Site-specific unnatural base excision via visible light.

Base excision (BE) is an important yet hard-to-control biological event. Unnatural base pairs are powerful tools to revolutionize biological studies in various areas. In this paper, we report a visible-light-induced method to construct site-specific unnatural BE and show the influence of its regulation on transcription and translation levels.

UV-induced hydrogen transfer in DNA base pairs promoted by dark nπ* states.

Dark nπ* states were shown to have substantial contribution to the destructive photochemistry of pyrimidine nucleobases. Based on quantum-chemical calculations, we demonstrate that the characteristic hydrogen bonding pattern of the GC base pair could facilitate the formation of a wobble excited-state charge-transfer complex. This entails a barrierless electron-driven proton transfer (EDPT) process which enables damageless photodeactivation of the base pair. These photostabilizing properties are retained even when guanine is exchanged to hypoxanthine. The inaccessibility of this process in the AT base pair sheds further light on the reasons why cytosine is less susceptible to the formation of photodimers in double-stranded DNA.

Efficiency of radiation-induced base lesion excision and the order of enzymatic treatment.

PURPOSE: To clarify whether initial base excision repair processes at clustered DNA damage sites comprising multiple base lesions affect subsequent excision processes via the formation of additional strand breaks by glycosylase and apurinic/apyrimidinic (AP) endonuclease base excision enzymes. MATERIALS AND METHODS: Plasmid DNA (pUC18) as a model DNA molecule was exposed to high-linear-energy-transfer (LET) ionizing radiation (He2+ or C6+ ions) or low-LET ionizing radiation (X-rays) under various conditions to produce varied radical-scavenging effects. pUC18 was then treated sequentially or simultaneously with two bacterial base excision enzymes (glycosylases), namely, endonuclease III and formamidopyrimidine-DNA glycosylase, which convert pyrimidine (or abasic [AP] site) and purine (or AP site) lesions to single-strand breaks (SSB), respectively. Yields of additional SSB or double-strand breaks (DSB) as digestion products were examined after changing the order of enzymatic treatment. RESULTS: There were few differences among the enzymatic treatments, indicating that treatment order did not affect the final yields of additional SSB or DSB formed by glycosylase activity. This suggests that of the total damage, the fraction of clustered damage sites with a persistent base lesion dependent on the order of glycosylase treatment was insignificant if present. CONCLUSION: Base lesion clusters induced by high- or low-LET radiation appear three or more base pairs apart, and are promptly converted to a DSB by glycosylase, regardless of the order of enzymatic treatment.

Ultraviolet Absorption Induces Hydrogen-Atom Transfer in G⋠C Watson-Crick DNA Base Pairs in Solution.

Ultrafast deactivation pathways bestow photostability on nucleobases and hence preserve the structural integrity of DNA following absorption of ultraviolet (UV) radiation. One controversial recovery mechanism proposed to account for this photostability involves electron-driven proton transfer (EDPT) in Watson-Crick base pairs. The first direct observation is reported of the EDPT process after UV excitation of individual guanine-cytosine (G⋅C) Watson-Crick base pairs by ultrafast time-resolved UV/visible and mid-infrared spectroscopy. The formation of an intermediate biradical species (G[-H]⋅C[+H]) with a lifetime of 2.9 ps was tracked. The majority of these biradicals return to the original G⋅C Watson-Crick pairs, but up to 10% of the initially excited molecules instead form a stable photoproduct G*⋅C* that has undergone double hydrogen-atom transfer. The observation of these sequential EDPT mechanisms across intermolecular hydrogen bonds confirms an important and long debated pathway for the deactivation of photoexcited base pairs, with possible implications for the UV photochemistry of DNA.

Excited states in DNA strands investigated by ultrafast laser spectroscopy.

Ultrafast laser experiments on carefully selected DNA model compounds probe the effects of base stacking, base pairing, and structural disorder on excited electronic states formed by UV absorption in single and double DNA strands. Direct π-orbital overlap between two stacked bases in a dinucleotide or in a longer single strand creates new excited states that decay orders of magnitude more slowly than the generally subpicosecond excited states of monomeric bases. Half or more of all excited states in single strands decay in this manner. Ultrafast mid-IR transient absorption experiments reveal that the long-lived excited states in a number of model compounds are charge transfer states formed by interbase electron transfer, which subsequently decay by charge recombination. The lifetimes of the charge transfer states are surprisingly independent of how the stacked bases are oriented, but disruption of π-stacking, either by elevating temperature or by adding a denaturing co-solvent, completely eliminates this decay channel. Time-resolved emission measurements support the conclusion that these states are populated very rapidly from initial excitons. These experiments also reveal the existence of populations of emissive excited states that decay on the nanosecond time scale. The quantum yield of these states is very small for UVB/UVC excitation, but increases at UVA wavelengths. In double strands, hydrogen bonding between bases perturbs, but does not quench, the long-lived excited states. Kinetic isotope effects on the excited-state dynamics suggest that intrastrand electron transfer may couple to interstrand proton transfer. By revealing how structure and non-covalent interactions affect excited-state dynamics, on-going experimental and theoretical studies of excited states in DNA strands can advance understanding of fundamental photophysics in other nanoscale systems.

Photoinduced processes in nucleic acids.

Photoinduced processes in nucleic acids are phenomena of fundamental interest in diverse fields, from prebiotic studies, through medical research on carcinogenesis, to the development of bioorganic photodevices. In this contribution we survey many aspects of the research across the boundaries. Starting from a historical background, where the main milestones are identified, we review the main findings of the physical-chemical research of photoinduced processes on several types of nucleic-acid fragments, from monomers to duplexes. We also discuss a number of different issues which are still under debate.

UV-excitation from an experimental perspective: frequency resolved.

Electronic spectroscopy of DNA bases in the gas phase provides detailed information about the electronic excitation, which places the molecule in the Franck-Condon region in the excited state and thus prepares the starting conditions for excited-state dynamics. Double resonance or hole-burning spectroscopy in the gas phase can provide such information with isomer specificity, probing the starting potential energy landscape as a function of tautomeric form, isomeric structure, or hydrogen bonded or stacked cluster structure. Action spectroscopy, such REMPI, can be affected by excited-state lifetimes.

Photoinduced charge-separation in DNA.

DNA site-specifically modified with a photosensitizer (Sens) was synthesized and the charge-separation and charge-recombination dynamics in DNA were studied. We specifically focused on the formation of the long-lived charge-separated state whose lifetime (τ) is longer than 0.1 µs. The quantum yields of the formation of the charge-separated states (Φ) upon the photoexcitation of the Sens, and the τ were measured using the laser flash photolysis technique. We utilized naphthalimide (NI), naphthaldiimide (ND), and anthraquinone (AQ) as a Sens to investigate the mechanism of the formation of the charge-separated state in DNA via rapid positive charge (hole) transfer between adenine and thymine (A-T) base-pairs. By replacing some T bases in the A-T stretch with 5-bromouracil ((br)U), the charge-separation was shown to occur via the photoinduced charge-injection into the second and further neighboring As to the Sens. On the other hand, the generation of a hole on A nearest to Sens ends up with the rapid charge-recombination within a contact ion pair. A long-lived charge-separated state was also generated in DNA when a commonly used fluorophore such asTAMRA, Alexa 532, and ATTO 655, which can only oxidize guanine-cytosine (G-C) base-pair, but not A-T, was used as a Sens. These results suggested that the charge-separation in DNA is a general phenonmenon for fluorescent dyes which fluorescence is quenched only by G-C.

Computational modeling of photoexcitation in DNA single and double strands.

The photoexcitation of DNA strands triggers extremely complex photoinduced processes, which cannot be understood solely on the basis of the behavior of the nucleobase building blocks. Decisive factors in DNA oligomers and polymers include collective electronic effects, excitonic coupling, hydrogen-bonding interactions, local steric hindrance, charge transfer, and environmental and solvent effects. This chapter surveys recent theoretical and computational efforts to model real-world excited-state DNA strands using a variety of established and emerging theoretical methods. One central issue is the role of localized vs delocalized excitations and the extent to which they determine the nature and the temporal evolution of the initial photoexcitation in DNA strands.

Attacking mechanism of hydroxyl radical to DNA base-pair: density functional study in vacuum and in water.

Recently, the influence of radiation on human body has been recognized as a serious problem. In particular, highly reactive hydroxyl radicals *OH produced by the radiation react with DNA, resulting in a great damage on its structure and electronic properties. It is thus important to investigate the reaction mechanism of *OH to DNA for elucidating the initial damage in DNA induced by the radiation. In the present study, we search for transition states (TS) of the reaction between G-C/A-T base-pair and [Formula: see text] in vacuum and in water, by the density functional theory (DFT) calculations. At first, we obtain the stable structures for the dehydrogenated G-C and A-T, in which the hydrogen atom of NH2 group of G or A base is abstracted by [Formula: see text]. From the structures of the dehydrogenated as well as the natural base-pairs, the TS between these structures is searched for and the activation free energy (AFE) is estimated for the reaction. In vacuum, AFEs for the G-C and A-T are almost the same each other, while the stabilization energy by the reaction for G-C is about 4.9 kcal/mol larger than that for A-T, indicating that the population of the dehydrogenated G-C is remarkably larger than that of the dehydrogenated A-T in vacuum. On the other hand, in water approximated by the continuum solvation model, the AFE for A-T is 2.6 kcal/mol smaller than that for G-C, indicating that the reaction dehydrogenated by [Formula: see text] occurs more frequently for the solvated A-T base-pair than G-C.

Excitation of nucleobases from a computational perspective I: reaction paths.

The main intrinsic photochemical events in nucleobases can be described on theoretical grounds within the realm of non-adiabatic computational photochemistry. From a static standpoint, the photochemical reaction path approach (PRPA), through the computation of the respective minimum energy path (MEP), can be regarded as the most suitable strategy in order to explore the electronically excited isolated nucleobases. Unfortunately, the PRPA does not appear widely in the studies reported in the last decade. The main ultrafast decay observed experimentally for the gas-phase excited nucleobases is related to the computed barrierless MEPs from the bright excited state connecting the initial Franck-Condon region and a conical intersection involving the ground state. At the highest level of theory currently available (CASPT2//CASPT2), the lowest excited (1)(ππ*) hypersurface for cytosine has a shallow minimum along the MEP deactivation pathway. In any case, the internal conversion processes in all the natural nucleobases are attained by means of interstate crossings, a self-protection mechanism that prevents the occurrence of photoinduced damage of nucleobases by ultraviolet radiation. Many alternative and secondary paths have been proposed in the literature, which ultimately provide a rich and constructive interplay between experimentally and theoretically oriented research.

Watson-Crick base pairing controls excited-state decay in natural DNA.

Excited-state dynamics are essential to understanding the formation of DNA lesions induced by UV light. By using femtosecond IR spectroscopy, it was possible to determine the lifetimes of the excited states of all four bases in the double-stranded environment of natural DNA. After UV excitation of the DNA duplex, we detected a concerted decay of base pairs connected by Watson-Crick hydrogen bonds. A comparison of single- and double-stranded DNA showed that the reactive charge-transfer states formed in the single strands are suppressed by base pairing in the duplex. The strong influence of the Watson-Crick hydrogen bonds indicates that proton transfer opens an efficient decay path in the duplex that prohibits the formation or reduces the lifetime of reactive charge-transfer states.

Analysis of mutant frequencies and mutation spectra in hMTH1 knockdown TK6 cells exposed to UV radiation.

Ultraviolet radiation is a highly mutagenic agent that damages the DNA by the formation of mutagenic photoproducts at dipyrimidine sites and by oxidative DNA damages via reactive oxygen species (ROS). ROS can also give rise to mutations via oxidation of dNTPs in the nucleotide pool, e.g. 8-oxo-dGTP and 2-OH-dATP and subsequent incorporation during DNA replication. Here we show that expression of human MutT homolog 1 (hMTH1) which sanitizes the nucleotide pool by dephosphorylating oxidized dNTPs, protects against mutagenesis induced by long wave UVA light and by UVB light but not by short wave UVC light. Mutational spectra analyses of UVA-induced mutations at the endogenous Thymidine kinase gene in human lymphoblastoid cells revealed that hMTH1 mainly protects cells from transitions at GC and AT base pairs.

Formation of N-N cross-links in DNA by reaction of radiation-produced DNA base pair diradicals: a DFT study.

This study employs DFT (density functional theory) to investigate the formation of hydrazine-like (N-N) cross-linked structures between DNA base pair diradicals that are likely to result from the interaction of high linear energy transfer (LET) radiation, such as ion-beam radiation, with DNA. In our calculations, we generated the guanine (G), cytosine (C), adenine (A), and thymine (T) radicals by removing one hydrogen atom from an N-H bond involved in the normal base pairing. The radical species formed are those that naturally result from one-electron oxidation of the bases followed by deprotonation. N-N cross-links between G and C or A and T diradicals were studied using the BHandHLYP, B3LYP, M06, and M06-2X density functionals and 6-31G* basis set. From a comparison to several test cases performed with the G3B3 method, which gives thermodynamically reliable values, we found that calculations employing the BHandHLYP/6-31G* method predict the best estimates of bonding energies for hydrazine-like structures. Our study shows that the N-N cross-link formed between guanine radical and a neutral cytosine is endothermic in nature but can form metastable structures. However, the reactions between two DNA base radicals (diradical) to form several N-N cross-linked structures are found to be highly exothermic in nature. The N-N cross-links formed between various G-C, G-G, and C-C diradicals have binding energies in the range of ca. -54 to -68, -41 to -47, and -67 to -75 kcal/mol, respectively, whereas A-T, A-A, and T-T have binding energies of -80, -60, and -98 kcal/mol, respectively. In all purine-pyrimidine N-N cross-linked structures, the highest occupied molecular orbital (HOMO) is found to be localized on the purine moiety and the lowest unoccupied molecular orbital (LUMO) is on the pyrimidine moiety.

Modeling DNA response to terahertz radiation.

Collective response of DNA to terahertz electric fields is studied in a simple pair bond model. We confirm, with some caveats, a previous observation of destabilizing DNA breather modes and explore the parameter dependence of these modes. It is shown that breather modes are eliminated under reasonable physical conditions and that thermal effects are significant.

Base pairing enhances fluorescence and favors cyclobutane dimer formation induced upon absorption of UVA radiation by DNA.

The photochemical properties of the DNA duplex (dA)(20)·(dT)(20) are compared with those of the parent single strands. It is shown that base pairing increases the probability of absorbing UVA photons, probably due to the formation of charge-transfer states. UVA excitation induces fluorescence peaking at ∼420 nm and decaying on the nanosecond time scale. The fluorescence quantum yield, the fluorescence lifetime, and the quantum yield for cyclobutane dimer formation increase upon base pairing. Such behavior contrasts with that of the UVC-induced processes.

A supra-photoswitch involving sandwiched DNA base pairs and azobenzenes for light-driven nanostructures and nanodevices.

A supra-photoswitch is designed for complete ON/OFF switching of DNA hybridization by light irradiation for the purpose of using DNA as a material for building nanostructures. Azobenzenes, attached to D-threoninols that function as scaffolds, are introduced into each DNA strand after every two natural nucleotides (in the form (NNX)n where N and X represent the natural nucleotide and the azobenzene moiety, respectively). Hybridization of these two modified strands forms a supra-photoswitch consisting of alternating natural base pairs and azobenzene moieties. In this newly designed sequence, each base pair is sandwiched between two azobenzene moieties and all the azobenzene moieties are separated by base pairs. When the duplex is irradiated by visible light, the azobenzene moieties take the trans form and this duplex is surprisingly stable compared to the corresponding native duplex composed of only natural oligonucleotides. On the other hand, when the azobenzene moieties are isomerized to the cis form by UV light irradiation, the duplex is completely dissociated. Based on this design, a DNA hairpin structure is synthesized that should be closed by visible light irradiation and opened by UV light irradiation at the level of a single molecule. Indeed, perfect ON/OFF photoregulation is attained. This is a promising strategy for the design of supra-photoswitches such as photoresponsive sticky ends on DNA nanodevices and other nanostructures.

NMR solution structures of bistranded abasic site lesions in DNA.

Ionizing radiation produces clustered lesions in DNA. Since the orientation of bistranded lesions affects their recognition by DNA repair enzymes, clustered damages are more difficult to process and thus more toxic than single oxidative lesions. In order to understand the structural determinants that lead to differential recognition, we used NMR spectroscopy and restrained molecular dynamics to solve the structure of two DNA duplexes, each containing two stable abasic site analogues positioned on opposite strands of the duplex and staggered in the 3' (-1 duplex, (AP) 2-1 duplex) or 5' (+1 duplex, (AP) 2+1 duplex) direction. Cross-peak connectivities observed in the nonexchangeable NOESY spectra indicate compression of the helix at the lesion site of the duplexes, resulting in the formation of two abasic bulges. The exchangeable proton spectra show the AP site partner nucleotides forming interstrand hydrogen bonds that are characteristic of a Watson-Crick G.C base pairs, confirming the extra helical nature of the AP residues. Restrained molecular dynamics simulations generate a set of converging structures in full agreement with the spectroscopic data. In the (AP) 2-1 duplex, the extra helical abasic site residues reside in the minor groove of the helix, while they appear in the major groove in the (AP) 2+1 duplex. These structural differences are consistent with the differential recognition of bistranded abasic site lesions by human AP endonuclease.

Photophysical characters of rationally designed hetero-ring-expanded guanine analogues and effect of cytosine pairing.

We present the results of the CIS and TDB3LYP calculations of the optical absorption and emission spectra of some newly designed guanine (G) analogues and their Watson-Crick base pairs. Compared with natural G, the onset absorption peaks of these newly designed analogues are red-shifted, while the fluorescence peaks are blue-shifted. In general, the first excited singlet states (pipi*) of these analogues are nonplanar for all bases considered here. But, the Stokes shifts for the designed G-analogues are much smaller than that of natural G, suggesting that they have stronger molecular rigidity and higher fluorescence quantum yields than those of natural G. The first excited states of these Watson-Crick base pairs essentially originate from those of their isolated purine moieties, as demonstrated from the S1 geometries of their Watson-Crick base pairs. For G and its analogues, A1 and A2 (they are ring-expanded with one-bond intercalation at the C5 site), the pairing with cytosine reduces the oscillator strengths of both the first absorption peak (by 27%-60%) and the fluorescent emission (by 19%-23%), while for the analogues A3, A4, and xG in which G is ring-expanded with a two-bond intercalation at the C5 site, the pairing, in contrast, increases the oscillator strengths of both the first absorption peak (by 11%-15%) and the fluorescent emission (by 3%-20%). These observations indicate that the pairing with cytosine can quench the fluorescence for G, A1, and A2 but enhance the fluorescence quantum yields for A3, A4, and xG. The significant shifts induced by ring-expansion in the ring-expanded G with a two-bond intercalation at the C5 site reveal a possibility for their fluorescent detections.

Charge separation in acridine- and phenothiazine-modified DNA.

The formation of the long-lived, charge-separated state in DNA upon visible light irradiation is of particular interest in molecular-scale optoelectronics, sensor design, and other areas of nanotechnology. However, the efficient generation of the charge-separated state is hampered by fast charge recombination within a contact ion pair, which limits the application of DNA for photoelectrochemical sensors and devices. In this study, a series of protonated 9-alkylamino-6-chloro-2-methoxyacridine (Acr+)- and phenothiazine (Ptz)-modified DNAs were synthesized for the further understanding of the mechanism of charge separation in DNA to generate a long-lived, charge-separated state with a high quantum yield (Phi). The Acr+ serves as a photosensitizer to produce a hole on guanine (G), and the G-C base pairs were used as a hole-transporting pathway to separate a hole from Acr* (the one-electron-reduced form of Acr+) to be trapped at Ptz. Since Acr+ oxides only G upon photoexcitation, the A-T base pair can be used as a spacer between Acr+ and the G-C base pair to avoid the formation of a contact ion pair. The charge injection dynamics was investigated by steady-state fluorescence spectra and fluorescence lifetime measurements, and the Phi and the lifetime of the charge-separated state produced upon photoirradiation were assessed by nanosecond laser flash photolysis of the Acr+- and Ptz-modified DNA. A long-lived, charge-separated state was successfully formed upon visible-light irradiation, and the Phi was the highest for the DNA having a single intervening A-T base pair between Acr+ and the G-C base pair. These results clearly demonstrated that the charge separation process in DNA can be refined by putting a redox-inactive intervening base pair as a spacer between a photosensitizer and the nucleobase to be oxidized to slow down the charge recombination rate.