UV-induced amino → imino hydrogen-atom transfer in 1-methylcytosine.

Monomers of 1-methylcytosine, isolated in low-temperature argon matrixes, were excited with narrowband tunable UV light. Irradiation at λ = 314 nm resulted in syn-anti photoisomerizations between the two minor imino-oxo forms of the compound, while the dominating amino-oxo form stayed intact. Subsequent irradiations at 308 nm (as well as at shorter wavelengths) led to the amino → imino hydrogen-atom transfer converting the biologically relevant amino-oxo tautomer into the imino-oxo forms. This is the first report on the amino → imino phototautomerism in 1-methylcytosine. The observed UV-induced syn-anti photoisomerizations within the imino-oxo forms of 1-methylcytosine were found to lead to photostationary states. The photostationary [syn]/[anti] population ratio depended on the wavelength of the exciting UV light. This dependence was not monotonous. Spectral indications of the open-ring isocyanate product, generated from the amino-oxo tautomer upon UV (λ ≤ 308 nm) irradiation, were also observed.