Kinetics and mechanism of carbon-8 methylation of purine bases and nucleosides by methyl radical.

Abstract

The kinetics of homolytic methylation of the model purine compound caffeine at carbon-8 were determined as a function of several reaction variables. The methyl radical was generated from tert-butyl peracetate (BPA) either thermally (65 to 95/sup 0/C) or photochemically (greater than 300 nm, 25/sup 0/C). The thermal reaction k (25/sup 0/C) was found to be 3.09 x 10/sup -8/ s/sup -1/ from the linear log k (pseudo-first-order) vs. l/T plot. The light reactions using the 450- and 1200-W mercury lamps proceeded with k (25/sup 0/C) 450- and 2160-fold greater, respectively. The derived activation energies are consistent with an S/sub E/Ar reaction. Increasing the concentration of caffeine from 0.25 M to 1.67 M in the presence of 3 molar equiv of BPA did not cause any side reaction. The pH-rate profile can be predicted by rate equations, which are derived on the basis of an electrophilic substitution occurring on the free base and conjugate acid of a heteroaromatic system. A competition study using tetrahydrofuran reveals the presence of a radical sigma complex IIIa and a charge transfer complex IIIb as intermediates for methylation under neutral and acidic conditions, respectively. Their rate-determining nature was indicated by the small positive kinetic isotope effectmore » and the inverse solvent isotope effects: k/sub H/sub 3/O/sup +///k/sub D/sub 3/O/sup +// = 0.87 and k/sub H/sub 2/O//k/sub D/sub 2/O/ = 0.32. Thus, in acidic medium, a preequilibrium proton transfer to form the caffeine conjugate acid precedes the rate-controlling formation of IIIb. In neutral solution, the rate-determining step appears to be the protonation of the radical nitrogen in IIIa converting it to III. The acid-catalyzed caffeine-BPA reaction was shown to be general for other purines such as adenine, adenosine, guanine, guanosine, hypoxanthine, and inosine.« less