Putative phototautomerization of 4-thiouridine in the S2 excited state revealed by fluorescence study using picosecond laser spectroscopy

The stationary and dynamic properties of S2 → S0 fluorescence of 2′,3′,5′-tri-O-acetyl-4-thiouridine (TU) in acetonitrile solution at room temperature have been studied. In the emission spectra of the sample, measured upon excitation with picosecond laser pulses, changes in fluorescence intensity and emission band shape have been observed with increasing time of excitation. The fluorescence intensity increases and the band maximum moves to the shorter wavelength until the steady-state condition is achieved. These changes are fully reversible indicating that they are not related to the formation of stable photoproducts. No such changes have been observed in the fluorescence spectrum recorded upon excitation of the same sample of TU with continuous light from xenon lamp. Under the latter conditions the fluorescence is very weak (ϕF = 1 × 10−4) and the band exhibit a maximum at λmaxF=420 nm. The decay kinetics was measured using a two-stage method in order to estimate the number of emitting species and determine the parameters characterizing their emission. Analysis of the experimental fluorescence decays at λF = 400, 430 and 450 nm measured upon various time intervals (<20 min) of the sample excitation with laser pulses has shown that the best fit of experimental to theoretical curve is obtained assuming the presence of three emitting species with lifetimes of τ1 = 5 ± 1 ps, τ2 = 80 ± 20 ps and τ3 = 600 ± 100 ps. In contrast to the lifetimes the values of fractional coefficients depend on λF and the time of measurement. The results of the fluorescence study of TU with picosecond laser spectroscopy are discussed in terms of phototautomerization of TU (2-keto-4-thione) in the S2 state leading to its two possible tautomers: 4-thione, 2-hydroxy form in the S2 state and 4-thiol, 2-keto form in the S1 state. The models of the tautomers of TU were synthesized and their absorption and emission features were determined.