxidation of [3 c] by H 2 O 2 results in the transient formylmethyl-hydroxy complex [(Bzbpa)-Rh III (OH){k 1 -C-CH 2 C(O)H}] [5 c] .