The mechanism that nature applies to dissipate excess energy from solar UV light absorption in DNA is fundamental, because its efficiency determines the vulnerability of all genetic material to photodamage and subsequent mutations. Using femtosecond time-resolved broadband spectroscopy, we have traced the electronic excitation in both time and space along the base stack in a series of single-stranded and double-stranded DNA oligonucleotides. The obtained results demonstrate not only the presence of delocalized electronic domains (excitons) as a result of UV light absorption, but also reveal the spatial extent of the excitons.excitons ͉ femtosecond ͉ spectroscopy ͉ photophysics S ince the early days of DNA photochemistry there has been considerable speculation about the nature of the excited singlet states of the bases, i.e., whether they are localized on a single base or delocalized over several bases (1-4). The answer to this question is paramount for understanding the mechanism of UV light-induced chemical reactions that may result in carcinogenic mutations (5-7). Although the rich photochemistry of nucleic acids has been extensively studied (5, 6), the precursor states for chemical reactions damaging the genome are not well characterized. In other words, the nature of these states and the concomitant electronic and nuclear dynamics must be investigated to understand how excitation energy is transformed and dissipated within the double-helix. With respect to the biological consequences of UV light-induced phenomena in DNA most efforts were spent on the investigation of charge migration processes through the base stack (8, 9). Although photoinduced charge transfer is initiated by electronic excitation, no clear picture has yet emerged of how to describe electronic excitations in DNA. Circular dichroism measurements on single-stranded homoadenines and A⅐T duplexes indicate significant electronic coupling between the adjacent bases in the stack (10). On the other hand, the fact that the DNA UV spectra closely resemble the sum of the spectra of the constituent bases was interpreted as evidence for strictly localized excited states (1). Since the pioneering work of Eisinger et al. (2), who reported a broad red-shifted emission in high-concentrated nucleoside solutions at low temperatures, excimers and exciplexes have been invoked in discussions on nucleic acid (11-17). What remains unclear, however, is the question of whether these states are formed as a result of dynamic quenching, i.e., through combined nuclear rearrangements of locally excited bases with nonexcited adjacent bases. Alternatively, the UV excitation of DNA could have excitonic character with shared oscillator strength between adjacent bases. In this article, we provide firm experimental results that indicate the presence of electronically delocalized domains in DNA base stacks as a result of UV light absorption. Our femtosecond spectroscopic data are interpreted and discussed in the framework of molecular exciton theory (18). The first theoretical ...

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The reductive electron transfer (ET) in DNA can be studied by ultrafast time-resolved measurements combined with chemically probed DNAstrand-cleavage experiments. Owing to the numerous conformations of DNA present the results show a variety of ET rates. For more information see the Communication by H.-A. Wagenknecht, T. Fiebig, et al. on the following pages.

Table S1. m/z values for oligomer sequences determined by MALDI-TOF mass spectrometry. Sequence Calculated Found Sequence Calculated Found APy-T 4886.3 4881.6 APy-1 Br U 4951.2 4948.2 APy-2 Br U 4951.2 4948.0 APy-2U 4872.3 4875.2 APy-3 Br U 4951.2 4948.3 APy-3U 4872.3 4869.5 APy-4 Br U 4951.2 4955.6 APy-4U 4872.3 4874.8 APy-5 Br U 5568.6 5576.7 APy-5U 5489.5 5489.1 APy-6 Br U 6186.0 6190.7 APy-6U 6107.1 6106.3 APy-7 Br U 6803.4 6805.4 APy-7U 6724.3 6726.9 APy-8 Br U 7420.9 7416.0 APy-8U 7341.8 7345.2 APy-2 I U 4998.3 4998.5 APy-2U 4872.3 4875.6 APy-3 I U 4998.3 4996.2 APy-3U 4872.3 4868.1 APy-4 I U 4998.3 4997.9 APy-4U 4872.3 4871.1 APy-5 I U 5615.7 5618.2 APy-5U 5489.6 5492.1