Singlet excited-state lifetimes of cytosine derivatives measured by femtosecond transient absorption

Lifetimes of the lowest excited singlet (SI) 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 S, 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 S-1 --> S-N transition. The S, lifetime of the important minor base 5-methylcytosine (m(5)Cyt) 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 m(5)Cyt at low pH led to a modest decrease in their S, lifetimes. On the other hand, deprotonation of Cyt and m(5)Cyt significantly increased the lifetime of their respective S, states. These trends support the intermediacy of the n,pi* state localized on the carbonyl oxygen in the nonradiative decay mechanism of Cyt. Longer S, lifetimes were observed for 5-fluorocytosine and N-4-acetylcytosine. Collectively, these results illustrate the great potential of femtosecond laser spectroscopy for investigating excited-state dynamics in DNA and DNA components.