Elucidating the photophysical mechanisms in sulfur-substituted nucleobases (thiobases) is essential for designing prospective drugs for photo- and chemotherapeutic applications. Although it has long been established that the phototherapeutic activity of thiobases is intimately linked to efficient intersystem crossing into reactive triplet states, the molecular factors underlying this efficiency are poorly understood. Herein we combine femtosecond transient absorption experiments with quantum chemistry and nonadiabatic dynamics simulations to investigate 2-thiocytosine as a necessary step to unravel the electronic and structural elements that lead to ultrafast and near-unity triplet-state population in thiobases in general. We show that different parts of the potential energy surfaces are stabilized to different extents via thionation, quenching the intrinsic photostability of canonical DNA and RNA nucleobases. These findings satisfactorily explain why thiobases exhibit the fastest intersystem crossing lifetimes measured to date among bio-organic molecules and have near-unity triplet yields, whereas the triplet yields of canonical nucleobases are nearly zero. Sulfur-substituted nucleobases are promising photo- and chemotherapeutic drugs. Here, the authors unravel the electronic and structural aspects that lead to the ultrafast population of triplet states in these molecules, providing an explanation for their efficiency as photosensitizers.

/pdf/the-origin-of-efficient-triplet-state-population-in-sulfur-23f69vj553.pdf

The origin of efficient triplet state population in sulfur-substituted nucleobases.

Sulfur-substituted nucleobases (a.k.a., thiobases) are among the world's leading prescriptions for chemotherapy and immunosuppression. Long-term treatment with azathioprine, 6-mercaptopurine and 6-thioguanine has been correlated with the photoinduced formation of carcinomas. Establishing an in-depth understanding of the photochemical properties of these prodrugs may provide a route to overcoming these carcinogenic side effects, or, alternatively, a basis for developing effective compounds for targeted phototherapy. In this review, a broad examination is undertaken, surveying the basic photochemical properties and excited-state dynamics of sulfur-substituted analogs of the canonical DNA and RNA nucleobases. A molecular-level understanding of how sulfur substitution so remarkably perturbs the photochemical properties of the nucleobases is presented by combining experimental results with quantum-chemical calculations. Structure-property relationships demonstrate the impact of site-specific sulfur substitution on the photochemical properties, particularly on the population of the reactive triplet state. The value of fundamental photochemical investigations for driving the development of ultraviolet-A chemotherapeutics is showcased. The most promising photodynamic agents identified thus far have been investigated in various carcinoma cell lines and shown to decrease cell proliferation upon exposure to ultraviolet-A radiation. Overarching principles have been elucidated for the impact that sulfur substitution of the carbonyl oxygen has on the photochemical properties of the nucleobases.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/php.12975

Photochemical and Photodynamical Properties of Sulfur-Substituted Nucleic Acid Bases.

The steady-state and time-resolved photochemistry of the natural nucleic acid bases and their sulfur- and nitrogen-substituted analogues in solution is reviewed. Emphasis is given to the experimental studies performed over the last 3–5 years that showcase topical areas of scientific inquiry and those that require further scrutiny. Significant progress has been made toward mapping the radiative and nonradiative decay pathways of nucleic acid bases. There is a consensus that ultrafast internal conversion to the ground state is the primary relaxation pathway in the nucleic acid bases, whereas the mechanism of this relaxation and the level of participation of the 1πσ*, 1 nπ*, and 3ππ* states are still matters of debate. Although impressive research has been performed in recent years, the microscopic mechanism(s) by which the nucleic acid bases dissipate excess vibrational energy to their environment, and the role of the N-glycosidic group in this and in other nonradiative decay pathways, are still poorly understood. The simple replacement of a single atom in a nucleobase with a sulfur or nitrogen atom severely restricts access to the conical intersections responsible for the intrinsic internal conversion pathways to the ground state in the nucleic acid bases. It also enhances access to ultrafast and efficient intersystem crossing pathways that populate the triplet manifold in yields close to unity. Determining the coupled nuclear and electronic pathways responsible for the significantly different photochemistry in these nucleic acid base analogues serves as a convenient platform to examine the current state of knowledge regarding the photodynamic properties of the DNA and RNA bases from both experimental and computational perspectives. Further investigations should also aid in forecasting the prospective use of sulfur- and nitrogen-substituted base analogues in photochemotherapeutic applications.

Photochemistry of nucleic acid bases and their thio- and aza-analogues in solution.

Photocatalysts for a sustainable future: Innovations in large-scale environmental and energy applications.

Perovskite oxides as active materials in novel alternatives to well-known technologies: A review

This work reports the effectiveness of TiO2 photocatalysts modified by defect engineering methods. The prepared photocatalysts were characterized by X-ray diffraction (XRD), UV–Vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL) measurements and electron spin resonance (ESR) technique. The immobilized photocatalyst layer was prepared using an abrasive material as a support and a mineral binder. To the best of our knowledge, this is the first time that such type of the immobilized photocatalytic layer has been prepared and tested. A home-made flat-plate photoreactor (FPP) with artificial light sources was used for the degradation of an aqueous solution of imidacloprid using an immobilized TiO2. The effects of different operating variables such as irradiation source, recirculation flow rate, irradiated surface area and photoreactor volume on the photocatalytic performances were investigated. The hydrodynamic behavior of the flat-plate photoreactor (FPP) was studied utilizing a residence time distribution (RTD) technique using the axial dispersion model. Based on kinetic measurements it was found that the photocatalytic degradation of imidacloprid under the conditions used in this study can be described by a complex kinetic model that follows pseudo-zero order kinetics at the beginning of the degradation, and with the progress of the reaction at higher irradiation times changes to the pseudo-first order. Liquid chromatography - quadrupole time‐of‐flight mass spectrometry (LC‐QTOF‐MS) was used to identify imidacloprid degradation by-products as a function of irradiation time. Theoretical modeling of the geometry and electronic structure of imidacloprid in aqueous solution was performed using density functional theory (DFT) calculation. Based on the results of theoretical simulations and experimental measurements, the possible pathway for photodegradation of imidacloprid in aqueous phase after irradiation with UVA was proposed and discussed.

Photocatalytic degradation of imidacloprid in the flat-plate photoreactor under UVA and simulated solar irradiance conditions—The influence of operating conditions, kinetics and degradation pathway

Thioanalogs of the nucleic-acid bases are known photosensitive
probes and good traps of radiation energy. Radiation-induced sulfur-centered
free radicals stabilized on the base thioanalogs molecules are of the cationic
origin. Their electronic configuration is of the π type, unless an electron-donating
group, like Cl−, is added to the sulfur atom, forming the σσ*
“ three-electron” S∴Cl bond. The σ radical
observed in irradiated 2-thiothymine is formed simply by deprotonation
of the pristine cation radical. The 2σ character of the radical is determined
from the nature of the g tensor and the A(14N)
coupling tensor and their relation to the radical skeleton. The deprotonation
of the cation radical takes place at N(3) rather than at N(1), generally observed
in the irradiated pyrimidine natural bases and their other thioanalogs.

Sigma radicals in gamma-irradiated single crystals of 2-thiothymine

The excess electrons or holes transported in nucleic acids and related systems of stacked bases are trapped at the sites determined by their electronic properties. The trapping sites for holes are determined predominantly by the lowest ionization potentials (IPs) of the constituents. Here the calculations of IPs for a number of thioanalogs of the nucleic-acid bases and the base derivatives in the partial third-order electron propagator (P3) method are reported. It is shown that IPs of the thioanalogs are lower than the values of the respective bases, justifying the use of the thioanalogs as hole traps in conjunction with the natural bases. The mercaptoguanines have the lowest IPs from all the molecules studied. The comparison of the results derived in the P3 and Koopmans approaches shows that these two methods might predict different ordering of the energy levels.

Ionization potentials of nucleobase analogs using partial third-order electron propagator method

Experimental and theoretical (ReaxFF) study of manganese-based catalysts for low-temperature toluene oxidation

Thioanalogues of nucleic-acid bases have been shown to be the place of preferential charge or energy localization when incorporated in the system of respective regular bases. Most evidence of such behavior is based on experiments using γ-ray or other ionizing radiation. Here time dependent density functional theory (TDDFT) has been applied to investigate singlet and triplet excited state spectra of thioanalogues and some oxo-derivatives of nucleic-acid bases and compare the spectra to the calculated spectra of the regular bases as well as to the existing experimental data. It has been shown that thioanalogues have lower excitation singlet and triplet energies than are the respective energies in the regular bases. Energy localization on thioanalogues and their spectroscopic properties are related to the effect of sulfur substitution in such systems.

Tddft study of nucleobase thioanalogues and oxo-derivatives excited states

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