1H-NMR lifetimes of cytosine interactions with the DNA melting probe, methylmercury.

Studies on monomeric cytosine were undertaken to establish a kinetic foundation for the progressive melting of DNA by the mutagen, methylmercury. The reversible displacement of protons by methylmercury at the amino group of cytosine is slow on the 1H-NMR time scale at 100 and 360 MHz. Exchange coupled resonances are produced, not only for all protons of the free- and mercurated amino species, but for the rotational isomers of the latter. These spectra provide for assignment of all exchange-coupled resonances, selection of resonances providing mercuration rates from line shape and measurement of pH-dependent reciprocal lifetimes of the free-amino species (less than or equal to 6 s-1 at pH 3 and 15 s-1 at pH 4). Evidence is presented for the existence of an amino-mercurated species of cytidine thus far not reported (formation constant, 10.