CATALYSIS OF HYDROLYSIS AND NUCLEOPHILIC-SUBSTITUTION AT THE P-N BOND OF PHOSPHOIMIDAZOLIDE-ACTIVATED NUCLEOTIDES IN PHOSPHATE BUFFERS

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 less-than-or-equal-to pH less-than-or-equal-to 8.6. This degradation is been attributed to (i) nucleophilic substitution of the imidazolide and (ii) catalysis of the P-N bond hydrolysis by phosphate. 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 HPO-4(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 beta = 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 beta-nuc = 0.35 calculated for water, H2PO4-, HPO-4(2-), and hydroxide acting as nucleophiles indicates a more associative transition state for nucleotidyl (O2POR- with R = nucleoside) transfers than that observed for phosphoryl (PO-3(2-) transfers (beta-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 phosphate, assuming the concentrations of phosphate, P(i), on prebiotic Earth were similar to those in the present oceans ([P(i)] almost-equal-to 2.25 mu-M).