Incorporation of 2,3-Disubstituted-1,4-Naphthoquinones into the A1 Binding Site of Photosystem I Studied by EPR and ENDOR Spectroscopy

Transient electron paramagnetic resonance and pulsed electron-nuclear double resonance (ENDOR) spectra of the state $ P_{700}^{ \cdot + } A_{1}^{ \cdot - } $ in photosystem I containing a series of non-native naphthoquinones (NQs) are presented. Previous studies have shown that quinones bind to the A₁ site with only one of their carbonyl groups H-bonded to the protein and that the asymmetric H-bond produces an odd alternant distribution of the spin density within the quinone. It is known that the native phylloquinone binds with its methyl group meta and its phytyl tail ortho to the H-bonded carbonyl. Monosubstituted NQs with short alkyl chains have been found to bind preferentially with their alkyl side groups meta to the H-bonded carbonyl. The selectivity of the binding site toward methyl and short chain substituents is studied by incorporating disubstituted NQs that have a methyl group at the 2-position and a short chain at the 3-position of the quinone ring. The hyperfine couplings (hfcs) of the methyl group protons are sensitive to the spin density distribution on the quinone and are used to deduce the position of the methyl group relative to the H-bonded carbonyl. The measured methyl proton hfcs indicate that the disubstituted quinones bind exclusively with their methyl group in the meta position relative to the H-bonded carbonyl and no evidence for binding with the methyl group in the ortho position is found. The disubstituted quinones have also been chosen to study the effect of electron withdrawing substituents on the spin density distribution. When the short chain contains electronegative atoms such as sulfur or chlorine, the methyl proton hfcs of the quinone in the A₁ binding site are found to be significantly larger than those of 2-methyl-1,4-naphthoquinone and phylloquinone in the same environment. Solution ENDOR measurements of the quinone radical anions in isopropanol and density functional theory (DFT) calculations in vacuo show that this increase in the hfcs is mostly intrinsic to the quinones due to the electron-withdrawing ability of the short chain and is not a result of differences in the binding to the protein. The DFT calculations suggest that the main reason for the increased methyl proton hfcs is delocalization of the singly occupied molecular orbital onto the side chain, which leads to an increase of the spin density on the neighboring carbon, which carries methyl group.     

An EPR and DFT study on the primary radical formed in hydroxylation reactions of 2,6-dimethoxy-1,4-benzoquinone

The quinone compound 2,6-dimethoxy-1,4-benzoquinone is hydroxylated in alkaline aqueous solution with pH above 12. Electron paramagnetic resonance experiments showed that two transient radicals are formed in this reaction. The radical appearing first is assigned to a one electron reduced 2,6-dimethoxy-1,4-benzoquinone, receiving the electron from an intermediate anionic hydroxylated species. For this primary radical, all proton couplings were determined (quinoid ring protons: 1.453?G, methyl protons: 0.795?G). The density functional theory method was applied to obtain electronic and structural information of the primary radical and a solution structure is suggested. For approaching the experimental hyperfine couplings in theoretical models, it was necessary to consider effects of external polarisation arising from water molecules near one carbonyl group, and the orientation of methoxy groups towards the quinone ring. With this approach, the secondary radical formed in the hydroxylation reaction, and the transient radicals found for other biologically important quinones (including coenzymes Q) and their hydroxylated species may become accessible.     

Electron Spin Resonance of Alkali Metal Contact Ion Pairs of 2,6-di-tert-Butylbenzoquinone in Non Polar Solvent

The question whether excited triplet states of quinones react with a number of substrates such as alcohols, phenols, and amines, via electron transfer mechanism has attracted much attention in recent years. The existence of some triplet exciplex was postulated by Kobashi et al¹ in their study of hydrogen atom abstraction by p-chloranil using laser flash spectrocopic detection. In some recent e.s.r. and CIDEP studies^2,3, however, there is no compelling evidence that the photoreduction of quinones and benzophenones undergoes an initial electron transfer mechanism. It should be noted that most of the e.s.r. studies of quinone radical anions had been carried out in polar solvents. In flash photolysis studies some evidence has indeed been obtained^1,4 in which the efficiency of hydrogen abstraction by excited triplet quinones increases with solvent polarity and therefore it is possible that the initial primary process involves electron transfer followed immediately by proton transfer. On the other hand, we     

Chemically Induced Dynamic Magnetic Polarization. Part X. The Photoreduction of 2-Methyl-p-benzoquinone

While the photoreduction of quinones has been initially used by us as a model system to probe the role of excited triplet states in CIDEP¹ and CIDNP² phenomena, our continuing interest in the quinone photochemical systems is partly due to their photobiological importance. Alkyl substituted quinones have in fact been shown to be associated with chlorophyll³ and to be involved in photosynthesis⁴. The 2-methyl-p-benzoquinone (MQ) is the simplest unsymmetrical alkyl substituted p-quinone which, during photolysis, may lead to two isomeric semiquinone radicals. We are particularly interested in applying the CIDEP and CIDNP techniques to investigate the structure and reactivity of the intermediate semiquinone radicals as they may give some insight to the photochemical properties of the parent 2-methyl-p-quinone.     

Transient and pulsed EPR study of17O-substituted methyl-naphthoquinone as radical anion in the A1 binding site of photosystem I and in frozen solution

Quinones have been studied in considerable detail as functional cofactors in membrane-bound protein-cofactor systems, in particular in reaction centers (RCs) of photosynthesis. For both types of RCs, they act primarily as one-electron gates during light-induced charge separation but at very different redox potential. Hydrogen bonding between the RC protein and the two, 1,4-quinone carbonyl groups constitutes a major protein-cofactor interaction in control of function. In contrast to symmetric H-bonding for quinones in isotropic solution, asymmetric H-bonding is a characteristic feature of the quinone binding sites in RC proteins. A simple valence bond model correlates the asymmetry of respective H-bond strength with the asymmetric spin density distribution derived from observable hyperfine couplings of the quinone anion state. Among all quinone-protein systems studied so far, the A₁ acceptor site in photosystem (PS) I exhibits the highest asymmetry. Since the carbonyl groups carry most of the total unpaired electron spin density, isotopic labelling of the carbon (¹³C) and oxygen (¹⁷O) appears to be the proper way to characterize the H-bond asymmetry by hyperfine couplings. Indeed, recent¹³C hyperfine studies, together with data for protons in specific ring substituents, confirm the high asymmetry correlated with only one dominant H-bond in the A₁ site of PS I, which is consistent with the structure model derived from X-ray structure (1JB0) for the ground state of the PS I protein complex.¹⁷O hyperfine tensors measured for the A₁ site of PS I yield high hyperfine coupling constants but very small asymmetry for the two carbonyl groups. The asymmetry is even three times smaller than the already small one observed for the Q_A site of purple bacterial RCs. A small asymmetry is however consistent with previous studies on model systems which showed an insensitivity of the¹⁷O hyperfine coupling to H-bond-induced changes of the unpaired electron spin density. The large¹⁷O hyperfine coupling itself appears to depend on the electrostatics seen by the radical anion. It is slightly larger when A ₁ ^? is part of the functional transient radical ion pair state as compared with the photoaccumulated stable radical anion. Possible explanations and consequences of these results are discussed.     

The radioprotection of amino acids by quinones

G-values have been measured for some ammo acids and their quinone complexes radiolysed by cobalt 60 gamma rays. The G-values for the complexes are much lower than for the individual compounds. Free radical saturation occurs for the complexes at lower dosages than either the amino acids or quinones.     

Application of Optically Detected EPR to Study the Stability of Sterically Hindered Amine Radical Cations to Proton Transfer Reaction

Optically detected electron paramagnetic resonance technique has been applied to study the stability of sterically hindered amine radical cations with respect to the ion–molecule proton transfer reaction in liquid squalane solutions. The reaction has proved very sensitive to the steric effect of bulky tertiary substituents at the nitrogen atom. It has been established that steric hindrance is able to efficiently block the N–H proton transfer reaction, while the H-bonding may stabilize the distonic intermediate complex of aminyl radical and cation of ammonium type.     

Influence of ortho substituents on 17O NMR chemical shifts in phenyl esters of substituted benzoic acids

¹⁷O NMR spectra for 29 phenyl esters of ortho‐, para,‐ and meta‐substituted benzoic acids, X‐C₆H₄CO₂C₆H₅, at natural abundance in acetonitrile were recorded. The δ(¹⁷O) values of carbonyl and the single‐bonded oxygens for para derivatives gave good correlation with the σ⁺ constants. The δ(¹⁷O) values for meta derivatives correlated well with the σ_m constants. The influence of ortho substituents on the δ(¹⁷O) values of carbonyl oxygen and the single‐bonded oxygens was analyzed using the Charton equation containing the inductive, σ_I, resonance, σ⁺_R, and steric, E, substituent constants. For ortho derivatives, excellent correlations with the Charton equation were obtained when the data treatment was performed separately for derivatives containing electron‐donating +R and electron‐attracting ?R substituents. The electron‐donating substituents in ortho‐, meta‐, and para‐substituted esters resulted in shielding of the ¹⁷O signal and the electron‐withdrawing groups caused deshielding. In phenyl ortho‐substituted benzoates, the substituent‐induced positive inductive (ρ_I > 0), resonance (ρ_R > 0), and steric (δ_orthoE > 0) effects were found. The steric interaction of ortho substituents with ester group was found to produce a deshielding effect on the carbonyl and single‐bonded oxygens. For ortho derivatives with ?R substituents, the resonance term was insignificant and the steric term was ca. twice weaker as compared to that for derivatives with +R substituents. The δ(¹⁷O) values for ortho‐substituted nitrobenzenes, acetophenones, and benzoyl chlorides showed a good correlation with the Charton equation as well. In ortho‐substituted nitrobenzenes the inductive, resonance and steric effect were found to be ca. 1.7 times stronger as compared to that for phenyl ortho‐substituted benzoates. Copyright © 2010 John Wiley & Sons, Ltd.     

Investigation of the substituent specific cross‐interaction effects on 13C NMR of the CN bridging group in substituted benzylidene anilines

The substituent effect on ¹³C NMR of the C?N in benzylidene anilines XPhCH?NPhY was investigated, in which the substituents X and Y are in p‐position or in m‐position of the two aromatic rings. The substituent effects including the inductive effects of X and Y, the conjugative effects of X and Y, and the substituent specific cross‐interaction effect were put into one model to quantify the ¹³C NMR chemical shift δ_C(C?N) of the C?N in XPhCH?NPhY. A penta‐parameter correlation equation with correlation coefficient 0.9975 and standard error 0.17 ppm was obtained for 80 samples of compounds. The result shows that the substituents X and Y have an opposite effect on the δ_C(C?N). The electron‐withdrawing effects of X decrease the δ_C(C?N); while the electron‐donating effects of X increase the δ_C(C?N). In contrast, the electron‐withdrawing effects of Y increase the δ_C(C?N); while the electron‐donating effects of Y decrease the δ_C(C?N). A new substituent specific cross‐interaction effect parameter Δσ² was proposed, which indicates that the most substituent specific cross‐interaction effect exists in the pair of max electron‐withdrawing group (EWG) and max electron‐donating group (EDG) or the pair of max EDG and max EWG. Further to verify the obtained correlation equation, 15 samples of model compounds were prepared and their δ_C(C?N) was measured in this work. The predicted δ_C(C?N) values with the obtained equation are in good agreement with the measured ones for these prepared compounds, which confirmed the reliability of the obtained equation. Copyright © 2010 John Wiley & Sons, Ltd.     

Metal–Organic Framework Supported Palladium Nanoparticles: Applications and Mechanisms

Heterogeneous palladium (Pd)‐based catalysts are extensively applied to improve the catalytic performance and/or expand the reaction scope in many catalytic processes, involving the cross‐coupling, hydrogenation, reduction, and oxidation reactions. Among them, metal–organic framework (MOF)‐supported Pd nanoparticles (Pd NPs) are becoming the most popular one for their excellent catalytic performance and reusable property. To motivate the development of this technology, the applications of MOF‐supported Pd NPs (Pd NPs/MOFs) in heterogeneous catalysis are critically summarized herein, including the hydrogenation reduction of nitro‐ and polyunsaturated compounds, synthesis of carbon–carbon (C? C) bonds compounds, chromium (Cr(VI)) reduction, dehalogenation, alcohol oxidation, CO₂ conversion, and CO oxidation. The influences of base, solvents, electron character of substitutes, and type of halogen on the catalytic performance are comprehensively discussed. Finally, the application prospects of Pd NPs/MOFs and existing shortcomings in the catalytic field are proposed.     

13C−1H heteronuclear dipolar correlation studies of the hydrogen bonding of the quinones inRhodobacter sphaeroides R26 reaction centers

The photosynthetic reaction center (RC) of the photosynthetic bacteriumRhodobacter sphaeroides R26 contains two quinones, Q_A and Q_B. Solid-state heteronuclear (¹H?¹³C) dipolar correlation spectroscopy has been used to study the binding of the quinones in the ground state for RCs reconstituted with l-¹³C ubiquinone-10. Lee-Goldburg cross-polarization buildup curves are recorded to determine distancesr _CH between the l-¹³C carbon labels and the protons involved in the polarization transfer. The l-¹³C of both Q_A and Q_B have intermolecular correlations with protons that resonate downfield, in the region of hydrogen-bonding protons. The distances between the carbon labels and the correlated protons are short, 0.21±0.01 nm. Hence the nuclear magnetic resonance provides evidence for strong hydrogen-bonding interactions at the l-C=O of both Q_A and Q_B for RCs in the ground state. The environment of the l-¹³C of the Q_B is structurally heterogeneous compared to that of the Q_A. The data can be reconciled with a strong H-bonding interaction of the l-C=O of Q_A with Ala M260 NH, and with complex hydrogen bonding involving NH of Ile-L224 and of Gly-L225, and possibly the Ser-L223 hydroxyl group of the l-C=O of the Q_B, in the proximal site.     

C?H hydrogen bonding: 3. Hammett correlations for phenylacetonitriles and tetrafluorobenzenes with HMPA

Understanding the hydrogen bond, or H‐bond, arguably the strongest known intermolecular attractive force, is of great importance. Models of H‐bonding hold that both electrostatic and covalent components are at work. H‐bonding of C? H bonds, and thus presumably other A? H bonds, is greatly facilitated by polar substituents. We have found that equilibrium constants, K, for C? H H‐bonding in series of phenylacetonitriles and tetrafluorobenzenes with HMPA are closely correlated by the Hammett equation. That is, K values vary as substituents of different polarities are introduced at positions remote from the donor C? H bond. This result is consistent with a significant electrostatic component in these examples of H‐bonding. It may, however, also be explained by interaction with the σ* orbital of the C? H bond, which is involved in the covalent component of the H‐bond. Theoretical models need to be applied to this and other cases. Copyright © 2010 John Wiley & Sons, Ltd.     

Alkene ionization potentials: Part I: Quantitative determination of alkyl group structural effects

We have determined by photoelectron spectroscopy the ionization potentials of 63 alkenes of which 42 are reported for the first time, including 13 tetrasubstituted alkenes. The following conclusions can be deduced by a quantitative study of all the structural effects of the alkyl substituents, in order of decreasing importance: (a) There is a rapid non-linear decreasing of the ionization potential with increase in the number of substituents; (b) A dominant effect is the number of carbon atoms in the substituent, which expresses the stabilization of the ground state of the ion by all the σ electrons of the alkyl groups; (c) A small influence is the relative position of the substituents, except when these are numerous (tetrasubstituted alkenes) or very bulky (olefins with t-butyl substituents), in which case there is a steric effect superimposed on the preceding effects.     

In Silico study of carcinogenic o‐Quinone metabolites derived from polycyclic aromatic hydrocarbons (PAHs)

A computational density functional theory study on the structural and electronic properties of several polycyclic aromatic hydrocarbon (PAH) ortho‐quinones was performed and the possible mechanism of DNA‐adduct formation was analyzed to evaluate its thermodynamic viability. Molecular docking techniques were applied to examine the noncovalent interactions developed when a model PAH ortho‐quinone intercalates between the DNA double helix. Quantum‐chemical ONIOM (our Own N‐layer Integrated molecular Orbital molecular Mechanics) calculations within the structure of a DNA fragment were carried out to evaluate the significant steps of noncovalent complex and covalent adduct formation. The solvent effect was also considered by employing a continuum solvation model. The present calculations suggest that initial noncovalent interactions of the PAH o‐quinone within the DNA double helix could determine the feasibility of benzo[a]pyrene‐7,8‐dione‐DNA covalent adduct formation, and that dispersion‐corrected functionals are more suitable for locating the noncovalent complex. Copyright © 2012 John Wiley & Sons, Ltd.     

Transfer hydrogenation using recyclable polymer-supported formate (PSF): Efficient and chemoselective reduction of nitroarenes

Summary Nitroarenes can be reduced in high yields to the corresponding anilines by transfer hydrogenation using a stable H-donor, polymer-supported formate (PSF) in combination with palladium acetate (catalytic). The reactions occur at 100–120^∘C in dimethyl-formamide and the PSF can be recycled for at least three runs. The procedure is chemoselective for nitro group; ester, ketone, aldehyde, and halide substituents on aromatic ring remain unaffected.     

Effect of ortho substituents on carbonyl carbon 13C NMR chemical shifts in substituted phenyl benzoates

¹³C NMR spectra of 37 ortho‐, meta‐, and para‐substituted phenyl benzoates, containing substituents in benzoyl and phenyl moiety, 4 ortho‐substituted methyl and 5 ethyl benzoates as well as 9 R‐substituted alkyl benzoates have been recorded. The influence of the ortho substituents on the carbonyl carbon ¹³C NMR chemical shift, δ_CO, was found to be described by a linear multiple regression equation containing the inductive, σ_I, resonance, σ°_R, and steric, E, or υ substituent constants. For all the ortho‐substituted esters containing substituents in the acyl part as well as the phenyl part, the substituent‐induced reverse inductive effect (ρ_I < 0), the normal resonance effect (ρ_R > 0), and the negative steric effect (δ_ortho < 0) with the E were observed. In the case of ortho substituents in the phenyl part, the resonance effect was negligible. Due to inductive effect, the ortho electron‐withdrawing substituents showed an upfield shift or shielding of the carbonyl carbon, while the electron‐donating substituents had an opposite effect. Because of the sterical consequences, ortho substituents revealed a deshielding effect on the ¹³C NMR chemical shift of the carbonyl carbon. For all the meta‐ and para‐substituted esters, the reverse substituent‐induced inductive and resonance effects (ρ_I < 0, ρ_R < 0) were found to be significant. In alkyl benzoates, the alkyl substituents showed the reverse inductive and steric effects. The log k values for the alkaline hydrolysis in water, aqueous 0.5 M Bu₄NBr and 2.25 M Bu₄NBr, and the IR frequencies, ν_CO, for the ortho‐, meta‐, and para‐substituted phenyl benzoates and alkyl benzoates were correlated nicely with the corresponding ¹³C NMR substituent chemical shifts, Δδ_CO. Copyright © 2009 John Wiley & Sons, Ltd.     

Potential application of palladium nanoparticles as selective recyclable hydrogenation catalysts

The search for more efficient catalytic systems that might combine the advantages of both homogeneous (catalyst modulation) and heterogeneous (catalyst recycling) catalysis is one of the most exciting challenges of modern chemistry. More recently with the advances of nanochemistry, it has been possible to prepare soluble analogues of heterogeneous catalysts. These nanoparticles are generally stabilized against aggregation into larger particles by electrostatic or steric protection. Herein we demonstrate the use of room temperature ionic liquid for the stabilization of palladium nanoparticles that are recyclable catalysts for the hydrogenation of carbon–carbon double bonds and application of these catalysts to the selective hydrogenation of internal or terminal C=C bonds in unsaturated primary alcohols. The particles suspended in room temperature ionic liquid show no metal aggregation or loss of catalytic activity even on prolonged use.     

The importance of the ortho effect in the solvolyses of 2,6‐difluorobenzoyl chloride

The ortho effect of the chloro substituents in 2,6‐dichlorobenzoyl chloride sufficiently hindered attack on the acyl carbon such that an ionization mechanism was observed over the full range of solvents studied. We now compare this behavior with that of 2,6‐difluorobenzoyl chloride. The smaller fluoro substituents allow the dominant pathway to be addition–elimination (association–dissociation) in all solvents except those rich in fluoroalcohol, where ionization is dominant. Ranges of operation for both mechanisms had previously been observed for the parent benzoyl chloride but with a wider ionization range than for the 2,6‐difluoro derivative. This indicates that, relative to the parent, the electronic destabilizing influence of the fluorines on acyl cation formation outweighs the steric retardation to attack because of the presence of the two ortho‐fluorine atoms. An extended (two‐term) Grunwald–Winstein equation treatment of the solvolyses of 2,6‐difluorobenzoyl chloride is reported. Copyright © 2011 John Wiley & Sons, Ltd.     

Concerted SN2 mechanism for the hydrolysis of acid chlorides: comparisons of reactivities calculated by the density functional theory with experimental data

DFT computations have been performed in acetone and water solvents in order to investigate the mechanism of hydrolysis of acid chlorides. Acetyl chloride and chloroacetyl chloride hydrolyze via concerted, one‐step S_N2 mechanism, with the attack of water at the sp² hybridized carbon atom of the C?O group, and the transition state (TS) has distorted tetrahedral geometry. Solvent molecules act as general base and general acid catalysts. The TS of chloroacetyl chloride is tighter and less polar than the TS of acetyl chloride. The structure of the S_N2 TS for the hydrolysis of benzoyl chlorides changes with the substituents and the solvent. Tight and loose TSs are formed for substrates bearing electron withdrawing (e‐w) and electron donating (e‐d) groups, respectively. In acetone, only the e‐w effect of the substituents increase the reactivity of the substrates, and the change of the structure of the TSs with the substituents is small. In water, polar and very loose TSs are formed in the reactions of benzoyl chlorides bearing e‐d substituents, and the rate enhancing effect of both e‐d and e‐w groups can be computed at higher level of theory. Calculated reactivities and the changes of the structure of the TSs with substituents and solvent are in accordance with the results of kinetic studies. In S_N2 nucleophilic substitutions late/early TSs are formed if the attacking reagent is poorer/better nucleophile than the leaving group, and loose/tight TSs are formed for substrates bearing e‐d/e‐w substituents and in protic/aprotic solvents. Copyright © 2010 John Wiley & Sons, Ltd.     

The Raman noncoincidence effect of the ν(CO) band of ME6N liquid crystal across the nematic–isotropic phase transition studied by a micro‐spectroscopy experiment

The Raman spectroscopic noncoincidence effect (NCE) of the ν(CO) band of the liquid crystal ME6N (4‐cyanophenyl‐4′‐hexylbenzoate) has been measured at different temperatures (47–52 °C) around the nematic‐isotropic phase transition (47.8 °C) employing a micro‐Raman experiment under confocal conditions and performed on a homogeneously aligned thin sample. The low value of NCE (0.9 cm⁻¹) obtained over the whole temperature range suggests that the orientational structure of the liquid crystal in both phases is governed by the steric hindrances in the proximity of the carbonyl group, rather than by dipolar interactions. This hypothesis is supported by the results of a supplementary investigation of the NCE of the ν(CO) Raman band in liquid ketones and esters, made progressively more hampered by the insertion of bulky (phenyl) groups in proximity of the carbonyl group. The NCE of the ν(CO) band, in fact, decreases from 5.5 cm⁻¹ in acetone (the less hampered) to 0.7 cm⁻¹ in benzophenone (the most hampered among the studied ketones), and from 6.2 cm⁻¹ in methyl acetate (the less hampered) to 2.2 cm⁻¹ in phenyl benzoate (the most hampered among the studied esters). To our best knowledge, this represents the first attempt to analyze the NCE in terms of steric hindrance of the substituents around the target oscillator. A parallel analysis of the difference between the anisotropic and the isotropic bandwidths of the ν(CO) Raman band in these molecular liquids indicates that reorientational dynamics plays only a marginal role, if any. Copyright © 2006 John Wiley & Sons, Ltd.