Limiting concentrations of activated mononucleotides necessary for poly(C)-directed elongation of oligoguanylates

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Abstract
Selected imidazolide-activated nucleotides have been subjected to hydrolysis under conditions similar to those that favor their template-directed oligomerization. Rate constants of hydrolysis of the P−N bond in guanosine 5′-monophosphate 2-methylimidazolide (2-MeImpG) and in guanosine 5′-monophosphate imidazolide (ImpG), kh, have been determined in the presence/absence of magnesium ion as a function of temperature and polycytidylate [poly(C)] concentration. Using the rate constant of hydrolysis of 2-MeImpG and the rate constant of elongation, i.e., the reaction of an oligoguanylate with 2-MeImpG in the presence of poly(C) acting as template, the limiting concentration of 2-MeImpG necessary for oligonucleotide elongation to compete with hydrolysis can be calculated. The limiting concentration is defined as the initial concentration of monomer that results in its equal consumption by hydrolysis and by elongation. These limiting concentrations of 2-MeImpG are found to be 1.7 mM at 37°C and 0.36 mM at 1°C. Boundary conditions in the form of limiting concentration of activated nucleotide may be used to evaluate a prebiotic model for chemical synthesis of biopolymers. For instance, the limiting concentration of monomer can be used as a basis of comparison among catalytic, but nonenzymatic, RNA-type systems. We also determined the rate constant of dimerization of 2-MeImpG, k2=0.45±0.06 M−1 h−1 in the absence of poly(C), and 0.45±0.06≤k2≤0.97±0.13 M−1 h−1 in its presence at 37°C and pH 7.95. This dimerization, as well as the trimerization of 2-MeImpG, which represent the first steps in the oligomerization reaction, are markedly slower than the elongation of longer oligoguanylates, (pG) n n>6. This means that in the presence of low concentrations of 2-MeImpG (≤1.7 mM) the system directs the elongation of longer oligomers more efficiently than the formation of short oligomers such as dimers and trimers. These results will be discussed as a possible example of chemical selection in template-directed reactions of nucleotides.