Reaction of arylnitrenium ions with guanine derivatives:: N1-methylguanosine and N2, N2-dimethylguanosine

A prior flash photolysis study of the direct reaction of arylnitrenium ions with 2'-deoxyguanosine identified a second intermediate that grew in as the transient nitrenium ion reacted with the nucleoside. This intermediate was identified as the the product of the addition of the nitrenium ion to the C-8 position of guanine prior to loss of the C-8 proton - the C-8 intermediate. A feature of the C-8 intermediate is that it exists in acid-base forms. This behavior was evident in both a spectroscopic analysis as well as in the rate-pH profile, which showed a break around pH 4 from a pH-independent reaction to a reaction that was first-order in H+. The present study was designed to identify the structure of the conjugate base form. This involved a kinetic study of the decay of the C-8 intermediate derived from the reaction of the 2-fluorenylnitrenium ion with N-1-methylguanosine and N-2,N-2-dimethylguanosine. The rationale was that the former is unable to lose the N-1 proton, while the latter cannot deprotonate at the NH2 group. The rate-pH profiles clearly show that it is the N-1 proton that is acidic. The rate constants for the C-8 intermediate of N-2,N-2-dimethylguanosine show the same downward break observed with 2'-deoxyguanosine and guanosine associated with conversion to the conjugate base form. In contrast, the rate constants for the N-1-methylguanosine intermediate are independent of pH. Rate constants for the reaction forming the C-8 intermediate are also reported. These show that the reaction of nitrenium ions with the N-2,N-2-dimethylguanine derivative is significantly faster (except where the reactions are diffusion controlled). This is consistent with the initial step of the reaction of an arylnitrenium ion and guanine occurring by direct addition at C-8. The developing positive charge in such a reaction can be delocalized to the C-2 position where pi donors such as NH2 and NMe2 can exert a stabilizing effect.