Singlet Excited-state Lifetimes of Cytosine Derivatives Measured by Femtosecond Transient Absorption¶

Abstract

Lifetimes of the lowest excited singlet (S1) electronic states of various derivatives of the pyrimidine nucleobase cytosine (Cyt) were measured by the femtosecond transient absorption technique. The bases were excited in room-temperature aqueous solution at 265 nm using approximately 200 fs pump pulses from a titanium-sapphire laser system. The decay of excited-state absorption (ESA) at visible probe wavelengths was used to determine the S1 lifetimes of a variety of modified Cyt compounds at different pH values by global fitting. Identical lifetimes were observed for Cyt and cytidine (Cyd) within experimental uncertainty, but ESA by the ribonucleoside was considerably stronger, suggesting that the ribose group increases the oscillator strength of the S1 --> SN transition. The S1 lifetime of the important minor base 5-methylcytosine (m5Cyt) is 7.2 +/- 0.4 ps at pH 6.8. The same lifetime was measured for the ribonucleoside 5-methylcytidine, but sugar substitution again increased the strength of the ESA signal. Protonation of Cyd and m5Cyt at low pH led to a modest decrease in their S1 lifetimes. On the other hand, deprotonation of Cyt and m5Cyt significantly increased the lifetime of their respective S1 states. These trends support the intermediacy of the n,pi* state localized on the carbonyl oxygen in the nonradiative decay mechanism of Cyt. Longer S1 lifetimes were observed for 5-fluorocytosine and N4-acetylcytosine. Collectively, these results illustrate the great potential of femtosecond laser spectroscopy for investigating excited-state dynamics in DNA and DNA components.

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

“…This finding is partly supported by the experimental result that cytidine at low pH still has an excited-state lifetime in the subpicosecond range despite the loss of the 1 n N π* state by protonation at N3. 65 Our calculated barrier height is consistent with previously reported theoretical values of 0.14 eV by Kistler et al 30 and 0.2 eV by Blancafort 31 . The barrier height reported by Merchán and Serrano-Andrés 24 is relatively high (0.52 eV), which is possibly attributed to the fact that the CASSCF method is used in geometry optimization.…”

Ultrafast IR spectroscopy of the short-lived transients formed by UV excitation of cytosine derivatives

“…12 The possibility that this species might be due to a protonated or deprotonated excited state has also been examined. However, the ps-TRIR transient behaviour we observe is the same at pH 6.9 and pH 8.5, as also are the reported transient visible data, 13 …”

Ultrafast excited-state dynamics of RNA and DNA C tracts

“…3 - Fig. 7 demonstrate that excited electronic states in RNA and DNA C tracts decay one to three orders of magnitude more slowly than the excited singlet state of monomeric cytidine, which has a lifetime of ~1 ps [37]. Two classes of long-lived excitations are evident in these experiments.…”

“…This indicates that protonated cytosine residues are not inherently more fluorescent than the neutral molecules. Indeed, the lifetime of protonated cytidine is 630 ± 60 fs compared to ~1 ps for the neutral nucleoside [37]. In contrast to guanosine, where protonation dramatically increases the excited state lifetime [3], protonation of the cytidine monomer thus actually reduces the excited state lifetime in aqueous solution [37].…”