Gas-Phase Cytosine and Cytosine-N₁-Derivatives Have 0.1–1 ns Lifetimes Near the S₁ State Minimum

Abstract

Ultraviolet radiative damage to DNA is inefficient because of the ultrafast S1 ⇝ S0 internal conversion of its nucleobases. Using picosecond pump-ionization delay measurements, we find that the S1((1)ππ*) state vibrationless lifetime of gas-phase keto-amino cytosine (Cyt) is τ = 730 ps or ∼ 700 times longer than that measured by femtosecond pump-probe ionization at higher vibrational excess energy, Eexc. N1-Alkylation increases the S1 lifetime up to τ = 1030 ps for N1-ethyl-Cyt but decreases it to 100 ps for N1-isopropyl-Cyt. Increasing the vibrational energy to Eexc = 300-550 cm(-1) decreases the lifetimes to 20-30 ps. The nonradiative dynamics of S1 cytosine is not solely a property of the amino-pyrimidinone chromophore but is strongly influenced by the N1-substituent. Correlated excited-state calculations predict that the gap between the S2((1)nOπ*) and S1((1)ππ*) states decreases along the series of N1-derivatives, thereby influencing the S1 state lifetime.

The excited-state structure, vibrations, lifetimes, and nonradiative dynamics of jet-cooled 1-methylcytosine

“…The lowest-energy vibronic band in the ps R2PI spectrum at 0 0 0 + 66 cm −1 is also weak. We have recently reported the lifetime as τ = 600 ± 290 ps, 15 but by measuring 8 separate transients on different days we have decreased the uncertainty by nearly a factor of two, the revised value is τ = 450 ± 160 ps. At +200 cm −1 , the lifetime decreases to 230 ps and continues to decrease up to the shortest measurable lifetime τ = 15 ps at 400 cm −1 above the 0 0 0 band.…”

“…8 Using ps pump/ionization techniques, Blaser et al have recently measured the lifetime at the Cyt S 1 0 0 0 band as τ = 730 ps. 15 In the R2PI spectra, the short lifetimes are reflected in a break-off of the sharp vibronic bands at vibrational excess energies E exc ∼ 500 cm −1 above the 0 0 0 transition. [8][9][10] When exciting Cyt at its 0 0 0 band (313 nm, 31 900 cm −1 ) followed by ionization at 157 nm, an excited-state signal with a lifetime of ∼270 ns is also observed; this signal has been attributed to a triplet state.…”

How nature covers its bases

“…Leutwyler and co-workers reported the wavelength dependence of the excited state dynamics of cytosine and a series of derivatives, showing how the barrier for internal conversion is affected by structure. 45 The limited range of ns REMPI spectra of other nucleobases, such as adenine, 27,[93][94][95] is also indicative of the existence of barriers to internal conversion pathways.…”

Computing the Absorption and Emission Spectra of 5-Methylcytidine in Different Solvents: A Test-Case for Different Solvation Models

“…16,18,19 Besides the possible role of deamination processes, 15,20 hotspots containing 5-methylcytosine are up to 5 to 15-fold more sensitive to pyrimidine photodimerization upon ultraviolet irradiation compared to C. [21][22][23] The steady-state and Time-resolved (TR) spectroscopic properties of 5-methylcytosine have been tackled by several studies. [24][25][26][27] From a theoretical point of view, other C derivatives have received more attention [27][28][29][30][31][32][33][34] and the works on 5-methylcytosine are instead scarce and limited to the gas phase. 31,35,36 Eventually, it is also apparent that interaction with the environment (including the solvent as well as the sugar moiety) is mandatory to appreciate and understand all the effects recorded in vivo.…”