Long-distance charge transport in duplex DNA: the phonon-assisted polaron-like hopping mechanism

Dynamics of invariant solutions of the DNA model using Lie symmetry approach

The utilization of the Lie group method serves to encapsulate a diverse array of wave structures. This method, established as a robust and reliable mathematical technique, is instrumental in deriving precise solutions for nonlinear partial differential equations (NPDEs) across a spectrum of domains. Its applications span various scientific disciplines, including mathematical physics, nonlinear dynamics, oceanography, engineering sciences, and several others. This research focuses specifically on the crucial molecule DNA and its interaction with an external microwave field. The Lie group method is employed to establish a five-dimensional symmetry algebra as the foundational element. Subsequently, similarity reductions are led by a system of one-dimensional subalgebras. Several invariant solutions as well as a spectrum of wave solutions is obtained by solving the resulting reduced ordinary differential equations (ODEs). These solutions govern the longitudinal displacement in DNA, shedding light on the characteristics of DNA as a significant real-world challenge. The interactions of DNA with an external microwave field manifest in various forms, including rational, exponential, trigonometric, hyperbolic, polynomial, and other functions. Mathematica simulations of these solutions confirm that longitudinal displacements in DNA can be expressed as periodic waves, optical dark solitons, singular solutions, exponential forms, and rational forms. This study is novel as it marks the first application of the Lie group method to explore the interaction of DNA molecules.

Chemical Repair of Radical Damage to the GC Base Pair by DNA-Bound Bisbenzimidazoles

The migration of an electron-loss center (hole) in calf thymus DNA to bisbenzimidazole ligands bound in the minor groove is followed by pulse radiolysis combined with time-resolved spectrophotometry. The initially observed absorption spectrum upon oxidation of DNA by the selenite radical is consistent with spin on cytosine (C), as the GC• pair neutral radical, followed by the spectra of oxidized ligands. The rate of oxidation of bound ligands increased with an increase in the ratio (r) ligands per base pair from 0.005 to 0.04. Both the rate of ligand oxidation and the estimated range of hole transfer (up to 30 DNA base pairs) decrease with the decrease in one-electron reduction potential between the GC• pair neutral radical of ca. 1.54 V and that of the ligand radicals (E0', 0.90-0.99 V). Linear plots of log of the rate of hole transfer versus r give a common intercept at r = 0 and a free energy change of 12.2 ± 0.3 kcal mol-1, ascribed to the GC• pair neutral radical undergoing a structural change, which is in competition to the observed hole transfer along DNA. The rate of hole transfer to the ligands at distance, R, from the GC• pair radical, k2, is described by the relationship k2 = k0 exp(constant/R), where k0 includes the rate constant for surmounting a small barrier.

Development of a QM/MM(ABEEM) method for the deprotonation of neutral and cation radicals in the G-tetrad and GGX(8-oxo-G) tetrad

The rapid deprotonation of G˙+ in the DNA strand impedes positive charge (hole) transfer, whereas the slow deprotonation rate of G˙+ in the G-tetrad makes it a more suitable carrier for hole conduction. The QM/MM(ABEEM) combined method, which involves the integration of QM and the ABEEM polarizable force field (ABEEM PFF), was developed to investigate the deprotonation of neutral and cation free radicals in the G-tetrad and GGX(8-oxo-G) tetrad (xanthine and 8-oxoguanine dual substituted G-tetrad). By incorporating valence-state electronegativity piecewise functions χ*(r) and implementing charge local conservation conditions, QM/MM(ABEEM) possesses the advantage of accurately simulating charge transfer and polarization effect during deprotonation. The activation energy calculated by the QM method of X˙ is the lowest among other bases in the GGX(8-oxo-G) tetrad, which is supported by the computation of the average electronegativity calculated by ABEEM PFF. By utilizing QM/MM(ABEEM) with a two-way free energy perturbation method, the deprotonation activation energy of X˙ in the GGX(8-oxo-G) tetrad is determined to be 33.0 ± 2.1 kJ mol-1, while that of G˙+ in the G-tetrad is 20.7 ± 0.6 kJ mol-1, consistent with the experimental measurement of 20 ± 1.0 kJ mol-1. These results manifest that X˙ in the GGX(8-oxo-G) tetrad exhibits a slower deprotonation rate than G˙+ in the G-tetrad, suggesting that the GGX(8-oxo-G) tetrad may serve as a more favorable hole transport carrier. Furthermore, the unequal average electronegativities of bases in the GGX(8-oxo-G) tetrad impede the deprotonation rate. This study provides a potential foundation for investigating the microscopic mechanism of DNA electronic devices.

Capturing Protein-Nucleic Acid Interactions by High-Intensity Laser-Induced Covalent Crosslinking

Interactions of DNA with structural proteins such as histones, regulatory proteins, and enzymes play a crucial role in major cellular processes such as transcription, replication and repair. The in vivo mapping and characterization of the binding sites of the involved biomolecules are of primary importance for a better understanding of genomic deployment that is implicated in tissue and developmental stage-specific gene expression regulation. The most powerful and commonly used approach to date is immunoprecipitation of chemically cross-linked chromatin (XChIP) coupled with sequencing analysis (ChIP-seq). While the resolution and the sensitivity of the high-throughput sequencing techniques have been constantly improved little progress has been achieved in the crosslinking step. Because of its low efficiency the use of the conventional UVC lamps remains very limited while the formaldehyde method was established as the "gold standard" crosslinking agent. Efficient biphotonic crosslinking of directly interacting nucleic acid-protein complexes by a single short UV laser pulse has been introduced as an innovative technique for overcoming limitations of conventionally used chemical and photochemical approaches. In this survey, the main available methods including the laser approach are critically reviewed for their ability to generate DNA-protein crosslinks in vitro model systems and cells.

Quantum biology. π-π entanglement signatures in Protein-DNA interactions

DNA biological functions are carried out by individual proteins that interact with specific sequences along DNA to prime molecular processes required by cellular metabolism. Protein-DNA interactions include DNA replication, gene expression and its regulation, DNA repair, DNA restriction and modification by endonucleases, generally classified as enzymatic functions, or transcription factors functions. To find specific binding target sequences and finalize their activities, proteins must operate in symbiosis with cellular crowded environment identifying extremely small cognate sequences along the DNA chain, ranging from 15-20 bps for repressors to 4-6 bps for restriction enzymes in less than one second.

Bio-inspired strategies for next-generation perovskite solar mobile power sources

Smart electronic devices are becoming ubiquitous due to many appealing attributes including portability, long operational time, rechargeability and compatibility with the user-desired form factor. Integration of mobile power sources (MPS) based on photovoltaic technologies with smart electronics will continue to drive improved sustainability and independence. With high efficiency, low cost, flexibility and lightweight features, halide perovskite photovoltaics have become promising candidates for MPS. Realization of these photovoltaic MPS (PV-MPS) with unconventionally extraordinary attributes requires new 'out-of-box' designs. Natural materials have provided promising designing solutions to engineer properties under a broad range of boundary conditions, ranging from molecules, proteins, cells, tissues, apparatus to systems in animals, plants, and humans optimized through billions of years of evolution. Applying bio-inspired strategies in PV-MPS could be biomolecular modification on crystallization at the atomic/meso-scale, bio-structural duplication at the device/system level and bio-mimicking at the functional level to render efficient charge delivery, energy transport/utilization, as well as stronger resistance against environmental stimuli (e.g., self-healing and self-cleaning). In this review, we discuss the bio-inspired/-mimetic structures, experimental models, and working principles, with the goal of revealing physics and bio-microstructures relevant for PV-MPS. Here the emphasis is on identifying the strategies and material designs towards improvement of the performance of emerging halide perovskite PVs and strategizing their bridge to future MPS.

Remote Photodamaging of DNA by Photoinduced Energy Transport

Local DNA photodamaging by light is well-studied and leads to a number of structurally identified direct damage, in particular cyclobutane pyrimidine dimers, and indirect oxidatively generated damage, such as 8-oxo-7,8-hydroxyguanine. Similar damages have now been found at remote sites, at least more than 105 Å (30 base pairs) away from the site of photoexcitation. In contrast to the established mechanisms of local DNA photodamaging, the processes of remote photodamage are only partially understood. Known pathways include those to remote oxidatively generated DNA photodamages, which were elucidated by studying electron hole transport through the DNA about 20 years ago. Recent studies with DNA photosensitizers and mechanistic proposals on photoinduced DNA-mediated energy transport are summarized in this minireview. These new mechanisms to a new type of remote DNA photodamaging provide an important extension to our general understanding to light-induced DNA damage and their mutations.

DNA Electrochemistry: Charge-Transport Pathways through DNA Films on Gold

Over the past 25 years, collective evidence has demonstrated that the DNA base-pair stack serves as a medium for charge transport chemistry in solution and on DNA-modified gold surfaces. Since this charge transport depends sensitively upon the integrity of the DNA base pair stack, perturbations in base stacking, as may occur with DNA base mismatches, lesions, and protein binding, interrupt DNA charge transport (DNA CT). This sensitivity has led to the development of powerful DNA electrochemical sensors. Given the utility of DNA electrochemistry for sensing and in response to recent literature, we describe critical protocols and characterizations necessary for performing DNA-mediated electrochemistry. We demonstrate DNA electrochemistry with a fully AT DNA sequence using a thiolated preformed DNA duplex and distinguish this DNA-mediated chemistry from that of electrochemistry of largely single-stranded DNA adsorbed to the surface. We also demonstrate the dependence of DNA CT on a fully stacked duplex. An increase in the percentage of mismatches within the DNA monolayer leads to a linear decrease in current flow for a DNA-bound intercalator, where the reaction is DNA-mediated; in contrast, for ruthenium hexammine, which binds electrostatically to DNA and the redox chemistry is not DNA-mediated, there is no effect on current flow with mismatches. We find that, with DNA as a well hybridized duplex, upon assembly, a DNA-mediated pathway facilitates the electron transfer between a well coupled redox probe and the gold surface. Overall, this report highlights critical points to be emphasized when utilizing DNA electrochemistry and offers explanations and controls for analyzing confounding results.

Ballistic transport and quantum unfurling in molecular junctions via minimal representations of quantum master equations

Quantum furling and unfurling are inelastic transitions between localized and delocalized electronic states. We predict scenarios where these processes govern charge transport through donor-bridge-acceptor molecular junctions. Like in the case of ballistic transport, the resulting currents are nearly independent of the molecular bridge length. However, currents involving quantum furling and unfurling processes can be controlled by the coupling to vibrations in the intra-molecular and the extra-molecular environment, which can be experimentally tuned. Our study is based on rate equations for exchange of energy (bosons) and particles (fermions) between the molecular bridge and its environment. An efficient algorithm is introduced for a compact representation of the relevant rate equations, which utilizes the redundancies in the rate matrix and the sparsity of the creation and annihilation operators in the molecular Fock space.

Revisiting the Hole Size in Double Helical DNA with Localized Orbital Scaling Corrections

The extent of electronic wave function delocalization for the charge carrier (electron or hole) in double helical DNA plays an important role in determining the DNA charge transfer mechanism and kinetics. The size of the charge carrier's wave function delocalization is regulated by the solvation induced localization and the quantum delocalization among the π stacked base pairs at any instant of time. Using a newly developed localized orbital scaling correction (LOSC) density functional theory method, we accurately characterized the quantum delocalization of the hole wave function in double helical B-DNA. This approach can be used to diagnose the extent of delocalization in fluctuating DNA structures. Our studies indicate that the hole state tends to delocalize among 4 guanine-cytosine (GC) base pairs and among 3 adenine-thymine (AT) base pairs when these adjacent bases fluctuate into degeneracy. The relatively small delocalization in AT base pairs is caused by the weaker π-π interaction. This extent of delocalization has significant implications for assessing the role of coherent, incoherent, or flickering coherent carrier transport in DNA.

Clustered DNA Damage: Electronic Properties and Their Influence on Charge Transfer. 7,8-Dihydro-8-Oxo-2'-Deoxyguaosine Versus 5',8-Cyclo-2'-Deoxyadenosines: A Theoretical Approach

Approximately 3 × 10¹⁷ DNA damage events take place per hour in the human body. Within clustered DNA lesions, they pose a serious problem for repair proteins, especially for iron–sulfur glycosylases (MutyH), which can recognize them by the electron-transfer process. It has been found that the presence of both 5′,8-cyclo-2′-deoxyadenosine (cdA) diastereomers in the ds-DNA structure, as part of a clustered lesion, can influence vertical radical cation distribution within the proximal part of the double helix, i.e., d[~oxoGcAoxoG~] (7,8-dihydro-8-oxo-2′-deoxyguaosine - ^oxodG). Here, the influence of cdA, “the simplest tandem lesion”, on the charge transfer through ds-DNA was taken into theoretical consideration at the M062x/6-31+G** level of theory in the aqueous phase. It was shown that the presence of (5′S)- or (5′R)-cdA leads to a slowdown in the hole transfer by one order of magnitude between the neighboring dG→^oxodG in comparison to “native” ds-DNA. Therefore, it can be concluded that such clustered lesions can lead to defective damage recognition with a subsequent slowing down of the DNA repair process, giving rise to an increase in mutations. As a result, the unrepaired, ^oxodG: dA base pair prior to genetic information replication can finally result in GC → TA or AT→CG transversion. This type of mutation is commonly observed in human cancer cells. Moreover, because local multiple damage sites (LMSD) are effectively produced as a result of ionization factors, the presented data in this article might be useful in developing a new scheme of radiotherapy treatment against the background of DNA repair efficiency.

The Dynamics of Hole Transfer in DNA

High-energy radiation and oxidizing agents can ionize DNA. One electron oxidation gives rise to a radical cation whose charge (hole) can migrate through DNA covering several hundreds of Å, eventually leading to irreversible oxidative damage and consequent disease. Understanding the thermodynamic, kinetic and chemical aspects of the hole transport in DNA is important not only for its biological consequences, but also for assessing the properties of DNA in redox sensing or labeling. Furthermore, due to hole migration, DNA could potentially play an important role in nanoelectronics, by acting as both a template and active component. Herein, we review our work on the dynamics of hole transfer in DNA carried out in the last decade. After retrieving the thermodynamic parameters needed to address the dynamics of hole transfer by voltammetric and spectroscopic experiments and quantum chemical computations, we develop a theoretical methodology which allows for a faithful interpretation of the kinetics of the hole transport in DNA and is also capable of taking into account sequence-specific effects.

Molecular Junctions Inspired by Nature: Electrical Conduction through Noncovalent Nanobelts

Charge transport occurs in a range of biomolecular systems, whose structures have covalent and noncovalent bonds. Understanding from these systems have yet to translate into molecular junction devices. We design junctions which have hydrogen-bonds between the edges of a series of prototype noncovalent nanobelts (NCNs) and vary the number of donor-acceptors to study their electrical properties. From frontier molecular orbitals (FMOs) and projected density of state (DOS) calculations, we found these NCN dimer junctions to have low HOMO-LUMO gaps and states at the Fermi level, suggesting these are metallic-like systems. Their conductance properties were studied with nonequilibrium Green's functions density functional theory (NEGF-DFT) and was found to decrease with cooperative H-bonding, that is, the conductance decreased as the alternating donor-acceptors around the nanobelts attenuates to a uniform distribution in the H-bonding arrays. The latter gave the highest conductance of 51.3 × 10^-6 S and the Seebeck coefficient showed n-type (-36 to -39 μV K^-1) behavior, while the lower conductors with alternating H-bonds are p-type (49.7 to 204 μV K^-1). In addition, the NCNs have appreciable binding energies (19.8 to 46.1 kcal mol^-1), implying they could form self-assembled monolayer (SAM) heterojunctions leading to a polymeric network for long-range charge transport.

Theoretical modeling of DNA electron hole transport through polypyrimidine sequences: a QM/MM study

The phenomenon of DNA hole transport (HT) has attracted of scientists for several decades, mainly due to its potential application in molecular electronics. As electron holes mostly localize on purine bases in DNA, the majority of scientific effort has been invested into chemically modifying the structures of adenine and guanine in order to increase their HT-mediating properties. In this work we examine an alternative, never yet explored, way of affecting the HT efficiency by forcing electron holes to localize on pyrimidine bases and move between them. Using an enhanced and revised version of our previously developed QM/MM model, we perform simulations of HT through polyadenine, polycytosine, polyguanine, and polythymine stacks according to a multistep hopping mechanism. From these simulations, kinetic parameters for HT are obtained. The results indicate a particularly high efficiency of cytosine→cytosine hopping, which is about ten times higher than the G → G hopping. We also discuss possible improvement of cytosine HT by modifying the oxidoreductive properties of complementary guanine residues.

Artificial Sandwich Base for Monitoring Single-Nucleobase Changes and Charge-Transfer Rates in DNA

Developing a convenient method to discriminate among different types of DNA nucleotides within a target sequence of the human genome is extremely challenging. We herein report an artificial ferrocene-base (Fe-base) that was synthesized and incorporated into different loci of a DNA strand. The Fe-base replacement on a nucleobase can interact with DNA bases and efficiently discriminate among A, T, G, and C DNA bases of the complementary locus on the basis of interacting electrochemical properties. Furthermore, cyclic-voltammetry (CV) studies demonstrated the electrochemical stability of DNA strands incorporated with Fe-bases and the reversibility of the incorporation. Square-wave voltammetry (SWV) was performed to measure current changes between Fe-bases and bases of interest in the DNA duplex. The changes in the charge-transfer rates appeared to be correlated with the position of the Fe-base in the DNA strand, allowing rapid and efficient sensing of single-nucleobase changes in DNA and showing promise for the design of Fe-oligomer chip technology as a tool for DNA sequencing.

Biphotonic Ionization of DNA: From Model Studies to Cell

Oxidation reactions triggered by low-intensity UV photons represent a minor contribution with respect to the overwhelming pyrimidine base dimerization in both isolated and cellular DNA. The situation is totally different when DNA is exposed to high-intensity UVC radiation under conditions where biphotonic ionization of the four main purine and pyrimidine bases becomes predominant at the expense of singlet excitation processes. The present review article provides a critical survey of the main chemical reactions of the base radical cations thus generated by one-electron oxidation of nucleic acids in model systems and cells. These include oxidation of the bases with the predominant formation of 8-oxo-7,8-dihydroguanine as the result of preferential hole transfer to guanine bases that act as sinks in isolated and cellular DNA. In addition to hydration, other nucleophilic addition reactions involving the guanine radical cation give rise to intra- and interstrand cross-links together with DNA-protein cross-links. Information is provided on the utilization of high-intensity UV laser pulses as molecular biology tools for studying DNA conformational features, nucleic acid-protein interactions and nucleic acid reactivity through DNA-protein cross-links and DNA footprinting experiments.

Synthesis, Optoelectronic Properties, Self-Association, and Base Pairing of Nucleobase-Functionalized Oligothiophenes

Device-relevant π-conjugated oligothiophenes with the canonical nucleobases directly embedded into the π-framework have been designed, synthesized, and characterized. These oligomers offer the ability to tune optoelectronic properties via the intimate merging of the nucleobase molecular electronic structure with base-pairing fidelity. Analysis of their optical and electronic properties in a hydrogen-bond-disrupting solvent (DMF) indicates that the nucleobase identity influences the intrinsic electronic properties of the semiconductors. These differences are supported by DFT calculations which demonstrate that the HOMO/LUMO orbitals are distributed differently for each compound. The solubility and competition between self-association and base pairing in a hydrogen-bond-supporting solvent (chloroform) was studied to better understand the oligomer behavior under conditions relevant for downstream solution processing into thin-film devices. These solution studies reveal that in each case base-pairing is preferred to self-aggregation; the relatively weak heteroassociation of 1A-1U (35 ± 5 M^-1) should be amenable to facile solution processing and successive hydrogen bond formation in the solid state, while the strong heteroassociation between 1G and 1C (>10⁴ M^-1) should enable assemblies to be preformed in solution. These results are expected to enable the synthesis of more complex π-conjugated architectures and facilitate their extension to optoelectronic devices.

A Redox Role for the [4Fe4S] Cluster of Yeast DNA Polymerase δ

A [4Fe4S]2+ cluster in the C-terminal domain of the catalytic subunit of the eukaryotic B-family DNA polymerases is essential for the formation of active multi-subunit complexes. Here we use a combination of electrochemical and biochemical methods to assess the redox activity of the [4Fe4S]2+ cluster in Saccharomyces cerevisiae polymerase (Pol) δ, the lagging strand DNA polymerase. We find that Pol δ bound to DNA is indeed redox-active at physiological potentials, generating a DNA-mediated signal electrochemically with a midpoint potential of 113 ± 5 mV versus NHE. Moreover, biochemical assays following electrochemical oxidation of Pol δ reveal a significant slowing of DNA synthesis that can be fully reversed by reduction of the oxidized form. A similar result is apparent with photooxidation using a DNA-tethered anthraquinone. These results demonstrate that the [4Fe4S] cluster in Pol δ can act as a redox switch for activity, and we propose that this switch can provide a rapid and reversible way to respond to replication stress.

Multi-dimensional charge transport in supramolecular helical foldamer assemblies

Aromatic foldamers are bioinspired architectures whose potential use in materials remains largely unexplored. Here we report our investigation of vertical and horizontal charge transport over long distances in helical oligo-quinolinecarboxamide foldamers organized as single monolayers on Au or SiO2. Conductive atomic force microscopy showed that vertical conductivity is efficient and that it displays a low attenuation with foldamer length (0.06 Å-1). In contrast, horizontal charge transport is found to be negligible, demonstrating the strong anisotropy of foldamer monolayers. Kinetic Monte Carlo calculations were used to probe the mechanism of charge transport in these helical molecules and revealed the presence of intramolecular through-space charge transfer integrals approaching those found in pentacene and rubrene crystals, in line with experimental results. Kinetic Monte Carlo simulations of charge hopping along the foldamer chain evidence the strong contribution of multiple 1D and 3D pathways in these architectures and their dependence on conformational order. These findings show that helical foldamer architectures may provide a route for achieving charge transport over long distance by combining multiple charge transport pathways.

Microsecond Simulation of Electron Transfer in DNA: Bottom-Up Parametrization of an Efficient Electron Transfer Model Based on Atomistic Details

The transfer of electrons over long distances in complex molecular systems is a phenomenon of significance in both biochemistry and technology. In recent years, we have been developing efficient models to study ET in complex systems, including DNA as a prominent example. Ab initio and model approaches have been combined in an "on-the-fly" calculation of ET parameters, which can be used to propagate nuclear and electronic degrees of freedom simultaneously. These previous efforts have aimed at deriving an efficient nonadiabatic quantum mechanical-molecular mechanical (QM/MM) simulation scheme for ET, making nanosecond simulations of ET in realistic systems possible. This, however, is still insufficient for the treatment of large donor-bridge-acceptor systems, like the ET in DNA, overcoming long adenine bridges. Therefore, we have constructed a theoretical model in a bottom-up manner. All quantum-chemical as well as force-field calculations are substituted by theoretical models of the involved phenomena on a molecular level, including polarization and relaxation of the molecular environment, which are often omitted in other recently developed theoretical models of ET. A nonadiabatic simulation scheme is employed, and no assumptions regarding the ET mechanism are needed. Thus, the predictive power of the simulations is preserved, while pushing the limits of the accessible time scales beyond microseconds. This model-based simulation scheme is applied to ET in various DNA species. Good agreement with the "full" atomistic nonadiabatic QM/MM scheme is observed for the archetypal DNA ET systems, the polyA sequence, as well as the sequences GT_nGGG, containing adenines as bridge sites. Furthermore, ET in larger, more complex DNA sequences is simulated, and the results are discussed.

Metallic Nanostructures Based on DNA Nanoshapes

Metallic nanostructures have inspired extensive research over several decades, particularly within the field of nanoelectronics and increasingly in plasmonics. Due to the limitations of conventional lithography methods, the development of bottom-up fabricated metallic nanostructures has become more and more in demand. The remarkable development of DNA-based nanostructures has provided many successful methods and realizations for these needs, such as chemical DNA metallization via seeding or ionization, as well as DNA-guided lithography and casting of metallic nanoparticles by DNA molds. These methods offer high resolution, versatility and throughput and could enable the fabrication of arbitrarily-shaped structures with a 10-nm feature size, thus bringing novel applications into view. In this review, we cover the evolution of DNA-based metallic nanostructures, starting from the metallized double-stranded DNA for electronics and progress to sophisticated plasmonic structures based on DNA origami objects.

Ionization-Induced Structural Changes in Uracil Dimers and Their Spectroscopic Signatures

The electronic structure of the three representative isomers of the ionized uracil dimers is characterized by high-level electronic structure calculations. Noncovalent interactions between the fragments lower the vertical ionization energies by 0.13-0.35 eV, the largest drop being observed for the stacked and the T-shaped isomers. The initial hole is delocalized in the stacked and the H-bonded isomers and is localized in the T-shaped one. The ionization induces significant structural relaxation and increases the binding energies. The stacked dimer cation relaxes to the symmetric structure bound by 22.7 kcal/mol. The T-shaped dimer cation has a binding energy of 25.1 kcal/mol. Thus, the relative order of the stacked and T-shaped isomers is reversed upon ionization. Finally, the H-bonded isomer, which relaxes to the proton-transferred structure, is bound by 37.0 kcal/mol. The electronic spectra of all three isomers characterized at the vertical and the relaxed geometries show different patterns, which may be exploited in spectroscopic probing of ionization-induced dynamics in these species.

Dynamics of Excess-Electron Transfer through Alternating Adenine:Thymine Sequences in DNA

This paper presents the results of an investigation into the sequence-dependent excess-electron transfer (EET) dynamics in DNA, which plays an important role in DNA damage/repair. There are many published studies on EET in consecutive adenine:thymine (A:T) sequences (Tn), but those in alternating A:T sequences (ATn) remain limited. Here, two series of functionalized DNA oligomers, Tn and ATn, were synthesized with a strongly electron-donating photosensitizer, a trimer of ethylenedioxythiophene (3 E), and an electron acceptor, diphenylacetylene (DPA). Laser flash photolysis experiments showed that the EET rate constant of AT3 is two times lower than that of T3 due to the lack of π-stacking of Ts in AT3. Thus, it was indicated that excess-electron hopping is affected by the interaction between LUMOs of nucleotides.

Unbiased charge oscillations in B-DNA: monomer polymers and dimer polymers

We call monomer a B-DNA base pair and examine, analytically and numerically, electron or hole oscillations in monomer and dimer polymers, i.e., periodic sequences with repetition unit made of one or two monomers. We employ a tight-binding (TB) approach at the base-pair level to readily determine the spatiotemporal evolution of a single extra carrier along a N base-pair B-DNA segment. We study highest occupied molecular orbital and lowest unoccupied molecular orbital eigenspectra as well as the mean over time probabilities to find the carrier at a particular monomer. We use the pure mean transfer rate k to evaluate the easiness of charge transfer. The inverse decay length β for exponential fits k(d), where d is the charge transfer distance, and the exponent η for power-law fits k(N) are computed; generally power-law fits are better. We illustrate that increasing the number of different parameters involved in the TB description, the fall of k(d) or k(N) becomes steeper and show the range covered by β and η. Finally, for both the time-independent and the time-dependent problems, we analyze the palindromicity and the degree of eigenspectrum dependence of the probabilities to find the carrier at a particular monomer.

Induction of action-at-a-distance mutagenesis by 8-oxo-7,8-dihydroguanine in DNA pol λ-knockdown cells

Introduction

In DNA, 8-oxo-7,8-dihydroguanine (G^O, 8-hydroxyguanine) is one of the most pivotal oxidatively damaged bases and induces G:C → T:A transversion mutations. DNA polymerase λ preferentially inserts dCTP opposite G^Oin vitro, and this error-free bypass function is considered to be important after A base removal from G^O:A pairs by the MUTYH DNA glycosylase. To examine the effects of reduced levels of DNA polymerase λ on the G^O-induced mutations, the polymerase was knocked-down in human U2OS cells, and a shuttle plasmid DNA containing a G^O:C pair at position 122 in the supF gene was transfected into the cells. The plasmid DNA replicated in the cells was introduced into an Escherichia coli indicator strain, to measure the supF mutant frequency.

Results

The knockdown of DNA polymerase λ significantly enhanced the mutant frequency of the G^O plasmid DNA. Contrary to our expectations, the knockdown did not promote the targeted G:C → T:A transversion. Instead, substitution mutations at G:C sites other than position 122 were enhanced in the cells.

Conclusions

These results suggested that the knockdown of DNA polymerase λ induced action-at-a-distance mutagenesis in human cells when the G^O:C pair was present in the DNA.

How Does Guanine-Cytosine Base Pair Affect Excess-Electron Transfer in DNA?

Charge transfer and proton transfer in DNA have attracted wide attention due to their relevance in biological processes and so on. Especially, excess-electron transfer (EET) in DNA has strong relation to DNA repair. However, our understanding on EET in DNA still remains limited. Herein, by using a strongly electron-donating photosensitizer, trimer of 3,4-ethylenedioxythiophene (3E), and an electron acceptor, diphenylacetylene (DPA), two series of functionalized DNA oligomers were synthesized for investigation of EET dynamics in DNA. The transient absorption measurements during femtosecond laser flash photolysis showed that guanine:cytosine (G:C) base pair affects EET dynamics in DNA by two possible mechanisms: the excess-electron quenching by proton transfer with the complementary G after formation of C(•-) and the EET hindrance by inserting a G:C base pair as a potential barrier in consecutive thymines (T's). In the present paper, we provided useful information based on the direct kinetic measurements, which allowed us to discuss EET through oligonucleotides for the investigation of DNA damage/repair.

Electron Transfer Proteins as Electronic Conductors: Significance of the Metal and Its Binding Site in the Blue Cu Protein, Azurin

Electron transfer (ET) proteins are biomolecules with specific functions, selected by evolution. As such they are attractive candidates for use in potential bioelectronic devices. The blue copper protein azurin (Az) is one of the most-studied ET proteins. Traditional spectroscopic, electrochemical, and kinetic methods employed for studying ET to/from the protein's Cu ion have been complemented more recently by studies of electrical conduction through a monolayer of Az in the solid-state, sandwiched between electrodes. As the latter type of measurement does not require involvement of a redox process, it also allows monitoring electronic transport (ETp) via redox-inactive Az-derivatives. Here, results of macroscopic ETp via redox-active and -inactive Az derivatives, i.e., Cu(II) and Cu(I)-Az, apo-Az, Co(II)-Az, Ni(II)-Az, and Zn(II)-Az are reported and compared. It is found that earlier reported temperature independence of ETp via Cu(II)-Az (from 20 K until denaturation) is unique, as ETp via all other derivatives is thermally activated at temperatures >≈200 K. Conduction via Cu(I)-Az shows unexpected temperature dependence >≈200 K, with currents decreasing at positive and increasing at negative bias. Taking all the data together we find a clear compensation effect of Az conduction around the Az denaturation temperature. This compensation can be understood by viewing the Az binding site as an electron trap, unless occupied by Cu(II), as in the native protein, with conduction of the native protein setting the upper transport efficiency limit.

DNA-mediated oxidation of p53

Transcription factor p53 is the most commonly altered gene in human cancer. As a redox-active protein in direct contact with DNA, p53 can directly sense oxidative stress through DNA-mediated charge transport. Electron hole transport occurs over long distances through the π-stacked bases and leads to the oxidative dissociation of p53. The extent of protein dissociation depends upon the redox potential of the DNA in direct contact with each p53 monomer. The DNA sequence dependence of p53 oxidative dissociation was examined by electrophoretic mobility shift assays using oligonucleotides containing both synthetic and human p53 consensus sequences with an appended photooxidant, anthraquinone. Greater p53 dissociation is observed from sequences containing low-redox potential purine regions, particularly guanine triplets. Using denaturing polyacrylamide gel electrophoresis of irradiated anthraquinone-modified DNA, the DNA damage sites corresponding to sites of preferred electron hole localization were determined. The resulting DNA damage preferentially localizes to guanine doublets and triplets. Oxidative DNA damage is inhibited in the presence of p53, but only at sites in direct contact with p53. From these data, predictions about the sensitivity of human p53-binding sites to oxidative stress as well as possible biological implications have been made. On the basis of our data, the guanine pattern within the purine region of each p53-binding site determines the response of p53 to DNA oxidation, yielding for some sequences the oxidative dissociation of p53 from a distance and thereby providing another potential role for DNA charge transport chemistry within the cell.

Distance dependence of hole transfer rates from G radical cations to GGG traps in DNA

Relative reaction rates for hole transfer between G radical cations and GGG triplets in DNA, through different bridges of varying lengths, are numerically calculated and the obtained results are compared with corresponding experimental observations [Giese et al., 2001, Nature, 412, 318; Angew. Chem., Int. Ed., 1999, 38, 996]. Hole donors and acceptors are separated either by (T-A)n bridges or by N repeated barriers consisting of (T-A,T-A) double base-pairs which are connected through single G-C base-pairs. In the former case, hole transfer rates show a strong exponential decrease with the length of the bridge for short bridges, while a switching to weak distance dependence has been observed for longer bridges. In the latter case, a power law seems to better describe the distance dependence of charge transfer rates. All these experimental observations are qualitatively reproduced by our simulations without any adjustable parameter, considering only tunneling as the charge transfer mechanism. Physical insights into the mechanism providing the switching behavior in the case of (T-A)n bridges are presented through an analysis of the eigenfunctions of the system.

Scaling of temperature dependence of charge mobility in molecular Holstein chains

The temperature dependence of a charge mobility in a model DNA based on a Holstein Hamiltonian is calculated for four types of homogeneous sequences It has turned out that upon rescaling all four types are quite similar. Two types of rescaling, i.e., those for low and intermediate temperatures, are found. The curves obtained are approximated on a logarithmic scale by cubic polynomials. We believe that for model homogeneous biopolymers with parameters close to the designed ones, one can assess the value of the charge mobility without carrying out resource-intensive direct simulation, just by using a suitable approximating function.