Is the DPT tautomerization of the long A·G Watson-Crick DNA base mispair a source of the adenine and guanine mutagenic tautomers? A QM and QTAIM response to the biologically important question

How proton transfer affects the helical parameters in DNA:DNA microhelices

Proton transfer reactions are a widespread phenomenon in many areas of the life sciences and it is one of the origins of the spontaneous point mutations during DNA replication. Because of its importance, many studies have been reported on these reactions. However, the present work is the first one focused on the structural geometrical changes by double proton transfer (DPT). Thus, different Watson-Crick (WC) pairs were optimized first in a simple model with one nucleoside base pair, and in a microhelix form with three nucleoside base pairs. The canonical and few tautomeric forms were considered in DNA:DNA microhelices with A-type and B-type helical forms. The stability of these structures and how the DPT process affects the main geometrical parameters was analyzed, in particular the deformation of the helical parameters. The M06-2X DFT method was used for this purpose. The purine/pyrimidine ring in the keto form appears easier to be deformed than when it is in the enol form. The weaker WC base pair formed with mixed microhelices than with nucleobases alone and the significant deformation of the helical and backbone parameters with the DPT appears to complicate this process in microhelices.Communicated by Ramaswamy H. Sarma.

Structure-activity features of purines and their receptors: implications in cell physiopathology

The purine molecular structure consists of fused pyrimidine and imidazole rings. Purines are main pieces that conform the structure of nucleic acids which rule the inheritance processes. Purines also work as metabolic intermediates in different cell functions and as messengers in the signaling pathways throughout cellular communication. Purines, mainly ATP and adenosine (ADO), perform their functional and pharmacological properties because of their structural/chemical characteristics that make them either targets of mutagenesis, mother frameworks for designing molecules with controlled effects (e.g. anti-cancer), or chemical donors (e.g., of methyl groups, which represent a potential chemoprotective action against cancer). Purines functions also come from their effect on specific receptors, channel-linked and G-protein coupled for ATP, and exclusively G-coupled receptors for ADO (also known as ADORAs), which are involved in cell signaling pathways, there, purines work as chemical messengers with autocrine, paracrine, and endocrine actions that regulate cell metabolism and immune response in tumor progression which depends on the receptor types involved in these signals. Purines also have antioxidant and anti-inflammatory properties and participate in the cell energy homeostasis. Therefore, purine physiology is important for a variety of functions relevant to cellular health; thus, when these molecules present a homeostatic imbalance, the stability and survival of the cellular systems become compromised.

Intramolecular tautomerization of the quercetin molecule due to the proton transfer: QM computational study

Quercetin molecule (3, 3', 4', 5, 7-pentahydroxyflavone, C15H10O7) is an important flavonoid compound of natural origin, consisting of two aromatic A and B rings linked through the C ring with endocyclic oxygen atom and five hydroxyl groups attached to the 3, 3', 4', 5 and 7 positions. This molecule is found in many foods and plants, and is known to have a wide range of therapeutic properties, like an anti-oxidant, anti-toxic, anti-inflammatory etc. In this study for the first time we have revealed and investigated the pathways of the tautomeric transformations for the most stable conformers of the isolated quercetin molecule (Brovarets' & Hovorun, 2019) via the intramolecular proton transfer. Energetic, structural, dynamical and polar characteristics of these transitions, in particular relative Gibbs free and electronic energies, characteristics of the intramolecular specific interactions-H-bonds and attractive van der Waals contacts, have been analysed in details. It was demonstrated that the most probable process among all investigated is the proton transfer from the O3H hydroxyl group of the C ring to the C2' carbon atom of the C2'H group of the B ring along the intramolecular O3H…C2' H-bond with the further formation of the C2'H2 group. It was established that the proton transfer from the hydroxyl groups to the carbon atoms of the neighboring CH groups is assisted at the transition states by the strong intramolecular HCH…O H-bond (~28.5 kcal∙mol-1). The least probable path of the proton transfer-from the C8H group to the endocyclic O1 oxygen atom-causes the decyclization of the C ring in some cases. It is shortly discussed the biological importance of the obtained results.

Novel Tautomerisation Mechanisms of the Biologically Important Conformers of the Reverse Löwdin, Hoogsteen, and Reverse Hoogsteen G*·C* DNA Base Pairs via Proton Transfer: A Quantum-Mechanical Survey

For the first time, in this study with the use of QM/QTAIM methods we have exhaustively investigated the tautomerization of the biologically-important conformers of the G*·C* DNA base pair-reverse Löwdin G*·C*(rWC), Hoogsteen G*'·C*(H), and reverse Hoogsteen G*'·C*(rH) DNA base pairs-via the single (SPT) or double (DPT) proton transfer along the neighboring intermolecular H-bonds. These tautomeric reactions finally lead to the formation of the novel G· C O 2 * (rWC), G N 2 * · C(rWC), G*'N2·C(rWC), G N 7 * · C(H), and G*'N7·C(rH) DNA base mispairs. Gibbs free energies of activation for these reactions are within the range 3.64-31.65 kcal·mol-1 in vacuum under normal conditions. All TSs are planar structures (Cs symmetry) with a single exception-the essentially non-planar transition state TSG*·C*(rWC)↔G+·C-(rWC) (C1 symmetry). Analysis of the kinetic parameters of the considered tautomerization reactions indicates that in reality only the reverse Hoogsteen G*'·C*(rH) base pair undergoes tautomerization. However, the population of its tautomerised state G*'N7·C(rH) amounts to an insignificant value-2.3·10-17. So, the G*·C*(rWC), G*'·C*(H), and G*'·C*(rH) base pairs possess a permanent tautomeric status, which does not depend on proton mobility along the neighboring H-bonds. The investigated tautomerization processes were analyzed in details by applying the author's unique methodology-sweeps of the main physical and chemical parameters along the intrinsic reaction coordinate (IRC). In general, the obtained data demonstrate the tautomeric mobility and diversity of the G*·C* DNA base pair.

Unexpected Routes of the Mutagenic Tautomerization of the T Nucleobase in the Classical A·T DNA Base Pairs: A QM/QTAIM Comprehensive View

In this paper using quantum-mechanical (QM) calculations in combination with Bader's quantum theory of "Atoms in Molecules" (QTAIM) in the continuum with ε = 1, we have theoretically demonstrated for the first time that revealed recently highly-energetic conformers of the classical A·T DNA base pairs - Watson-Crick [A·T(wWC)], reverse Watson-Crick [A·T(wrWC)], Hoogsteen [A·T(wH)] and reverse Hoogsteen [A·T(wrH)] - act as intermediates of the intrapair mutagenic tautomerization of the T nucleobase owing to the novel tautomerisation pathways: A·T(wWC)↔A·T*(w⊥ WC); A·T(wrWC)↔A·T O 2 * (w⊥ rWC); A·T(wH)↔A·T*(w⊥ H); A·T(wrH)↔A·T O 2 * (w⊥ rH). All of them occur via the transition states as tight ion pairs (A+, protonated by the N6H2 amino group)·(T-, deprotonated by the N3H group) with quasi-orthogonal geometry, which are stabilized by the participation of the strong (A)N6+H···O4-/O2-(T) and (A)N6+H···N3-(T) H-bonds. Established tautomerizations proceed through a two-step mechanism of the protons moving in the opposite directions along the intermolecular H-bonds. Initially, proton moves from the N3H imino group of T to the N6H2 amino group of A and then subsequently from the protonated N6+H3 amino group of A to the O4/O2 oxygen atom of T, leading to the products - A·T*(w⊥ WC), A·T O 2 * (w⊥ rWC), A·T*(w⊥ H), and A·T O 2 * (w⊥ rH), which are substantially non-planar, conformationally-labile complexes. These mispairs are stabilized by the participation of the (A)N6H/N6H'···N3(T) and (T)O2H/O4H···N6(A) H-bonds, for which the pyramidalized amino group of A is their donor and acceptor. The Gibbs free energy of activation of these mutagenic tautomerizations lies in the range of 27.8-29.8 kcal·mol-1 at T = 298.15 K in the continuum with ε = 1.

Non-dissociative structural transitions of the Watson-Crick and reverse Watson-Crick А·Т DNA base pairs into the Hoogsteen and reverse Hoogsteen forms

In this study it was theoretically shown that discovered by us recently (Brovarets' et al., Frontiers in Chemistry, 2018, 6:8; doi: 10.3389/fchem.2018.00008) high-energetical, significantly non-planar (symmetry C₁), short-lived wobbled conformers of the classical Watson-Crick А·Т(WC), reverse Watson-Crick А·Т(rWC), Hoogsteen А·Т(Н) and reverse Hoogsteen А·Т(rН) DNA base pairs are the intermediates of their pairwise А∙Т(WC)/А∙Т(rWC) ↔ А∙Т(H)/А∙Т(rH) conformational transformations. These transitions do not require for their realization the energy-consumable anisotropic rotation of the amino group of A around the exocyclic C6-N6 bond. They are controlled by the non-planar transition states with quasi-orthogonal geometry (symmetry C₁) joined by the single intermolecular (Т)N3H···N6(А) H-bond (~4 kcal∙mol^-1). The Gibbs free energies of activation for these non-dissociative, dipole-active conformational transitions consist 7.33 and 7.81 kcal∙mol^-1, accordingly. Quantum-mechanical (QM) calculations in combination with Bader's quantum theory of "Atoms in Molecules" (QTAIM) have been performed at the MP2/aug-cc-pVDZ//B3LYP/6-311++G(d,p) level of QM theory in the continuum with ε = 4 under normal conditions.

Novel pathway for mutagenic tautomerization of classical А∙Т DNA base pairs via sequential proton transfer through quasi-orthogonal transition states: A QM/QTAIM investigation

In this paper we have theoretically predicted a novel pathway for the mutagenic tautomerization of the classical A∙T DNA base pairs in the free state, the Watson-Crick A·Т(WC), reverse Watson-Crick A·Т(rWC), Hoogsteen A·Т(H) and reverse Hoogsteen A·Т(rH) pairs, via sequential proton transfer accompanied by a significant change in the mutual orientation of the bases. Quantum-mechanical (QM) calculations were performed at the MP2/aug-cc-pVDZ//B3LYP/6-311++G(d,p) level in vacuum phase, along with Bader's quantum theory of Atoms in Molecules (QTAIM). These processes involve transition states (TSs) with quasi-orthogonal structures (symmetry C1), which are highly polar, tight ion pairs (A-, N6H2-deprotonated)∙(T+, O4/O2-protonated). Gibbs free energies of activation for the A∙T(WC) / A∙T(rWC) ↔ A*∙Т(rwWC) / A*∙Т(wWC) tautomeric transitions (~43.5 kcal∙mol-1) are lower than for the A∙T(H) / A∙T(rH) ↔ A*N7∙Т(rwH) / A*N7∙Т(wH) tautomerisations (~53.0 kcal∙mol-1) (rare tautomers are marked by an asterisk; w-wobble configured tautomerisation products). The (T)N3+H⋯N1-(A), (T)O4+H⋯N1-(A) / (T)N3+H⋯N1-(A) and (T)O2+H⋯N1-(A) H-bonds are found in the transition states TSA-·T+A·T(WC)↔A*·T(rwWC) / TSA-·T+A·T(rWC)↔A*·T(wWC). However, in the transition state TSA-·T+A·Т(H)↔A*N7·T(rwH) / TSA-·T+A·Т(rH)↔A*N7·T(wH), the (T)N3+H⋯N7-(A), (T)O4+H⋯N7-(A) / (T)N3+H⋯N7-(A) and (T)O2+H⋯N7-(A) H-bonds are supplemented by the attractive (T)O4+/O2+⋯N6-(A) van der Waals contacts. It was demonstrated that the products of the tautomerization of the classical A∙T DNA base pairs-A*∙Т(rwWC), A*N7∙Т(rwH) and A*N7∙Т(wH) (symmetry Cs)-further transform via double proton transfer into the energetically favorable wobble A∙T*(rwWC), A∙T*(rwH) and A∙T*O2(wH) base mispairs (symmetry Cs).

The influence of Cu+ binding to hypoxanthine on stabilization of mismatches involving hypoxanthine and DNA bases: a DFT study

In the present work, the influence of Cu⁺ binding to N3- and N7-positions of hypoxanthine on energetic, geometrical and topological properties of hypoxanthine-guanine, hypoxanthine-adenine, hypoxanthine-cytosine, hypoxanthine-thymine and hypoxanthine-hypoxanthine mismatches is theoretically investigated. The calculations, in gas phase, are performed at B3LYP/6-311++G(3df,3pd) level of theory. Unlike the other mispairs, Cu⁺ binding to N3-position of hypoxanthine causes the proton transfer process from enol form of hypoxanthine to imino forms of adenine and cytosine. This process also occurs in all mismatches having enol form of hypoxanthine when Cu⁺ binds to N7-position of hypoxanthine. The mismatches are stabilized by hydrogen bonds. The influence of Cu⁺ on hydrogen bonds is also examined by atoms in molecules (AIM) and natural bond orbital (NBO) analyses. Communicated by Ramaswamy H. Sarma.

Surprising Conformers of the Biologically Important A·T DNA Base Pairs: QM/QTAIM Proofs

For the first time novel high-energy conformers-A·T(wWC) (5.36), A·T(wrWC) (5.97), A·T(wH) (5.78), and A·T(wrH) (ΔG = 5.82 kcal·mol-1) (See Graphical Abstract) were revealed for each of the four biologically important A·T DNA base pairs - Watson-Crick A·T(WC), reverse Watson-Crick A·T(rWC), Hoogsteen A·T(H) and reverse Hoogsteen A·T(rH) at the MP2/aug-cc-pVDZ//B3LYP/6-311++G(d,p) level of quantum-mechanical theory in the continuum with ε = 4 under normal conditions. Each of these conformers possesses substantially non-planar wobble (w) structure and is stabilized by the participation of the two anti-parallel N6H/N6H'…O4/O2 and N3H…N6 H-bonds, involving the pyramidalized amino group of the A DNA base as an acceptor and a donor of the H-bonding. The transition states - TSA·T(WC)↔A·T(wWC), TSA·T(rWC)↔A·T(wrWC), TSA·T(H)↔A·T(wH), and TSA·T(rH)↔A·T(wrH), controlling the dipole-active transformations of the conformers from the main plane-symmetric state into the high-energy, significantly non-planar state and vice versa, were localized. They also possess wobble structures similarly to the high-energy conformers and are stabilized by the participation of the N6H/N6H'…O4/O2 and N3H…N6 H-bonds. Discovered conformers of the A·T DNA base pairs are dynamically stable short-lived structures [lifetime τ = (1.4-3.9) ps]. Their possible biological significance and future perspectives have been briefly discussed.

Physico-chemical profiles of the wobble ↔ Watson-Crick G*·2AP(w) ↔ G·2AP(WC) and A·2AP(w) ↔ A*·2AP(WC) tautomerisations: a QM/QTAIM comprehensive survey

This study is intended to clarify in detail the tautomeric transformations of the wobble (w) G*·2AP(w) and A·2AP(w) nucleobase mispairs involving 2-aminopurine (2AP) into the Watson-Crick (WC) G·2AP(WC) and A*·2AP(WC) base mispairs (asterisks denote mutagenic tautomers of the DNA bases), respectively, by quantum-mechanical methods and Bader's Quantum Theory of Atoms in Molecules. Our previously reported methodology has been used, which allows the evolution of the physico-chemical parameters to be tracked along the entire internal reaction coordinate (IRC), not exclusively in the stationary states of these reactions. These biologically important G*·2AP(w) ↔ G·2AP(WC) and A·2AP(w) ↔ A*·2AP(WC) w ↔ WC tautomerisations, which are involved in mutagenic tautomerically-conformational pathways, determine the origin of the transitions and transversions induced by 2AP. In addition, it is established that they proceed through planar, highly stable, zwitterionic transition states and they exhibit similar physico-chemical profiles and stages of sequential intrapair proton transfer, followed by spatial rearrangement of the nucleobases relative to each other within the base pairs. These w ↔ WC tautomerisations occur non-dissociatively and are accompanied by a significant alteration in geometry (from wobble to Watson-Crick and vice versa) and redistribution of the specific intermolecular interactions, which can be divided into 10 patterns including AHB H-bonds and loosened A-H-B covalent bridges along the IRC of tautomerisation. Based on the redistribution of the geometrical and electron-topological parameters of the intrapair hydrogen bonds, exactly 9 key points have been allocated to characterize the evolution of these reactions.

The A·T(rWC)/A·T(H)/A·T(rH) ↔ A·T*(rwWC)/A·T*(wH)/A·T*(rwH) mutagenic tautomerization via sequential proton transfer: a QM/QTAIM study

In this study for the first time we have revealed by QM and QTAIM calculations at the MP2/aug-cc-pVDZ//B3LYP/6-311++G(d,p) level of QM theory the novel routes of the mutagenic tautomerization of three biologically important A·T DNA base pairs – reverse Watson–Crick A·T(rWC), Hoogsteen A·T(H) and reverse Hoogsteen A·T(rH) – followed by their rebuilding into the wobble (w) A·T*(rw_WC), A·T*(w_H) and A·T*(rw_H) base mispairs by the participation of the mutagenic tautomers of the DNA bases (denoted by asterisk) and vice versa, thus complementing the physico-chemical property of the canonical A·T(WC) Watson–Crick DNA base pair reported earlier (Brovarets' et al., RSC Adv., 2015, 5, 99594–99605). These non-dissociative tautomeric transformations in the classical A·T(rWC), A·T(H) and A·T(rH) DNA base pairs proceed similarly to the canonical A·T(WC) DNA base pair via the intrapair sequential proton transfer with shifting towards major or minor grooves of DNA followed by further double proton transfer along the intermolecular H-bonds and are controlled by the plane symmetric and highly stable transition states – tight ion pairs formed by the A⁺ nucleobase, protonated by the N1/N7 nitrogen atoms, and T⁻ nucleobase, deprotonated by the N3H imino group. Comparison of the estimated populations of the tautomerised states (10⁻²¹ to 10⁻¹⁴) with similar characteristics for the canonical A·T(WC) DNA base pair (10⁻⁸ to 10⁻⁷) leads authors to the conclusion, that only a base pair with WC architecture can be a building block of the DNA macromolecule as a genetic material, which is able for the evolutionary self-development. Among all four classical DNA base pairs, only A·T(WC) DNA base pair can ensure the proper rate of the spontaneous point errors of replication in DNA.

We discovered tautomeric wobbling of the classical A·T DNA base pairs. This data evidence, that only a base pair with Watson–Crick architecture can be a building block of the DNA macromolecule as a genetic material, which is able for the evolutionary self-development.

Effects of Environmental Factors and Metallic Electrodes on AC Electrical Conduction Through DNA Molecule

BACKGROUND

Deoxyribonucleic acid (DNA) is one of the best candidate materials for various device applications such as in electrodes for rechargeable batteries, biosensors, molecular electronics, medical- and biomedical-applications etc. Hence, it is worthwhile to examine the mechanism of charge transport in the DNA molecule, however, still a question without a clear answer is DNA a molecular conducting material (wire), semiconductor, or insulator? The answer, after the published data, is still ambiguous without any confirmed and clear scientific answer. DNA is found to be always surrounded with different electric charges, ions, and dipoles. These surrounding charges and electric barrier(s) due to metallic electrodes (as environmental factors (EFs)) play a substantial role when measuring the electrical conductivity through λ-double helix (DNA) molecule suspended between metallic electrodes. We found that strong frequency dependence of AC-complex conductivity comes from the electrical conduction of EFs. This leads to superimposing serious incorrect experimental data to measured ones.

METHODS

At 1 MHz, we carried out a first control experiment on electrical conductivity with and without the presence of DNA molecule. If there are possible electrical conduction due to stray ions and contribution of substrate, we will detected them. This control experiment revealed that there is an important role played by the environmental-charges around DNA molecule and any experiment should consider this role.

RESULTS AND DISCUSSION

We have succeeded to measure both electrical conductivity due to EFs (σ _ENV) and electrical conductivity due to DNA molecule (σ _DNA) independently by carrying the measurements at different DNA-lengths and subtracting the data. We carried out measurements as a function of frequency (f) and temperature (T) in the ranges 0.1 Hz < f < 1 MHz and 288 K < T < 343 K. The measured conductivity (σ _MES) portrays a metal-like behavior at high frequencies near 1 MHz. However, we found that σ _DNA was far from this behavior because the conduction due to EFs superimposes σ _DNA, in particular at low frequencies. By measuring the electrical conductivity at different lengths: 40, 60, 80, and 100 nm, we have succeeded not only to separate the electrical conduction of the DNA molecule from all EFs effects that surround the molecule, but also to present accurate values of σ _DNA and the dielectric constant of the molecule ε'_DNA as a function of temperature and frequency. Furthermore, in order to explain these data, we present a model describing the electrical conduction through DNA molecule: DNA is a classical semiconductor with charges, dipoles and ions that result in creation of localized energy-states (LESs) in the extended bands and in the energy gap of the DNA molecule.

CONCLUSIONS

This model explains clearly the mechanism of charge transfer mechanism in the DNA, and it sheds light on why the charge transfer through the DNA can lead to insulating, semiconducting, or metallic behavior on the same time. The model considers charges on DNA, in the extended bands, either could be free to move under electric field or localized in potential wells/hills. Localization of charges in DNA is an intrinsic structural-property of this solitaire molecule. At all temperatures, the expected increase in thermal-induced charge is attributed to the delocalization of holes (or/and electrons) in potential hills (or/and potential wells) which accurately accounts for the total electric and dielectric behavior through DNA molecule. We succeeded to fit the experimental data to the proposed model with reasonable magnitudes of potential hills/wells that are in the energy range from 0.068 eV.

Transcription-associated mutational pressure in the Parvovirus B19 genome: Reactivated genomes contribute to the variability of viral populations

In this study we used non-overlapping parts of the two long open reading frames coding for nonstructural (NS) and capsid (VP) proteins of all available sequences of the Parvovirus B19 subgenotype 1a genome and found out that the rates of A to G, C to T and A to T mutations are higher in the first long reading frame (NS) of the virus than in the second one (VP). This difference in mutational pressure directions for two parts of the same viral genome can be explained by the fact of transcription of just the first long reading frame during the lifelong latency in nonerythroid cells. Adenine deamination (producing A to G and A to T mutations) and cytosine deamination (producing C to T mutations) occur more frequently in transcriptional bubbles formed by DNA "plus" strand of the first open reading frame. These mutations can be inherited only in case of reactivation of the infectious virus due to the help of Adenovirus that allows latent Parvovirus B19 to start transcription of the second reading frame and then to replicate its genome by the rolling circle mechanism using the specific origin. Results of this study provide evidence that the genomes reactivated from latency make significant contributions to the variability of Parvovirus B19.

A QM/QTAIM detailed look at the Watson-Crick↔wobble tautomeric transformations of the 2-aminopurine·pyrimidine mispairs

This work is devoted to the careful QM/QTAIM analysis of the evolution of the basic physico-chemical parameters along the intrinsic reaction coordinate (IRC) of the biologically important 2AP·T(WC)↔2AP·T*(w) and 2AP·C*(WC)↔2AP·C(w) Watson-Crick(WC)↔wobble(w) tautomeric transformations obtained at each point of the IRC using original authors' methodology. Established profiles reflect the high similarity between the courses of these processes. Basing on the scrupulous analysis of the profiles of their geometric and electron-topological parameters, it was established that the dipole-active WC↔w tautomerizations of the Watson-Crick-like 2AP·T(WC)/2AP·C*(WC) mispairs, stabilized by the two classical N3H⋯N1, N2H⋯O2 and one weak C6H⋯O4/N4 H-bonds, into the wobble 2AP·T*(w)/2AP·C(w) base pairs, respectively, joined by the two classical N2H⋯N3 and O4/N4H⋯N1 H-bonds, proceed via the concerted stepwise mechanism through the sequential intrapair proton transfer and subsequent large-scale shifting of the bases relative each other, through the planar, highly stable, zwitterionic transition states stabilized by the participation of the four H-bonds - N1⁺H⋯O4^-/N4^-, N1⁺H⋯N3^-, N2⁺H⋯N3^-, and N2⁺H⋯O2^-. Moreover, it was found out that the 2AP·T(WC)↔2AP·T*(w)/2AP·C*(WC)↔2AP·C(w) tautomerization reactions occur non-dissociatively and are accompanied by the consequent replacement of the 10 unique patterns of the specific intermolecular interactions along the IRC. Obtained data are of paramount importance in view of their possible application for the control and management of the proton transfer, e.g. by external electric or laser fields.

Molecular dynamic simulation and DFT study on the Drug-DNA interaction; Crocetin as an anti-cancer and DNA nanostructure model

In this research, the interaction of Crocetin as an anti-cancer drug and a Dickerson DNA has been investigated. 25 ns molecular dynamic simulations of Crocetin and DNA composed of 12 base pairs and a sequence of d(CGCGAATTCGCG)₂ were done in water. Three definite parts of the B-DNA were considered in analyzing the best interactive site from the thermodynamic point of view. Binding energy analysis showed that van der Waals interaction is the most important part related to the reciprocal O and H atoms of the Crocetin and DNA. Stabilizing interactions, obtained by ΔG calculations, showed that maximum and minimum interactions are related to the S1 and S3 regions, respectively. This means that the most probable van der Waals interaction site of the Dickerson B-DNA and Crocetin is located in the minor groove of DNA. Two sharp peaks at 2.55 and 1.75 Å in radial distribution functions of the PO⋯HO and NH⋯OC parts are related to new hydrogen bonds between the Crocetin and DNA in the complex which can be considered as the driving force of the anti-cancer mechanism of the Crocetin. Average values of 0.3 au and zero for the electron densities of the H⋯O bonds for DNA and complex, obtained by Quantum theory of atoms in molecules (QTAIM), means that the origin of DNA instability after complexation may be related to the H-bond denaturation by Crocetin. Finally, the evaluation of the dispersion interactions using the dispersion functional, -148.76 kcal.mol^-1, confirmed the importance of the dispersion interaction in drug-DNA complex.

Nucleotides and nucleoside signaling in the regulation of the epithelium to mesenchymal transition (EMT)

The epithelium-mesenchymal transition (EMT) is an important process of cell plasticity, consisting in the loss of epithelial identity and the gain of mesenchymal characteristics through the coordinated activity of a highly regulated informational program. Although it was originally described in the embryonic development, an important body of information supports its role in pathology, mainly in cancerous and fibrotic processes. The purinergic system of inter-cellular communication, mainly based in ATP and adenosine acting throughout their specific receptors, has emerged as a potent regulator of the EMT in several pathological entities. In this context, cellular signaling associated to purines is opening the understanding of a new element in the complex regulatory network of this phenotypical differentiation process. In this review, we have summarized recent information about the role of ATP and adenosine in EMT, as a growing field with high therapeutic potential.

A QM/QTAIM research under the magnifying glass of the DPT tautomerisation of the wobble mispairs involving 2-aminopurine

In this study, a comprehensive survey of the changes of the physico-chemical parameters at each point of the IRC for the biologically important T·2AP*(w) ↔ T*·2AP(w) and G·2AP*(w) ↔ G*·2AP(w) DPT tautomerisation reactions involved in the point mutations (transitions and transversions) induced by 2-aminopurine (2AP) in DNA is provided.

A ratiometric fluorescent pH probe based on keto–enol tautomerization for imaging of living cells in extreme acidity

A ratiometric fluorescent pH probe (DDXC) has been developed for extreme acidity, the sensing mechanism of which is based on the reversible protonation of the carbonyl oxygen followed by keto–enol tautomerization.

Whether 2-aminopurine induces incorporation errors at the DNA replication? A quantum-mechanical answer on the actual biological issue

In this paper, we consider the mutagenic properties of the 2-aminopurine (2AP), which has intrigued molecular biologists, biophysicists and physical chemists for a long time and been widely studied by both experimentalists and theorists. We have shown for the first time using QM calculations, that 2AP very effectively produces incorporation errors binding with cytosine (C) into the wobble (w) C·2AP(w) mispair, which is supported by the N4H⋯N1 and N2H⋯N3 H-bonds and is tautomerized into the Watson-Crick (WC)-like base mispair C*·2AP(WC) (asterisk denotes the mutagenic tautomer of the base), that quite easily in the process of the thermal fluctuations acquires enzymatically competent conformation. 2AP less effectively produces transversions forming the wobble mispair with A base - A·2AP(w), stabilized by the participation of the N6H⋯N1 and N2H⋯N1 H-bonds, followed by further tautomerization A·2AP(w) → A*·2AP(WC) and subsequent conformational transition A*·2AP(WC) → A*·2AP_syn thus acquiring enzymatically competent structure. In this case, incorporation errors occur only in those case, when 2AP belongs to the incoming nucleotide. Thus, answering the question posed in the title of the article, we affirm for certain that 2AP induces incorporation errors at the DNA replication. Obtained results are consistent well with numerous experimental data.

Conformation analysis of 1″,4″-Dispirocyclohexane-6,6'-bis(benzothiazoline): Combined IR, Raman, XRD and DFT approach

The subject of the study is the structure and conformation of 1″,4″-Dispiro-cyclohexane-6,6'-bis(benzothiazoline), a dispiro compound that has a cyclohexyl ring flanked by two benzothiazoline rings on either side. Using single crystal X-ray diffraction measurements, Infra-red absorption, and Raman spectroscopy techniques, it is found that the central cyclohexyl ring assumes the chair conformation and the sulfur, nitrogen atoms in both the benzothiazole rings are in the trans configurations. The experimental findings are further corroborated by geometry optimization and frequency calculations at B3LYP/6-311++G** level of theory using Gaussian 09 suite of program.

Rotational spectroscopy of the atmospheric photo-oxidation product o-toluic acid and its monohydrate

o-Toluic acid, a photo-oxidation product in the atmosphere, and its monohydrate were characterized in the gas phase by pure rotational spectroscopy. High-resolution spectra were measured in the range of 5-14 Hz using a cavity-based molecular beam Fourier-transform microwave spectrometer. Possible conformers were identified computationally, at the MP2/6-311++G(2df,2pd) level of theory. For both species, one conformer was identified experimentally, and no methyl internal rotation splittings were observed, indicative of relatively high barriers to rotation. In the monomer, rocking of the carboxylic acid group is a large amplitude motion, characterized by a symmetrical double-well potential. This and other low-lying out-of-plane vibrations contribute to a significant (methyl top-corrected) inertial defect (-1.09 amu Å(2)). In the monohydrate, wagging of the free hydrogen atom of water is a second large amplitude motion, so the average structure is planar. As a result, no c-type transitions were observed. Water tunneling splittings were not observed, because the water rotation coordinate is characterized by an asymmetrical double-well potential. Since the minima are not degenerate, tunneling is precluded. Furthermore, a concerted tunneling path involving simultaneous rotation of the water moiety and rocking of the carboxylic acid group is precluded, because the hilltop along this coordinate is a virtual, rather than a real, saddle-point. Inter- and intramolecular non-covalent bonding is discussed in terms of the quantum theory of atoms in molecules. The percentage of o-toluic acid hydrated in the atmosphere is estimated to be about 0.1% using statistical thermodynamics.

In silico studies with substituted adenines to achieve a remarkable stability of mispairs with thymine nucleobase

The modified adenine and thymine mispair achieves a remarkable stability, which can presumably help the DNA lesions to be less cytotoxic.

Whether the amino–imino tautomerism of 2-aminopurine is involved into its mutagenicity? Results of a thorough QM investigation

2AP* mutagenic tautomer is able to induce only one incorporation error – transversion – by pairing through the H-bonds into the G·2AP* mispair.

Structural grounds for the 2-aminopurine mutagenicity: a novel insight into the old problem of the replication errors

Mutagenic pressure of the 2AP molecule on DNA during its replication is realized via the more intensive generation of the T* mutagenic tautomers through the reaction 2AP·T(WC) → 2AP·T*(w).

The computational prediction of Raman and ROA spectra of charged histidine tautomers in aqueous solution

Histidine is a key component of a number of enzymatic mechanisms, and undertakes many functionalities in biochemical systems.

Proton tunneling in the A∙T Watson-Crick DNA base pair: myth or reality?

The results and conclusions reached by Godbeer et al. in their recent work, that proton tunneling in the A∙T(WC) Watson-Crick (WC) DNA base pair occurs according to the Löwdin's (L) model, but with a small (~10(-9)) probability were critically analyzed. Here, it was shown that this finding overestimates the possibility of the proton tunneling at the A∙T(WC)↔A*∙T*(L) tautomerization, because this process cannot be implemented as a chemical reaction. Furthermore, it was outlined those biologically important nucleobase mispairs (A∙A*↔A*∙A, G∙G*↔G*∙G, T∙T*↔T*∙T, C∙C*↔C*∙C, H∙H*↔H*∙H (H - hypoxanthine)) - the players in the field of the spontaneous point mutagenesis - where the tunneling of protons is expected and for which the application of the model proposed by Godbeer et al. can be productive.

Rotational spectroscopy of methyl benzoylformate and methyl mandelate: structure and internal dynamics of a model reactant and product of enantioselective reduction

Pure rotational spectra of a prototypical prochiral ester, methyl benzoylformate (MBF), and the product of its enantioselective reduction, (R)-(-)-methyl mandelate (MM), were measured in the range of 5-16 GHz, using a cavity-based molecular beam Fourier-transform microwave spectrometer. Potential conformers were located using density functional theory calculations, and one conformer of each species was identified experimentally. The minimum energy conformer of MBF, in which the ester group is in a Z orientation, was observed for the first time. Based on an atoms-in-molecules analysis, MBF contains a weak CH···O=C hydrogen bond between the carbonyl oxygen atom of the ester group and the nearest hydrogen atom of the aromatic ring. In the minimum energy conformer of MM, the ester group is oriented to accommodate a hydrogen bond between the hydrogen atom of the hydroxyl group and the carbonyl oxygen atom (OH···O=C), rather than the sp(3) oxygen atom (OH···O-C). For both species, splittings of the rotational transitions were observed, which are attributed to methyl internal rotation, and the orientations and barrier heights of the methyl tops were determined precisely. The barrier heights for MBF and MM are 4.60(2) and 4.54(3) kJ mol(-1), respectively, which are consistent with values predicted by high-level wavefunction-based calculations. On the basis of an atoms-in-molecules analysis, we propose that destabilization of the sp(3) oxygen atom of the ester group most directly dictates the barrier height.

Unusual H-Bond Topology and Bifurcated H-bonds in the 2-Fluoroethanol Trimer

By using a combination of rotational spectroscopy and ab initio calculations, an unusual H-bond topology was revealed for the 2-fluoroethanol trimer. The trimer exhibits a strong heterochiral preference and adopts an open OH⋅⋅⋅OH H-bond topology while utilizing two types of bifurcated H-bonds involving organic fluorine. This is in stark contrast to the cyclic OH⋅⋅⋅OH H-bond topology adopted by trimers of water and other simple alcohols. The strengths of different H-bonds in the trimer were analyzed by using the quantum theory of atoms in molecules. The study showcases a remarkable example of a chirality-induced switch in H-bond topology in a simple transient chiral fluoroalcohol. It provides important insight into the H-bond topologies of small fluoroalcohol aggregates, which are proposed to play a key role in protein folding and in enantioselective reactions and separations where fluoroalcohols serve as a (co)solvent.