Excited-State Dynamics of Cytosine Tautomers

Abstract

We report the relaxation dynamics of keto and enol or keto-imino cytosine, photoexcited in the wavelength range of 260-290 nm. Three transients with femtosecond to hundreds of picoseconds lifetimes are observed for the biologically relevant keto tautomer and are assigned to internal conversion and excited-state tautomerization. Only two transients with femtosecond and picosecond lifetimes are identified for the enol or keto-imino tautomer and are assigned to internal conversion processes. The results are discussed in the context of published ab initio theory.

Photophysics of cytosine tautomers: new insights into the nonradiative decay mechanisms from MS-CASPT2 potential energy calculations and excited-state molecular dynamics simulations

“…The molecular structure of this minimum is close to the ( 1 π N8 π*/gs) CI structure, but the S 0 and S 1 energies exhibit a large energy gap of about 1.5 eV. Based on this result, Kosma et al 7 attributed the long-time component of τ > 19 ps observed in their experimental spectra to a population trap in the S 1 minimum. However, our calculations contradict this assumption, and instead show that an extremely efficient deactivation pathway exists for the imino form.…”

“…The pump-probe ionization spectrum by Kosma et al 7 can also be interpreted basically in the same way. As mentioned in the Introduction, there is one notable difference from the experiments by Ho et al 8 ; Kosma et al 7 observed another slow component of the decay with lifetime in the range from 19 up to ≥ 150 ps.…”

“…These experimental studies by Kosma et al 7 and Ho et al 8 motivate us to theoretically investigate the excited-state potential energy surfaces of the keto, imino, and enol tautomers of cytosine on the equal footing and scrutinize the proposed scenarios on the multiple components of the observed spectra. Most of the previous computational studies have focused on the keto form of cytosine and its derivatives such as 5-fluorocytosine because of its biological significance.…”

“…3 The photophysics of isolated cytosine has been the subject of extensive experimental studies with time-resolved spectroscopy. [4][5][6][7][8] Cytosine in the gas phase is of special interest among the nucleic acid bases in various environments, because several tautomeric forms are likely to coexist. Experimental [9][10][11][12][13][14][15][16][17] and theoretical [11][12][13]18 studies have revealed the coexistence of at least three lowest-energy tautomers of isolated cytosine: keto (amino-keto), imino (imino-keto), and enol (amino-enol) forms.…”

“…These findings are in contrast to cytosine in aqueous solution, where the canonical keto form is calculated to be much more stable than other tautomers 12,19 and only this form is likely to contribute to the experimental spectrum. 20,21 Recently, Kosma et al 7 observed appreciable dependence of decay profiles on the excitation wavelength in the femtosecond time-resolved pump-probe ionization spectroscopy of isolated cytosine, and attributed this remarkable observation to the coexistence of the keto, imino, and enol tautomers. With the excitation wavelengths of 290, 280, 270, and 267 nm, the observed transients were decomposed into three components with lifetimes of τ 1 < 0.25 ps, τ 2 = 1.1 − 2.3 ps, and τ 3 ≥ 19 ps.…”