Catalysis of Hydrolysis and Nucleophilic Substitution at the P-N Bond of Phosphoimidazolide-Activated Nucleotides in Phosphate Buffers

Article abstract of DOI:10.1021/jo00004a032

Phosphoimidazolide-activated derivatives of guanosine and cytidine 5'-monophosphates, henceforth called ImpN's, exhibit enhanced rates of degradation in the presence of aqueous inorganic phosphate in the range 4.0 (*) pH (*) 8.6.This degradation is been attributed to (i) nucleophilic substitution of the imidazolide and (ii) catalysis of the P-N bond hydrolysis by phophate.The first reaction results in the formation of nucleoside 5'-diphosphate and the second in nucleoside 5'-monophosphate.Analysis of the observed rates as well as the product ratios as a function of pH and phosphate concentration allow distinction between various mechanistic possibilities.The results show that both H2PO4(-) and HPO4(2-) participate in both hydrolysis and nucleophilic substitution.Statistically corrected bimolecular rate constants indicate that the dianion is 4 times more effective as a general base than the monoanion, and 8 times more effective as nucleophile.The low Bronsted value β = 0.15 calculated for these phosphate species, presumed to act as general bases in facilitating water attack, is consistent with the fact that catalysis of the hydrolysis of the P-N bond in ImpN's has not been detected before.The β_nuc = 0.35 calculated for water, H2PO4(-), HPO4(2-), and hydroxide acting as nucleophiles indicates a more associative transition state for nucleotidyl (O2POR(-) with R = nucleoside) transfers than that observed for phosphoryl (PO3(2-)) transfers (β_nuc = 0.25).With respect to the stability/reactivity of ImpN's under prebiotic conditions, our study shows that these materials would not suffer additional degradation due to inorganic phophate, assuming the concentrations of phosphate, P_j, on prebiotic Earth were similar to those in the present oceans (j> ca. 2.25 μM).