Effect of secondary MO interaction on the regiospecificity in the reactions of α-ionones with singlet oxygen

The eme reactions of α-ionones with singlet oxygen were examined to ascertain theeffect of secondary Me interaction between the reactants on the reaction regiospecificity.Exclusive formation of 3-hydroxy-γ-ionones found in the reactions reveals favorable interaction ofsinglet oxygen with the acyclic α-hydrogen atom,On the other hand,no formation of 3-hy-droxy-β-ionones implies that the steric requirement was not met for the bond formation betweenzwitterionic perepoxide with C_1-hydrogen in the process.The MMP21 and MNDO calculationsindicate minus value of the secondary interaction energy for the aeyelie α-hydrogen abstrac-tion and a repulsion between the oxygen with C_1-hydrogen atom.Twisting tilting of the doublebond may account for favorable attack of singlet oxygen on C_3.An explanation of the excellentregiospecificity was addressed and placed in proper mechanic prospective.     

On the dimers of β-tocopherol

Upon oxidation in aprotic media, β-tocopherol (2) forms a spiro-dimer (10) as the main product. The reaction mechanism is a hetero-Diels-Alder process with inverse electron demand of two intermediate ortho-quinone methide molecules. The spiro-dimer can be reduced to the corresponding symmetric ethano-dimer (11). In contrast to the well-studied α-tocopherol case, spiro-dimer and ethano-dimer do not form a reversible redox pair, their interconversion is accompanied by coupling reactions at C-7 with 7a-(β-tocopher-5a-yl)-β-tocopherol (13) as the main byproduct besides some oligomeric material. The full NMR assignments (¹H, ¹³C) of the β-tocopherol oxidation products are given.     

Ab initio study on mechanisms of singlet oxygen reaction with ethylene

The reaction of singlet oxygen with ethylene has been studied at the ab initio level with both HF/3-21G and HF/6-31G basis sets, fully optimizing the geometries of the critical points. The transition state leading to the intermediate peroxirane from the initial reactants is found,Iocated at 81.71 kJ/mol above the dissociation limit. The vibrational analysis is done with two basis sets. From the results it can be seen that the frequencies have not been made an improvement obviously with augmented 6-31G compared to 3-21G basis; it follows that main reason for too high HF/3-21G frequency could mainly be the vibrational anharmonicity. The eigenvector corresponding to the single imaginary vibrational frequency is dominated by the larger O-O distance. The finding of the transition state confirms that the peroxirane minimum can be reachable passing through a peroxirane-like saddle point. In addition , the mechanisms of the reaction forming oxirane are discussed as well.     

EPR study on the photosensitized generation of reactive oxygen species by actinomycin D

Actinomycin D (AMD) is an anticancer antibiotic that can bind selectively to both double-stranded and single-stranded DNA, and this binding greatly enhances DNA photosensitization. Using electron paramagnetic resonance (EPR) in combination with spin trapping techniques, a systematic study was carried out on the reactive oxygen species generated in the photosensitization process of AMD. It was found that 1O2 and O2- are important reactive intermediates either insolution or in DNA complexes, and the generation of these species is in competition. This finding suggests that the photodynamic action of AMD proceeds via two pathways: energy transfer (type Ⅰ mechanism) and electron transfer (type Ⅱ mechanism). 1O2 is the main product formed via energy transfer reaction in solution while electron transfer between the excited states of AMD and DNA becomes the predominant pathway in DNA complexes.     

Solvent- and temperature-controlled inversion of π-facial selectivity in the 1,2-cycloaddition of singlet oxygen to hydroxyphenyl-substituted cyclohexadihydrofurans

Singlet-oxygenation of 3-hydroxyphenyl-substituted dihydrofurans fused with a cyclohexane 1a?c exclusively gave the corresponding syn/anti-stereoisomeric mixtures of dioxetanes 2a?c. The syn/anti-π-facial selectivity in the 1,2-cycloaddition of singlet oxygen (¹O₂) was found to be remarkably sensitive to the solvent as well as the reaction temperature. In fact, the solvent effect was so conspicuous that inversion of the syn/anti-π-facial selectivity was observed in different solvents, such as chloroform and toluene. An LSER (linear solvation energy relationships) analysis suggested that the Lewis-acidity/basicity and HBD (hydrogen-bond donor)/HBA (hydrogen-bond acceptor) ability as well as dipolarity/polarizability of the solvent played an important role in this change in syn/anti-π-facial selectivity. An investigation of the temperature-dependency of the singlet-oxygenation suggested that the syn/anti-π-facial-selective 1,2-cycloaddition of ¹O₂ to 1 was a conformationally-(entropy-) controlled process.     

Improving the Cβ Stereoselectivity of l-Threonine Aldolase for the Synthesis of l-threo-4-Methylsulfonylphenylserine by Modulating the Substrate-Binding Pocket To Control the Orientation of the Substrate Entrance

l -Threonine aldolase from Actinocorallia herbida (AhLTA) is an ideal catalyst for producing l -threo-4-methylsulfonylphenylserine [(2S,3R)- 1 b ], a key chiral precursor for florfenicol and thiamphenicol. The moderate C_β stereoselectivity is the main obstacle to the industrial application of AhLTA. To address this issue, a combinatorial active-site saturation test (CAST) together with sequence conservatism analysis was applied to engineer the AhLTA toward improved C_β stereoselectivity. The optical mutant Y314R could asymmetrically synthesize l -threo-4-methylsulfonylphenylserine with 81 % diastereomeric excess (de), which is 23 % higher than wild-type AhLTA. Molecular dynamic (MD) simulations revealed that the mechanism for the improvement in C_β stereoselectivity of Y314R is due to the acylamino group of residues Arg314 controlling the orientation of substrate 4-methylsulfonyl benzaldehyde ( 1 a ) in the active pocket by directed interaction with the methylsulfonyl group; this leads to asymmetric synthesis of l -threo-4-methylsulfonylphenylserine. The success in this study demonstrates that direct control of substrates in an active pocket is an attract strategy to address the C_β stereoselectivity problem of LTA and contribute to the industrial application of LTA.     

Quantum-chemical study of the singlet potential energy surface of the nitrene—dioxygen system

The potential energy surfaces of the HN—O₂ and PhN—O₂ systems were calculated by the MP2 and B3LYP methods. The mechanism of photooxidation of azides was refined. Photooxidation produces the nitrene—O₂ adducts with dioxaziridine and non-cyclic structures. The parameters of IR spectra of the adducts were calculated. The rearrangement of dioxaziridine to a nitro compound is likely a reason for chemiluminescence accompanying the photooxidation of azides.     

Synthesis of C6-to-C6 and C3-to-C3 diamide linked γ-cyclodextrin dimers

The syntheses of three new diamide-linked γ-cyclodextrin dimers joined by substitution at either a glucopyranose C6^A or C3^A carbon are reported. The syntheses involve the reaction of either C6^A or C3^A amino-substituted γ-cyclodextrin with bis(4-nitrophenyl)succinate to form succinamide linked γ-cyclodextrin dimers or reaction of C6^A azide-substituted γ-cyclodextrin with carbon dioxide to form a urea linked γ-cyclodextrin dimer.     

N-Methylanilinium trifluoroacetate-promoted Prins reaction of α-methylene-α-tetralone dimers: generation of new molecular scaffolds

α-Methylene-α-tetralone dimers (8-10) are shown to undergo a diastereoselective Prins reaction with paraformaldehyde in the presence of N-methylanilinium trifluoroacetate resulting in 2, 4, and 5. The structure of the new scaffold 2 was confirmed by X-ray crystallographic analysis.     

Is a strain an order parameter in the α-β phase transition of solid oxygen?

Theoretical arguments have been presented supporting an elastic strain as an order parameter of the α-β phase transition. Simple numerical calculations have been made to illustrate such a model.     

Photokinetics in tetraphenylporphyrin – molecular oxygen system at gas/solid interfaces: effect of singlet oxygen quenchers on oxygen-induced delayed fluorescence

The oxygen quenching of excited states and concomitant delayed fluorescence (DF) of meso-tetraphenylporphyrin (TPP) adsorbed on alumina (Al₂O₃) were studied at different TPP and O₂ concentrations and temperatures by the diffuse-reflectance laser flash technique. The formation of ¹O₂ in the course of ³TPP quenching by ³O₂ is followed by the energy transfer from ¹O₂ to ³TPP (¹O₂ feedback) with the generation of TPP fluorescent state. The global kinetic analysis of DF revealed variations on kinetic parameters with surface loading which match the aggregation of TPP on the surface. In concentrated samples the energy exchange between ¹O₂ and ³O₂ accelerates the ¹O₂ feedback more than 10 times. The key role of ¹O₂ in oxygen-induced DF is confirmed by the DF quenching by coadsorbed ¹O₂ quenchers (NaN₃, 3-methylindole, 1,4-diazabicyclo[2.2.2]octane). This process is in part controlled by the surface which enhances the efficiencies of amine quenchers but reduces that of NaN₃ when compared with the corresponding efficiencies in solution.     

Stacking of nucleic acid bases: optimization of the computational approach—the case of adenine dimers

Structural Chemistry - Stacking interactions play an important role in stabilizing DNA and RNA secondary structure. To select a computational level to study the stacking interactions, both energy...     

Efficient singlet oxygen generation by luminescent 2-(2′-thienyl)pyridyl cyclometalated platinum(II) complexes and their calixarene derivatives

Luminescent cyclometalated platinum(II) complexes, namely [Pt(Thpy)(PPh₃)X]ⁿ⁺ (HThpy = 2-(2′-thienyl)pyridine; X = Cl⁻ ( 1 ), n = 0; X = CH₃CN ( 2 ), pyridine ( 3 ), n = 1) and [Pt(Thpy)(HThpy)Y] ^{n +} (Y = Cl⁻ ( 4 ), n = 0; Y = pyridine ( 5 ), n = 1), exhibit structured emission with peak maximum at ∼556 and 598 nm in degassed acetonitrile and with emission quantum yield and lifetime of up to 0.38 and 26 μs, respectively. These complexes are efficient photosensitizers of singlet oxygen with yields up to >90%. Complex 5 exhibited photocytotoxicity towards cancer cells and fluorescence microscopic images of cells incubated with 5 reveal substantial uptake at the nucleus and mitochondria.     

Chiroptical spectra of bicyclic selenolactams. Optical activity of the singlet–triplet transition

Two selenolactams with rigid 2-azabicyclo[2.2.1]heptane skeletons were prepared by reaction of the corresponding lactams with P₄Se₁₀. A comparison of their UV–vis and CD spectra with those of the carbonyl and thiocarbonyl analogues showed a similar character of the lowest-energy electronic transitions and the same signs of the corresponding Cotton effects. The MCD spectra of selenolactams revealed that their n–π* absorption band is dominated by the singlet–triplet component. A very weak CD corresponding to this component has an opposite sign to its much stronger singlet–singlet counterpart observed at the blue edge of the n–π* band.     

Hydrogenation of α-methylstyrene linear dimers in the presence of Pd- and Ni-containing catalysts

Hydrogenation of α-methylstyrene linear dimers and of 4-methyl-2,4-diphenylpentane on commercial Pd-containing and Ni (GM-3 grade) catalysts was studied under the following conditions: T = 75–190°C, P(H₂) = 0.2–0.8 MPa, G _c = 2.5–10 wt %, and τ = 1–4 h.     

Complete active space analysis of a reaction pathway: Investigation of the oxygen–oxygen bond formation

Water nucleophilic attack is an important step in water oxidation reactions, which have been widely studied using density functional theory (DFT). Nevertheless, a single-determinant DFT picture may be insufficient for a deeper insight into the process, in particular during the oxygen–oxygen bond formation. In this work, we use complete active space self-consistent field calculations and describe an approach for a complete active space analysis along a reaction pathway. This is applied to the water nucleophilic attack at a Ru-based catalyst, which has successfully been used for efficient water oxidation and in silico design of new water oxidation catalysts recently.     

Theoretical study of the mechanism for the ClOO + NO reaction on the singlet potential energy surface

A detailed quantum chemical study is performed on the mechanism of ClOO + NO reaction at the B3LYP/6-311+G (2d) level of theory combined with CCSD (T) single point energy calculation. The possible product channels for the reaction are obtained and discussed on the basis of the singlet [ClNO₃] potential energy surface. The calculation indicates that the dominant product for the title reaction is ClO + NO₂ by the direct dissociation of the initial adduct, and the formation of the other products is much less likely since they are unfavorable kinetically. A comparison is also made between the title reaction and the analogous reaction of FO₂ + NO to gain a deeper insight into the mechanism of the XO₂ + NO reactions.     

Modulating the Biofunctionality of Metal–Organic-Framework-Encapsulated Enzymes through Controllable Embedding Patterns

Embedding an enzyme within a MOF as exoskeleton (enzyme@MOF) offers new opportunities to improve the inherent fragile nature of the enzyme, but also to impart novel biofunctionality to the MOF. Despite the remarkable stability achieved for MOF-embedded enzymes, embedding patterns and conversion of the enzymatic biofunctionality after entrapment by a MOF have only received limited attention. Herein, we reveal how embedding patterns affect the bioactivity of an enzyme encapsulated in ZIF-8. The enzyme@MOF can maintain high activity when the encapsulation process is driven by rapid enzyme-triggered nucleation of ZIF-8. When the encapsulation is driven by slow coprecipitation and the enzymes are not involved in the nucleation of ZIF-8, enzyme@MOF tends to be inactive owing to unfolding and competing coordination caused by the ligand, 2-methyl imidazole. These two embedding patterns can easily be controlled by chemical modification of the amino acids of the enzymes, modulating their biofunctionality.