| Abstract
| - The DNA base adenine and four monomethylated adenines were studied in solution at roomtemperature by femtosecond pump−probe spectroscopy. Transient absorption at visible probe wavelengthswas used to directly observe relaxation of the lowest excited singlet state (S1 state) populated by a UVpump pulse. In H2O, transient absorption signals from adenine decay biexponentially with lifetimes of 0.18± 0.03 ps and 8.8 ± 1.2 ps. In contrast, signals from monomethylated adenines decay monoexponentially.The S1 lifetimes of 1-, 3-, and 9-methyladenine are similar to one another and are all below 300 fs, while7-methyladenine has a significantly longer lifetime (τ = 4.23 ± 0.13 ps). On this basis, the biexponentialsignal of adenine is assigned to an equilibrium mixture of the 7H- and 9H-amino tautomers. Excited-stateabsorption (ESA) by 9-methyladenine is 50% stronger than by 7-methyladenine. Assuming that ESA bythe corresponding tautomers of adenine is unchanged, we estimate the population of 7H-adenine in H2Oat room temperature to be 22 ± 4% (estimated standard deviation). To understand how the environmentaffects nonradiative decay, we performed the first solvent-dependent study of nucleobase dynamics onthe ultrafast time scale. In acetonitrile, both lowest energy tautomers of adenine are present in roughlysimilar proportions as in water. The lifetimes of the 9-substituted adenines depend somewhat more sensitivelyon the solvent than those of the 7-substituted adenines. Transient signals for adenine in H2O and D2O areidentical. These solvent effects strongly suggest that excited-state tautomerization is not an importantnonradiative decay pathway. Instead, the data are most consistent with electronic energy relaxation due tostate crossings between the optically prepared 1ππ* state and one or more 1nπ* states and the electronicground state. The pattern of lifetimes measured for the monomethylated adenines suggests a special rolefor the 1nπ* state associated with the N7 electron lone pair.
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