Computational Biotransformation Profile of Paracetamol Catalyzed by Cytochrome P450

The P450-catalyzed biotransformation of the analgesic drug paracetamol (PAR) is a long-debated topic, involving different mechanistic hypotheses as well as experimental evidence for the metabolites <i>N</i>-acetyl-<i>p</i>-benzoquinone imine (NAPQI), <i>p</i>-benzoquinone, acetamide, and 3-hydroxy-PAR. During the catalytic cycle of P450, a high-valent iron­(IV)-oxo species known as Compound I (Cpd I) is formed as the ultimate oxidant, featuring two energetically close-lying ground states in the doublet (low-spin) and quartet (high-spin) spin states, respectively. In order to clarify the catalytic mechanism, a computational chemistry analysis has been undertaken for both the high- and low-spin routes, employing density functional theory (DFT) including PCM (polarized continuum-solvation model) that yields an approximate simulation of the bulk polarization exerted through the protein. The results demonstrate that hydrogen abstraction transfer (HAT) by the P450 oxidant Cpd I (FeO) is kinetically strongly preferred over the alternative pathways of an oxygen addition reaction (OAR) or two consecutive single-electron transfers (SET). Moreover, only the respective high-spin route yields <i>N</i>-acetyl-<i>p</i>-semiquinone imine (NAPSQI) as an intermediate that is converted to the electrophile <i>N</i>-acetyl-<i>p</i>-benzoquinone imine (NAPQI). By contrast, 3-hydroxy-PAR, acetamide, and <i>p</i>-benzoquinone as electrophilic and redox-active agent are formed predominantly in the low-spin state through reactions that do not involve NAPSQI. Thus, all experimentally observed PAR metabolites are in accord with an initial HAT from the phenolic oxygen, and NAPSQI should indeed be the precursor of NAPQI, both of which are generated only via the high-spin pathway

Computational Evidence for α-Nitrosamino Radical as Initial Metabolite for Both the P450 Dealkylation and Denitrosation of Carcinogenic Nitrosamines

The mutagenic and carcinogenic potency of α-CH_<i>n</i>-nitrosamines such as <i>N</i>-nitrosodimethylamine (NDMA) is caused by their P450-catalyzed α-hydroxylation and subsequent dealkylation, yielding alkyl diazonium ions (R−NN⁺) as potent electrophiles. Alternatively, P450s may also catalyze their denitrosation as metabolic detoxification. DFT calculations at the UB3LYP/LANL2DZ(Fe)/6-31G+**(H,C,N,O,S)//LANL2DZ(Fe)/6-31G(H,C,N,O,S) level of theory show that H-abstraction from the α-C of NDMA as initial metabolic step yields an α-nitrosamino radical (•CH₂N(CH₃)NO) as common first intermediate for both the oxidative dealkylation (toxification) and denitrosation (detoxification) pathways. In particular, the calculated kinetic isotope effect for the P450-mediated dealkylation of NDMA is in good agreement with experimental information. The results show further that the initial α-hydroxylation of NDMA may proceed in two spin states. Besides a stepwise high-spin (HS, quartet) route with a separate rebound barrier, there is a concerted low-spin (LS, doublet) pathway. Interestingly, the resultant two-state reactivity appears to discriminate between metabolic toxification and detoxification: Evaluation of calculated free energy barriers of the H-abstraction (Δ<i>G</i>^‡) through the Eyring equation suggests that the dealkylation:denitrosation product ratio is governed by the LS:HS ratio of the overall metabolic process. Moreover, inclusion of three further α-CH_<i>n</i>-nitrosamines in the computational analysis demonstrates that the initial H-abstraction barrier is proportional to the C–H bond dissociation enthalpy (BDE) of the substrates, which enables the estimation of spin-averaged reaction barriers through ground-state BDE calculations. The discussion includes also reductive denitrosation pathways that according to current computational evidence appear to be unlikely for aliphatic nitrosamines

<i>In Silico</i> Prediction of Cytochrome P450-Mediated Biotransformations of Xenobiotics: A Case Study of Epoxidation

Predicting the biotransformation of xenobiotics is important in toxicology; however, as more compounds are synthesized than can be investigated experimentally, powerful computational methods are urgently needed to prescreen potentially useful candidates. Cytochrome P450 enzymes (P450s) are the major enzymes involved in xenobiotic metabolism, and many substances are bioactivated by P450s to form active compounds. An example is the conversion of olefinic substrates to epoxides, which are intermediates in the metabolic activation of many known or suspected carcinogens. We have calculated the activation energies for epoxidation by the active species of P450 enzymes (an iron-oxo porphyrin cation radical oxidant, compound I) for a diverse set of 36 olefinic substrates with state-of-the-art density functional theory (DFT) methods. Activation energies can be estimated by the computationally less demanding method of calculating the ionization potentials of the substrates, which provides a useful and simple predictive model based on the reaction mechanism; however, the preclassification of these diverse substrates into weakly polar and strongly polar groups is a prerequisite for the construction of specific predictive models with good predictability for P450 epoxidation. This approach has been supported by both internal and external validations. Furthermore, the relation between the activation energies for the regioselective epoxidation and hydroxylation reactions of P450s and experimental data has been investigated. The results show that the computational method used in this work, single-point energy calculations with the B3LYP functional including zero-point energy and solvation and dispersion corrections based on B3LYP-optimized geometries, performs well in reproducing the experimental trends of the epoxidation and hydroxylation reactions

Modeling of Toxicity-Relevant Electrophilic Reactivity for Guanine with Epoxides: Estimating the Hard and Soft Acids and Bases (HSAB) Parameter as a Predictor

According to the electrophilic theory in toxicology, many chemical carcinogens in the environment and/or their active metabolites are electrophiles that exert their effects by forming covalent bonds with nucleophilic DNA centers. The theory of hard and soft acids and bases (HSAB), which states that a toxic electrophile reacts preferentially with a biological macromolecule that has a similar hardness or softness, clarifies the underlying chemistry involved in this critical event. Epoxides are hard electrophiles that are produced endogenously by the enzymatic oxidation of parent chemicals (e.g., alkenes and PAHs). Epoxide ring opening proceeds through a S_N2-type mechanism with hard nucleophile DNA sites as the major facilitators of toxic effects. Thus, the quantitative prediction of chemical reactivity would enable a predictive assessment of the molecular potential to exert electrophile-mediated toxicity. In this study, we calculated the activation energies for reactions between epoxides and the guanine N7 site for a diverse set of epoxides, including aliphatic epoxides, substituted styrene oxides, and PAH epoxides, using a state-of-the-art density functional theory (DFT) method. It is worth noting that these activation energies for diverse epoxides can be further predicted by quantum chemically calculated nucleophilic indices from HSAB theory, which is a less computationally demanding method than the exacting procedure for locating the transition state. More importantly, the good qualitative/quantitative correlations between the chemical reactivity of epoxides and their bioactivity suggest that the developed model based on HSAB theory may aid in the predictive hazard evaluation of epoxides, enabling the early identification of mutagenicity/carcinogenicity-relevant S_N2 reactivity

Optimization of PbI₂/MAPbI₃ Perovskite Composites by Scanning Electrochemical Microscopy

A variety of PbI₂/MAPbI₃ perovskites were prepared and investigated by a rapid screening technique utilizing a modified scanning electrochemical microscope (SECM) in order to determine how excess PbI₂ affects its photoelectrochemical (PEC) properties. An optimum ratio of 2.5% PbI₂/MAPbI₃ was found to enhance photocurrent over pristine MAPbI₃ on a spot array electrode under irradiation. With bulk films of various PbI₂/MAPbI₃ composites prepared by a spin-coating technique of mixed precursors and a one-step annealing process, the 2.5% PbI₂/MAPbI₃ produced an increased photocurrent density compared to pristine MAPbI₃ for 2 mM benzoquinone (BQ) reduction at −0.4 V vs Fc/Fc⁺. As a result of the relatively high quantum yield of MAPbI₃, a time-resolved photoluminescence quenching experiment could be applied to determine electron–hole diffusion coefficients and diffusion lengths of PbI₂/MAPbI₃ composites, respectively. The diffusion coefficients combined with the exciton lifetime of the pristine 2.5% PbI₂/MAPbI₃ (τ_PL = 103.3 ns) give the electron and hole exciton diffusion lengths, ∼300 nm. Thus, the 2.5% PbI₂/MAPbI₃ led to an approximately 3.0-fold increase in the diffusion length compared to a previous report of ∼100 nm for the pristine MAPbI₃ perovskite. We then demonstrated that the efficiency of liquid-junction solar cells for 2.5% excess PbI₂ of p-MAPbI₃ was improved from 6.0% to 7.3%

A Liquid Junction Photoelectrochemical Solar Cell Based on p‑Type MeNH₃PbI₃ Perovskite with 1.05 V Open-Circuit Photovoltage

A liquid junction photoelectrochemical (PEC) solar cell based on p-type methylammonium lead iodide (p-MeNH₃PbI₃) perovskite with a large open-circuit voltage is developed. MeNH₃PbI₃ perovskite is readily soluble or decomposed in many common solvents. However, the solvent dichloromethane (CH₂Cl₂) can be employed to form stable liquid junctions. These were characterized with photoelectrochemical cells with several redox couples, including I₃^–/I^–, Fc/Fc⁺, DMFc/DMFc⁺, and BQ/BQ^•– (where Fc is ferrocene, DMFc is decamethylferrocene, BQ is benzoquinone) in CH₂Cl₂. The solution-processed MeNH₃PbI₃ shows cathodic photocurrents and hence p-type behavior. The difference between the photocurrent onset potential and the standard potential for BQ/BQ^•– is 1.25 V, which is especially large for a semiconductor with a band gap of 1.55 eV. A PEC photovoltaic cell, with a configuration of p-MeNH₃PbI₃/CH₂Cl₂, BQ (2 mM), BQ^•– (2 mM)/carbon, shows an open-circuit photovoltage of 1.05 V and a short-circuit current density of 7.8 mA/cm² under 100 mW/cm² irradiation. The overall optical-to-electrical energy conversion efficiency is 6.1%. The PEC solar cell shows good stability for 5 h under irradiation

Electrochemical Formation of a <i>p–n</i> Junction on Thin Film Silicon Deposited in Molten Salt

Herein we report the demonstration of electrochemical deposition of silicon <i>p–n</i> junctions all in molten salt. The results show that a dense robust silicon thin film with embedded junction formation can be produced directly from inexpensive silicates/silicon oxide precursors by a two-step electrodeposition process. The fabricated silicon <i>p–n</i> junction exhibits clear diode rectification behavior and photovoltaic effects, indicating promise for application in low-cost silicon thin film solar cells

High-Performance Photodetectors Based on Solution-Processed Epitaxial Grown Hybrid Halide Perovskites

Hybrid organic–inorganic halide perovskites (HOIPs) have recently attracted tremendous attention because of their excellent semiconducting and optoelectronic properties, which exist despite their morphology and crystallinity being far inferior to those of more mature semiconductors, such as silicon and III–V compound semiconductors. Heteroepitaxy can provide a route to achieving high-performance HOIP devices when high crystalline quality and smooth morphology are required, but work on heteroepitaxial HOIPs has not previously been reported. Here, we demonstrate epitaxial growth of methylammonium lead iodide (MAPbI₃) on single crystal KCl substrates with smooth morphology and the highest carrier recombination lifetime (∼213 ns) yet reported for nonsingle crystalline MAPbI₃. Experimental Raman spectra agree well with theoretical calculations, presenting in particular a sharp peak at 290 cm^–1 for the torsional mode of the organic cations, a marker of orientational order and typically lacking in previous reports. Photodetectors were fabricated showing excellent performance, confirming the high quality of the epitaxial MAPbI₃ thin films. This work provides a new strategy to enhance the performance of all HOIPs-based devices

Down-Regulated Receptor Interacting Protein 140 Is Involved in Lipopolysaccharide-Preconditioning-Induced Inactivation of Kupffer Cells and Attenuation of Hepatic Ischemia Reperfusion Injury

<div><p>Background</p><p>Lipopolysaccharide (LPS) preconditioning is known to attenuate hepatic ischemia/reperfusion injury (I/RI); however, the precise mechanism remains unclear. This study investigated the role of receptor-interacting protein 140 (RIP140) on the protective effect of LPS preconditioning in hepatic I/RI involving Kupffer cells (KCs).</p><p>Methods</p><p>Sprague—Dawley rats underwent 70% hepatic ischemia for 90 minutes. LPS (100 μg/kg) was injected intraperitoneally 24 hours before ischemia. Hepatic injury was observed using serum and liver samples. The LPS/NF-κB (nuclear factor-κB) pathway and hepatic RIP140 expression in isolated KCs were investigated.</p><p>Results</p><p>LPS preconditioning significantly inhibited hepatic RIP140 expression, NF-κB activation, and serum proinflammatory cytokine expression after I/RI, with an observation of remarkably reduced serum enzyme levels and histopathologic scores. Our experiments showed that protection effects could be effectively induced in KCs by LPS preconditioning, but couldn’t when RIP140 was overexpressed in KCs. Conversely, even without LPS preconditioning, protective effects were found in KCs if RIP140 expression was suppressed with siRNA.</p><p>Conclusions</p><p>Down-regulated RIP140 is involved in LPS-induced inactivation of KCs and hepatic I/RI attenuation.</p></div

The influence of LPS preconditioning dose on hepatic ischemia/reperfusion injury.

<p>Animals subjected to 90 minutes of 70% hepatic ischemia, followed by 6 h reperfusion, then the hepatic I/RI was evaluated. <b>(A)</b> The H&E staining of the liver from sham group, LPS + I/RI group and I/RI group. <b>(B)</b> ALT serum level. <b>(C)</b> Suzuki’s pathological score (*, <i>P</i> < 0.01).</p