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The photophysical properties of the natural pigment violacein extracted from an Antarctic organism adapted to high exposure levels of UV radiation were measured in a combined steady-state and time-resolved spectroscopic study for the first time. In the low-viscosity solvents methanol and acetone, violacein exhibits low fluorescence quantum yields on the order of 1 × 10-4, and femtosecond transient absorption measurements reveal excited-state lifetimes of 3.2 ± 0.2 and 4.5 ± 0.2 ps in methanol and acetone, respectively. As solvent viscosity is increased, both the fluorescence quantum yield and excited-state lifetime of this intensely colored pigment increase dramatically, and stimulated emission decays 30-fold more slowly in glycerol than in methanol at room temperature. Excited-state deactivation is suggested to occur via a molecular-rotor mechanism in which torsion about an interring bond leads to a conical intersection with the ground state. [ABSTRACT FROM AUTHOR]

The excited-state dynamics of two cyclic DNA miniduplexes, each containing just two base pairs, are investigated using time-resolved infrared spectroscopy. As in longer DNA duplexes, intrastrand electron transfer induced by UV excitation triggers interstrand proton transfer in the alternating miniduplex containing two out-of-phase G·C base pairs. The resulting excited state decays on a time scale of several tens of picoseconds. This state is absent when one of the two G residues is substituted by 8-oxo-7,8-dihydroguanine, a modification that is suggested to disrupt base stacking, while maintaining base pairing. These findings demonstrate that a nucleobase tetramer arranged as two stacked base pairs accurately captures the interplay between intrastrand and interstrand decay channels. The similar signals seen in the miniduplexes and longer sequences suggest that excited states in the latter rapidly localize on two adjacent base pairs. [ABSTRACT FROM AUTHOR]

Femtosecond time-resolved IR spectroscopyis used to investigatethe excited-state dynamics of a dinucleotide containing an 8-oxoguanineanion at the 5′-end and neutral adenine at the 3′-end.UV excitation of the dinucleotide transfers an electron from deprotonated8-oxoguanine to its π-stacked neighbor adenine in less than1 ps, generating a neutral 8-oxoguanine radical and an adenine radicalanion. These species are identified by the excellent agreement betweenthe experimental and calculated IR difference spectra. The quantumefficiency of this ultrafast charge shift reaction approaches unity.Back electron transfer from the adenine radical anion to the 8-oxguanineneutral radical occurs in 9 ps, or approximately 6 times faster thanbetween the adenine radical anion and the 8-oxoguanine radical cation(Zhang, Y. et al. Proc. Natl. Acad. Sci. U.S.A.2014, 111, 11612–11617). The largeasymmetry in forward and back electron transfer rates is fully rationalizedby semiclassical nonadiabatic electron transfer theory. Forward electrontransfer is ultrafast because the driving force is nearly equal tothe reorganization energy, which is estimated to lie between 1 and2 eV. Back electron transfer is highly exergonic and takes place muchmore slowly in the Marcus inverted region. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]