Ultraviolet light is strongly absorbed by DNA, producing excited electronic states that sometimes initiate damaging photochemical reactions. Fully mapping the reactive and nonreactive decay pathways available to excited electronic states in DNA is a decades-old quest. Progress toward this goal has accelerated rapidly in recent years, in large measure because of ultrafast laser experiments. Here we review recent discoveries and controversies concerning the nature and dynamics of excited states in DNA model systems in solution. Nonradiative decay by single, solvated nucleotides occurs primarily on the subpicosecond timescale. Surprisingly, excess electronic energy relaxes one or two orders of magnitude more slowly in DNA oligo- and polynucleotides. Highly efficient nonradiative decay pathways guarantee that most excited states do not lead to deleterious reactions but instead relax back to the electronic ground state. Understanding how the spatial organization of the bases controls the relaxation of excess electronic energy in the double helix and in alternative structures is currently one of the most exciting challenges in the field.

Excited electronic states created by UV excitation of the diribonucleoside monophosphates ApA, ApG, ApC, ApU, and CpG were studied by the femtosecond transient-absorption technique. Bleach recovery signals recorded at 252 nm show that long-lived excited states are formed in all five dinucleosides. The lifetimes of these states exceed those measured in equimolar mixtures of the constituent mononucleotides by one to two orders of magnitude, indicating that electronic coupling between proximal nucleobases dramatically slows the relaxation of excess electronic energy. The decay rates of the long-lived states decrease with increasing energy of the charge-transfer state produced by transferring an electron from one base to another. The charge-transfer character of the long-lived states revealed by this analysis supports their assignment to excimer or exciplex states. Identical bleach recovery signals were seen for ApA, (A) 4, and poly(A) at delay times >10 ps after photoexcitation. This indicates that excited states localized on a stack of just two bases are the common trap states independent of the number of stacked nucleotides. The fraction of initial excitations that decay to long-lived exciplex states is approximately equal to the fraction of stacked bases determined by NMR measurements. This supports a model in which excitations associated with two stacked bases decay to exciplex states, whereas excitations in unstacked bases decay via ultrafast internal conversion. These results establish the importance of charge transferquenching pathways for UV-irradiated RNA and DNA in roomtemperature solution.DNA excited states ͉ DNA photodamage ͉ electronic structure ͉ femtosecond spectroscopy E xcited electronic states created in DNA by UV light have been studied since the 1960s but have received a great deal of attention recently because of advances in experiment and theory (1). These efforts are motivated by the desire to understand the photoreactions behind the genetic damage induced by UV light. There is also increasing interest in DNA and other arrays of -stacked chromophores as materials for optoelectronic applications (2-4).Striking differences have emerged between the dynamics of excited states in single bases and those in base assemblies. Excited states of single nucleobases and mononucleotides decay to the ground state primarily by ultrafast internal conversion in several hundred femtoseconds (5, 6). The additional degrees of freedom in polymeric DNA might be expected to quench singlet excited states even more rapidly, but the lifetimes actually increase dramatically. Femtosecond transient absorption measurements reveal that excited states in oligo-and polynucleotides relax in tens to hundreds of picoseconds (1, 7-10).Although there is consensus that electronic relaxation can take place orders of magnitude more slowly in DNA polymers, contradictory assignments have been proposed for the long-lived states. assigned the long-lived excited states seen in (dA) 18 , (dA) 18 ⅐(dT) 18 , and (dAdT) 9 ⅐(dAdT) 9 , to exci...

UV radiation creates excited states in DNA that lead to mutagenic photoproducts. Photoexcitation of single-stranded DNA can transfer an electron between stacked bases, but the fate of excited states in the double helix has been intensely debated. Here, photoinduced interstrand proton transfer (PT) triggered by intrastrand electron transfer (ET) is detected for the first time by time-resolved vibrational spectroscopy and quantum mechanical calculations. Long-lived excited states are shown to be oppositely charged base pair radical ions. In two of the duplexes, the base pair radical anions are present as tautomers formed by interstrand PT. Charge recombination occurs on the picosecond time scale preventing the accumulation of damaging radicals or mutagenic tautomers.

Understanding how base pairing and stacking mediate the dissipation of electronic energy in DNA is essential for understanding the initial steps in UV photodamage. Excited states of individual DNA bases decay in solution to the ground state either directly by ultrafast internal conversion or, in the case of pyrimidine bases, indirectly via longer-lived triplet and 1 nπ*states. 1 The base stacking present in single-and double-stranded DNA causes these systems to have dramatically longer excited-state lifetimes compared to monomeric bases. [2][3][4][5] Femtosecond transient absorption experiments have detected long-lived excited states in numerous π-stacked systems from dinucleosides 5 to G-quadruplexes. 1 These long-lived states are formed in high yields only when π-stacking is present and are observed in stacks comprised of both AT and GC base pairs. 3,4,6 A recent model assigns these states to charge-transfer (CT) excited states or exciplexes formed between π-stacked bases that arise from initially populated Frenkel exciton states. [3][4][5] Although base stacking is clearly a requirement for the formation of these long-lived states, the consequences of base pairing on DNA excited-state dynamics is still highly uncertain and is the motivation for this study.The possibility that UV mutagenicity is a consequence of proton transfer between paired bases was proposed many years ago. 7 More recently, proton transfer was suggested to be responsible for the photostability of DNA. [8][9][10] Using IR-UV hole-burning spectroscopy, Abo-Riziq et al. observed a broad UV spectrum for isolated Watson Crick (WC) GC base pairs in the gas phase, whereas sharp UV spectra were observed for non-WC GC base pairing combinations. 11 The broad UV spectrum unique to GC base pairs in the WC conformation was suggested to result from lifetime shortening due to a proton transfer mechanism. Subsequent ab initio calculations implicated an ultrafast deactivation pathway between the excited 1 ππ* state and ground state mediated by proton transfer. 9,12 Recently, Schwalb and Temps reported shortened fluorescence lifetimes in isolated WC GC base pair analogs relative to the monomers in chloroform using fluorescence up-conversion spectroscopy. 13 However, because these model systems lack π-stacking interactions, their relevance to duplex DNA is uncertain.Here, we investigate the effect of base pairing on excited-state dynamics in GC-containing duplexes when base stacking is also present. This study is timely given recent reports of interand intrastrand charge transfer states in computational studies of excited states in doublestranded DNA. 9,12,[14][15][16] We report the discovery of a pronounced isotope effect on excited- Figure 1. Figure 1 shows that there is significantly faster ground-state recovery for d(GC) 9 ·d(GC) 9 in H 2 O compared to D 2 O. In contrast, only a minor isotope effect (Figure 1 inset) is observed on the more rapidly decaying signals for an equimolar mixture of the 5′-mononucleotides, CMP and GMP. Circular dichroism spectr...

Excited states in double-stranded oligonucleotides containing G.C base pairs were studied by femtosecond transient absorption spectroscopy. Relaxation to the electronic ground state occurs about 10 times more slowly in the duplexes and hairpins studied on average than in the individual mononucleotides of G and C. Detection of long-lived excited states in G.C oligonucleotides complements the earlier observation of slow ground-state recovery in A.T DNA, showing that excited states with picosecond lifetimes are formed in DNAs containing either kind of base pair. The results show further that Watson-Crick G.C base pairs in these base-paired and base-stacked duplexes do not enable subpicosecond relaxation to the electronic ground state. A model is proposed in which fluorescent exciton states decay rapidly and irreversibly to dark exciplex states. This model explains the seemingly contradictory observations of femtosecond fluorescence and slower, picosecond recovery of the ground-state population.