ABSTRACT
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.
Subject(s)
Indoles/chemistry , Oxalobacteraceae/chemistry , Quantum Theory , Fluorescence , Molecular StructureABSTRACT
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 10-4, and femtosecond transient absorption measurements reveal excited-state lifetimes of 3.2 ± 0.2 and 4.5 ± 0.2 picoseconds 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
Melamine may have been an important prebiotic information carrier, but its excited-state dynamics, which determine its stability under UV radiation, have never been characterized. The ability of melamine to withstand the strong UV radiation present on the surface of the early Earth is likely to have affected its abundance in the primordial soup. Here, we studied the excited-state dynamics of melamine (a proto-nucleobase) and its lysine derivative (a proto-nucleoside) using the transient absorption technique with a UV pump, and UV and infrared probe pulses. For melamine, the excited-state population decays by internal conversion with a lifetime of 13 ps without coupling significantly to any photochemical channels. The excited-state lifetime of the lysine derivative is slightly longer (18 ps), but the dominant deactivation pathway is otherwise the same as for melamine. In both cases, the vast majority of excited molecules return to the electronic ground state on the aforementioned time scales, but a minor population is trapped in a long-lived triplet state.
Subject(s)
Lysine/analogs & derivatives , Lysine/chemistry , Triazines/chemistry , Drug Stability , Kinetics , Prebiotics , Quantum Theory , ThermodynamicsABSTRACT
UV radiation creates excited electronic states in DNA that can decay to mutagenic photoproducts. When excited states return to the electronic ground state, photochemical injury is avoided. Understanding of the available relaxation pathways has advanced rapidly during the past decade, but there has been persistent uncertainty, and even controversy, over how to compare results from transient absorption and time-resolved emission experiments. Here, emission from single- and double-stranded AT DNA compounds excited at 265â nm was studied in aqueous solution using the time-correlated single photon counting technique. There is quantitative agreement between the emission lifetimes ranging from 50 to 200â ps and ones measured in transient absorption experiments, demonstrating that both techniques probe the same excited states. The results indicate that excitations with lifetimes of more than a few picoseconds are weakly emissive excimer and charge transfer states. Only a minute fraction of excitations persist beyond 1â ns in AT DNA strands at room temperature.
Subject(s)
DNA/chemistry , Oligonucleotides/chemistry , Thermodynamics , Time Factors , Ultraviolet RaysABSTRACT
Photophysical investigations of the canonical nucleobases that make up DNA and RNA during the past 15 years have revealed that excited states formed by the absorption of UV radiation decay with subpicosecond lifetimes (i.e., <10(-12) s). Ultrashort lifetimes are a general property of absorbing sunscreen molecules, suggesting that the nucleobases are molecular survivors of a harsh UV environment. Encoding the genome using photostable building blocks is an elegant solution to the threat of photochemical damage. Ultrafast excited-state deactivation strongly supports the hypothesis that UV radiation played a major role in shaping molecular inventories on the early Earth before the emergence of life and the subsequent development of a protective ozone shield. Here, we review the general physical and chemical principles that underlie the photostability, or "UV hardiness", of modern nucleic acids and discuss the possible implications of these findings for prebiotic chemical evolution. In RNA and DNA strands, much longer-lived excited states are observed, which at first glance appear to increase the risk of photochemistry. It is proposed that the dramatically different photoproperties that emerge from assemblies of photostable building blocks may explain the transition from a world of molecular survival to a world in which energy-rich excited electronic states were eventually tamed for biological purposes such as energy transduction, signaling, and repair of the genetic machinery.
Subject(s)
DNA/chemistry , RNA/chemistry , Ultraviolet Rays , Electron Transport/radiation effects , Nucleotides/chemistry , Nucleotides/radiation effects , Photolysis/radiation effectsABSTRACT
The IR spectrum of a charge transfer (CT) excited electronic state in DNA has been computed for the first time, enabling assignment of the long-lived component of the transient IR spectrum of a d(AT)9 single strand to an A â T CT state. Experimentally, the CT state lifetime is much shorter than in the double strand, and our calculations explain this result using Marcus Theory.
Subject(s)
DNA , Electron Transport , Adenine , Animals , Humans , ThymineABSTRACT
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.
Subject(s)
Base Pairing , DNA/chemistry , Electron Transport , Guanine/analogs & derivatives , Protons , Ultraviolet Rays , Cyclization , Guanine/chemistry , Nucleic Acid ConformationABSTRACT
The excited-state dynamics of three distinct forms of the d(GC)9·d(GC)9 DNA duplex were studied by combined time-resolved infrared experiments and quantum mechanical calculations. In the B- and Z-forms, bases on opposite strands form Watson-Crick (WC) base pairs but stack differently because of salt-induced changes in backbone conformation. At low pH, the two strands associate by Hoogsteen (HG) base pairing. Ultraviolet-induced intrastrand electron transfer (ET) triggers interstrand proton transfer (PT) in the B- and Z-forms, but the PT pathway is blocked in the HG duplex. Despite the different decay mechanisms, a common excited-state lifetime of â¼ 30 ps is observed in all three duplex forms. The ET-PT pathway in the WC duplexes and the solely intrastrand ET pathway in the HG duplex yield the same pair of π-stacked radicals on one strand. Back ET between these radicals is proposed to be the rate-limiting step behind excited-state deactivation in all three duplexes.
Subject(s)
DNA, B-Form/chemistry , DNA, Z-Form/chemistry , Base Pairing , DNA, B-Form/radiation effects , DNA, Z-Form/radiation effects , Hydrogen Bonding , Kinetics , Models, Chemical , Spectrophotometry, Infrared , Ultraviolet RaysABSTRACT
8-Oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dGuo) is one of the most common forms of DNA oxidative damage. Recent studies have shown that 8-oxo-dGuo can repair cyclobutane pyrimidine dimers in double-stranded DNA when photoexcited, making its excited state dynamics of particular interest. The excited state lifetimes of 8-oxo-dGuo and its anion have been previously probed using transient absorption spectroscopy; however, more information is required to understand the decay mechanisms. In this work, excited state potential energy surfaces for the neutral and deprotonated forms of the free base, 8-oxoguanine (8-oxo-G), are explored theoretically using multireference methods while the nucleoside is experimentally studied using steady-state fluorescence spectroscopy. It is determined that the neutral species exhibits ultrafast radiationless decay via easy access to conical intersections. The relatively long lifetime for the anion can be explained by the existence of sizable barriers between the Franck-Condon region and two S1/S0 minimum energy conical intersections. A Strickler-Berg analysis of the experimentally measured fluorescence quantum yields and lifetimes is consistent with emission from ππ* excited states in line with theoretical predictions.
Subject(s)
Guanine/analogs & derivatives , Models, Chemical , 8-Hydroxy-2'-Deoxyguanosine , Deoxyguanosine/analogs & derivatives , Deoxyguanosine/chemistry , Guanine/chemistry , Hydrogen-Ion Concentration , Solvents , Spectrometry, FluorescenceABSTRACT
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.
Subject(s)
DNA/chemistry , Protons , Ultraviolet RaysABSTRACT
Femtosecond time-resolved IR spectroscopy is used to investigate the excited-state dynamics of a dinucleotide containing an 8-oxoguanine anion at the 5'-end and neutral adenine at the 3'-end. UV excitation of the dinucleotide transfers an electron from deprotonated 8-oxoguanine to its π-stacked neighbor adenine in less than 1 ps, generating a neutral 8-oxoguanine radical and an adenine radical anion. These species are identified by the excellent agreement between the experimental and calculated IR difference spectra. The quantum efficiency of this ultrafast charge shift reaction approaches unity. Back electron transfer from the adenine radical anion to the 8-oxguanine neutral radical occurs in 9 ps, or approximately 6 times faster than between 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 large asymmetry in forward and back electron transfer rates is fully rationalized by semiclassical nonadiabatic electron transfer theory. Forward electron transfer is ultrafast because the driving force is nearly equal to the reorganization energy, which is estimated to lie between 1 and 2 eV. Back electron transfer is highly exergonic and takes place much more slowly in the Marcus inverted region.
Subject(s)
Adenine/chemistry , DNA/chemistry , Guanosine/analogs & derivatives , Quantum Theory , Anions/chemistry , Electron Transport , Guanosine/chemistry , Photochemical ProcessesABSTRACT
During the early evolution of life, 8-oxo-7,8-dihydro-2'-deoxyguanosine (O) may have functioned as a proto-flavin capable of repairing cyclobutane pyrimidine dimers in DNA or RNA by photoinduced electron transfer using longer wavelength UVB radiation. To investigate the ability of O to act as an excited-state electron donor, a dinucleotide mimic of the FADH2 cofactor containing O at the 5'-end and 2'-deoxyadenosine at the 3'-end was studied by femtosecond transient absorption spectroscopy in aqueous solution. Following excitation with a UV pulse, a broadband mid-IR pulse probed vibrational modes of ground-state and electronically excited molecules in the double-bond stretching region. Global analysis of time- and frequency-resolved transient absorption data coupled with ab initio quantum mechanical calculations reveal vibrational marker bands of nucleobase radical ions formed by electron transfer from O to 2'-deoxyadenosine. The quantum yield of charge separation is 0.4 at 265 nm, but decreases to 0.1 at 295 nm. Charge recombination occurs in 60 ps before the O radical cation can lose a deuteron to water. Kinetic and thermodynamic considerations strongly suggest that all nucleobases can undergo ultrafast charge separation when π-stacked in DNA or RNA. Interbase charge transfer is proposed to be a major decay pathway for UV excited states of nucleic acids of great importance for photostability as well as photoredox activity.