Toward the Selection of Ribozymes for 1,3-Dipolar Cycloaddition Reactions

In vitro selection from combinatorial RNA libraries has repeatedly been used to study the catalytic and binding potential of nucleic acids. These selections not only led to RNA sequences catalyzing transformations known from metabolic pathways but also generated novel ribozymes for typical organic reactions. We were interested in 1,3-dipolar cycloaddition reactions, which are important tools for the formation of heterocyclic systems in organic chemistry and might also be found in the hypothetic RNA world. Here we describe our strategy and experiments to isolate RNA molecules catalyzing a 1,3-dipolar cycloaddition between nitrile oxides and an acrylate conjugated to RNA. We used direct selection with linker-coupled reactants, which has previously allowed the generation of true trans-acting catalysts for bimolecular reactions. A photocleavable linker was introduced to provide for a more stringent selection criterion. The 1,3-dipolar cycloaddition reaction was established in aqueous solution using a modified dinucleotide that was tethered to the dipolarophilic substrate. Two selection protocols were established, namely, a low-stringency affinity-based selection protocol, and a high-stringency procedure using the photocleavable moiety. In neither case was an increased activity toward the desired reaction obtained after 15 and 11 selection rounds, respectively. The resulting pools of RNA from several rounds were investigated both in cis and in trans. The limitations of this selection methodology are discussed in comparison with other catalysts for dipolar cycloadditions and, also, with respect to the unconventional substrates used.     

Reversal of Copper Inhibition in Chloroplast Reactions by Manganese

In the Mehler reaction, a Hill reaction utilizing molecular oxygen as the electron acceptor, rates of net oxygen uptake are stimulated by added manganous ions. Both whole cell photosynthesis and the Mehler reaction are inhibited by copper. Copper inhibition of the Mehler reaction can be reversed by manganese salts. Glutathione. which alone has no effect on Mehler reaction rates, enhances the effect of manganese in reversing copper inhibition. The effects of added Cu²⁺, Cu²⁺ and Mn²⁺, or Cu²⁺, Mn²⁺, and glutathione exhibit no induction phenomena when measured manometrically. Furthermore, the order of addition of these factors is unimportant: final rates are dependent only on the composition of reaction mixtures. Compared to the Mehler reaction, conventional Hill reactions are less sensitive to copper poisoning, while certain chloroplast mediated photoxidations (e.g. the photoxidation of diketogulonic acid) are far more sensitive. In all of the chloroplast mediated photoreactions tested, manganese is effective in reducing the sensitivity to copper poisoning.