The singlet excited state behavior of tetrakis(4-N-methylpyridyl)porphine (T4MPyP) in the presence of the four mononucleotides of DNA in aqueous solution has been examined. Addition of the nucleotides of adenine, thymine, or cytosine to a solution containing T4MPyP results in an increase in the fluorescence intensity of the porphyrin, while addition of guanine substantially quenches the intensity. Optical absorption measurements demonstrate noncovalent interactions of the nucleotides with the porphyrin, resulting in bathochromic shifts in the Soret region. Binding constants for the base:porphyrin complexes were determined from steady-state absorption data to be 1743 +/- 68, 385 +/- 15, 2433 +/- 150, and 198 +/- 7 M(-1) for dAMP, dTMP, dGMP, and dCMP, respectively. The T4MPyP singlet state lifetime increases from 5.29 to 11.3, 10.3, and 7.8 ns in the presence of dAMP, dTMP, and dCMP, respectively. In the case of dGMP, the lifetime data was best fit to a Lorentzian distribution centered at 0.69 ns and a discrete component at 3 ns. The quenching was modeled using an excited state reaction mechanism coupled with ground state complexation. The quenching is attributed to electron transfer between guanine (the most reducing of the bases) and T4MPyP (i.e., reductive quenching of the porphyrin singlet state).