Surface ECE mechanism in protein film voltammetry—a theoretical study under conditions of square-wave voltammetry

For the first time, the features of a surface electron transfer–chemical reaction–electron transfer (ECE) mechanism, relevant to protein-film set-up, have been studied theoretically under conditions of square-wave voltammetry. The considered surface ECE mechanism is presented by following reaction scheme:. The mathematical solutions of this complex redox mechanism are given in form of integral equations, and they can be applied to any chronoamperometric technique. Attention is given to two frequently met situations: (a) case where the energy for the reduction in the second electron transfer step is lower or equal to that of the first reduction step and (b) case where the energy for the reduction of the second electron transfer step is much higher than that of the first reduction step. The theoretical square-wave voltammograms feature various shapes, depending mainly on the energy difference between the two electron transfer steps, but they also depend on the kinetics of the first and the second electron transfer, as well as on the rate of the chemical reaction. Hints are given for qualitative recognition of the surface ECE mechanism and for its distinguishing from similar surface redox systems. Reliable methods are proposed for the estimation of kinetic parameters of the electron transfer steps and that of the chemical reaction. Since many biological compounds undergo this redox mechanism, the theoretical results presented in this work can be of help for the people dealing with organic electrochemistry or protein-film voltammetry. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Rubin Gulaboski is on leave from the Institute of Chemistry, Faculty of Natural Sciences and Mathematics, Skopje, Republic of Macedonia Dedicated to Professor John O. Bockris on the occasion of his 85th birthday     

苯甲酸、对甲苯胺、乙酰苯胺三组分分离实验教学设计

分离提纯是有机合成中关键的实验步骤,反应后处理这部分的工作内容要占到90%的工作量。为提高学生从混合物中分离目标化合物的实验技能,进一步锻炼学生综合运用萃取、蒸馏、重结晶等纯化实验操作,中南大学化学化工学院有机化学教学组在有机化学实验教学中安排了"三组分混合物分离"实验,面向的学生为医学临床五年制和八年制、化学化工学院的制药工程及应用化学专业。本文详细探讨了该实验的酸、碱分离两种教学方案,对先加酸分离得到的三个组分进行了1H NMR测试。从两个方案的实验教学效果来看,先加酸或先加碱对实验结果没有影响。     

Bis(diphenylphosphino)ethane nickel polychloridophenylthiolate complexes: synthesis and characterization

Transition Metal Chemistry - Reactions of (dppe)NiCl2 (dppe?=?1,2-bis(diphenylphosphino)ethane) with chlorido-substituted phenyl thiolates (RClS?) produced the corresponding...     

Early Industrial Roots of Green Chemistry and the history of the BHC Ibuprofen process invention and its Quality connection

Conventional wisdom and many published histories of “Green Chemistry” describe its start as being a result of governmental and/or regulatory actions at the US Environmental Protection Agency (“EPA”) during the early 1990’s. But there were many Real World industrial examples of environmentally friendly commercial processes in the oil and commodity chemicals industries for decades prior to the 1990s. Some early examples of commercial “Green Chemistry” are briefly described in this article. The Boots/Hoechst Celanese (“BHC”) Ibuprofen process was one of the earliest multiple-award-winning examples of industrial “Green Chemistry” in the fine chemical/pharmaceutical industry. The author, who conceived the BHC Ibuprofen synthetic strategy in 1984, reveals and documents that the BHC Ibuprofen process was not primarily a result of governmental or regulatory mandates, or environmentalist or political motivations. The BHC ibuprofen process, and probably many other early industrial “green” inventions, evolved from, and their development and commercialization motivated and guided by, a long prior industrial culture of both scientific and technical evolution. The invention and commercialization of these early industrial commercialized processes, and the BHC Ibuprofen process were also guided by both competitive and economic market needs, personal human motivations, and a low waste culture of “Quality” and “Continuous Improvement” that the commodity chemical industry internally promoted in the 1980’s. The author comments on some perceptions of the status of Green Chemistry now, and directions it should consider going in the future. The author recommends that young Green Chemists and/or Green Engineers reconsider “Quality” approaches in order to genuinely lead Society toward a Greener future.     

Modelling and graphic interpretation of response surface in chemical analysis

The complicated effects of several factors on an analytical signal can be studied comprehensively only by an experimental design in which the additive and non-additive effects of these factors are included in one experimental run, with simultaneous variation of the values of all factors. From the law of mass action, it is shown that systems in which the analytical signal is produced by a species formed during a chemical reaction (i.e., for spectrophotometry) can be described by a model based on quasi-linear regressions with squares and products of variables. The regression coefficients can be evaluated by means of factorial designs at three levels. Interpretation of results in the case of two or three independent variables is simplified by using a graphic method in which the complex effects of factors on the dependent variable are represented as an empirical response surface. In the case of regression with three independent variables, the response surface is four-dimensional; its dimensionality is reduced to three by several three-dimensional representations. The shapes of the response surfaces described by the response functions and the adequacy of the models are discussed.     

Reactivity indicators for degenerate states in the density-functional theoretic chemical reactivity theory

Density-functional-theory-based chemical reactivity indicators are formulated for degenerate and near-degenerate ground states. For degenerate states, the functional derivatives of the energy with respect to the external potential do not exist, and must be replaced by the weaker concept of functional variation. The resultant reactivity indicators depend on the specific perturbation. Because it is sometimes impractical to compute reactivity indicators for a specific perturbation, we consider two special cases: point-charge perturbations and Dirac delta function perturbations. The Dirac delta function perturbations provide upper bounds on the chemical reactivity. Reactivity indicators using the common used "average of degenerate states approximation" for degenerate states provide a lower bound on the chemical reactivity. Unfortunately, this lower bound is often extremely weak. Approximate formulas for the reactivity indicators within the frontier-molecular-orbital approximation and special cases (two or three degenerate spatial orbitals) are presented in the supplementary material. One remarkable feature that arises in the frontier molecular orbital approximation, and presumably also in the exact theory, is that removing electrons sometimes causes the electron density to increase at the location of a negative (attractive) Dirac delta function perturbation. That is, the energetic response to a reduction in the external potential can increase even when the number of electrons decreases.     

Synthesis of potentially photoactivatable coumarin derivatives via a 1,3‐dipolar cycloaddition

A copper (1)‐catalyzed 1,3‐dipolar cycloaddition reaction was used to prepare a series of mono and disubstituted 1,2,3‐triazolyl‐coumarins using a 1,3‐cycloaddition (“Click Chemistry”). Starting coumarins were synthesized using classical or modified Pechmann's reaction. The propargyl group was introduced as either propargylether or as a propargylamide. Azides were prepared in a three steps procedure. Cycloaddition products, containing a coumarin and a photoactivatable moiety, were obtained in good yields.     

Tunneling versus hopping in mixed-valence oligo-p-phenylenevinylene polychlorinated bis(triphenylmethyl) radical anions

Radical anions 1(-?)-5(-?), showing different lengths and incorporating up to five p-phenylenevinylene (PPV) bridges between two polychlorinated triphenylmethyl units, have been prepared by chemical or electrochemical reductions from the corresponding diradicals 1-5 which were prepared using Wittig-Horner-type chemistry. Such radical anions enabled us to study, by means of UV-vis-NIR and variable-temperature electron spin resonance spectroscopies, the long-range intramolecular electron transfer (IET) phenomena in their ground states, probing the influence of increasing the lengths of the bridges without the need of using an external bias to promote IET. The temperature dependence of the IET rate constants of mixed-valence species 1(-?)-5(-?) revealed the presence of two different regimes at low and high temperatures in which the mechanisms of electron tunneling via superexchange and thermally activated hopping are competing. Both mechanisms occur to different extents, depending on the sizes of the radical anions, since the lengths of the oligo-PPV bridges notably influence the tunneling efficiency and the activation energy barriers of the hopping processes, the barriers diminishing when the lengths are increased. The nature of solvents also modifies the IET rates by means of the interactions between the oligo-PPV bridges and the solvents. Finally, in the shortest compounds 1(-?) and 2(-?), the IET induced optically through the superexchange mechanism can also be observed by the exhibited intervalence bands, whose intensities decrease with the length of the PPV bridge.     

在化学教学中推广科普知识——以趣谈指纹显现技术为例

Fingerprint can be widely acknowledged as the "first evidence" of forensic science, in which fingerprint manifestation is the first and also the most important step to discover latent fingerprint. Flipped classroom and team-based learning modes have been implemented in the chapter of "chemistry and criminal science and technology" of Chemistry and Society course. Students were encouraged to consult relevant references in advance and participated in the team discussion in class. Herein, the fingerprint is personified into a "mysterious thief" with multiple faces, while various fingerprint display technologies are personified into investigators of the fingerprint department. Collectively, this teaching strategy can popularize the chemical principles of several fingerprint techniques to students, enrich their scientific understanding of the fingerprint display technology, and improve their scientific quality and learning interests.     

Mechanism of a chemical classic: quantum chemical investigation of the autocatalyzed reaction of the serendipitous wöhler synthesis of urea

The detailed reaction pathways for the ammonium cyanate transformation into urea (W?hler's reaction) in the gas phase, in solution, and in the solid state have exhaustively been explored by means of first-principles quantum chemical calculations at the B3LYP level of theory using the 6-31G(d,p) basis set. This serendipitous synthesis of urea is predicted to proceed in two steps; the first step involves the decomposition of the ammonium cyanate to ammonia and isocyanic or cyanic acid, and the second one, which is the main reaction step (and probably the rate-determining step), involves the interaction of NH(3) with either isocyanic or cyanic acid. Several alternative pathways were envisaged for the main reaction step of W?hler's reaction in a vacuum involving the formation of "four-center" transition states. Modeling W?hler's reaction in aqueous solution and in the solid state, it was found that the addition of NH(3) to both acids is assisted (autocatalyzed) by the active participation of extra H(2)O and/or NH(3) molecules, through a preassociative, cooperative, and hydrogen-transfer relay mechanism involving the formation of "six-center" or even "eight-center" transition states. The most energetically economic path of the rate-determining step of W?hler's reaction is that of the addition of NH(3) to the C=N double bond of isocyanic acid, directly affording urea. An alternative pathway corresponding to the anti-addition of ammonia to the Ctbd1;N triple bond of cyanic acid, yielding urea's tautomer HN=C(OH)NH(2), seems to be another possibility. In the last case, urea is formed through a prototropic tautomerization of its enolic form. The energies of the reactants, products, and all intermediates along with the barrier heights for each reaction path have been calculated at the B3LYP/6-31G(d,p) level of theory. The geometry optimization and characterization of all of the stationary points found on the potential energy hypersurfaces was performed at the same level of theory.     

Acid acting as redispersing agent to form stable colloids from photoactive crystalline aqueous sol–gel TiO<Subscript>2</Subscript> powder

In this work, the redispersion of three nanocrystalline TiO₂ colloids is studied: one pure and two Fe-doped titania. These three colloids are produced by an easy aqueous sol–gel synthesis using precipitation-acidic peptization of Ti precursor. For the two Fe-doped TiO₂, one is doped during synthesis (primary doping) and the other is doped after the synthesis (secondary doping). The initial colloids are composed of crystalline TiO₂ particles around 7?nm with good photocatalytic properties, tested on PNP degradation under visible light (wavelength >390?nm). The powders obtained by air drying of these three colloids are redispersed in water to produce colloids, which are compared to the initial colloid produced. For each colloid, five cycles of drying redispersion are achieved. The colloids are characterized by dynamic light scattering, zeta potential measurements, inductively coupled plasma–atomic emission spectroscopy, X-ray diffraction, nitrogen adsorption–desorption measurements, Mössbauer spectroscopy, diffuse reflectance spectroscopy, and photocatalytic tests. The results show that similar products are obtained between the cycles, maintaining homologous properties of colloids. This property of redispersion is mainly due to the acid (HNO₃, HCl, or H₂SO₄) which protonates the surface of the TiO₂ nanoparticle leading to high-surface charges and electrostatic repulsions between aggregates. This property can be very useful for industrial applications of this synthesis, especially as it allows the volume and weight to be reduced for transportation and storage. Moreover, results show that the pure TiO₂ powder can be doped during its redispersion step. The redispersion of the TiO₂ developed here is possible without surface functionalization or multiple step processes, contrary to commercial Degussa P25. A 2-year stability study of all the produced colloids has been performed by following the evolution of the macroscopic aspect and the physicochemical properties of these sols. This study showed high stability of the produced colloids.

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A comparison of direct methods to determine n-th order kinetic parameters of solid thermal decomposition for use in fire models

Pyrolysis models for burning solids in fire simulations are sensitive to the values of the activation energy, frequency factor, and reaction order that characterize the thermal decomposition of the solid to gaseous fuel, so direct measurement of these kinetic parameters is recommended, and simple methods are preferred. Three direct methods were evaluated with regard to the ability of their kinetic parameters to reproduce the thermal decomposition of five polymers measured by differential thermogravimetric analysis using the reaction order model. It was found that the two multiple heating rate methods produced identical, physically based kinetic parameters, while the peak property method produced non-physical kinetic parameters. However, all of these kinetic parameters in a single-step reaction order model gave reasonably good conversion histories for non-charring and moderately charring polymers. For a highly charring polymer, the conversion histories were poorly described without a multiple step reaction. The temperature at the maximum rate of conversion was found to be essentially independent of the reaction order, which decouples the frequency factor from the reaction order in the direct kinetic methods. Any of the direct methods are sufficiently accurate to obtain kinetic parameters for pyrolysis models because of the inherent spatial and temporal averaging of reaction rates at the burning surface of a thick solid and the uncertainty in the heat transfer mechanisms and thermo-physical parameters used in the models.     

A dielectric study of poly(vinyl acetate) using a pulse-probe technique

There is considerable literature available that describes our understanding of the viscoelastic properties of polymers subjected to mechanical stresses or deformations. What we refer to here as a pulse-probe technique is one method that is commonly used to study the time-dependent behavior of materials in histories, e.g., temperature-jump or step-deformations, that exhibit fading memory responses. In the linear case the behavior is well understood in the context of Boltzmann superposition ideas. However, there is only limited work available that investigates the dielectric response of materials within this same context. In the present study, we present an investigation of the dielectric behavior of poly(vinyl acetate) (PVAc) using a two-step pulse-probe technique. Time domain dielectric experiments were performed in the vicinity of the glass transition temperature. After establishing the linear response function in single-step experiments, two types of pulse-probe experiments were performed. In one, the time duration t ₁ of the first step in the probe was varied. In the second case, the magnitude of the field E ₁ applied to the sample for the first step was varied. We not only demonstrate the existence of the memory effect in the dielectric response, but also find that the responses are consistent with the linear Boltzmann superposition principle. Evidence of deviations from linear superposition at the highest electric fields is also presented.     

Photophysics and kinetics of naphthopyran derivatives,part 3: A general procedure to uniquely identify the kinetic and spectroscopic parameters of ABC(2k,2ϕ) systems and application to naphthopyran reactions

A new procedure has been developed with the aim of determining the kinetic and spectroscopic parameters of any photochemical systems of the generic ABC(2k,1?) and ABC(2k,2?) types. General expressions of the colorability and the equilibrium concentration ratio of two (of the three involved) species at a photostationary state or a state of thermal equilibrium have been derived. The procedure has been successfully applied to achieve unique identifiability for the seven sequences of the ABC(2k,1? and 2?) type that may represent naphthopyran dynamics. It is demonstrated that the reactivity of this particular compound is certainly described by a higher kinetic level than those (three or four reaction steps) considered in the present study. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 421–430, 2006     

颜色背后的化学——一款以颜色为科普主题的实物展架的开发

北京大学化学国家级实验教学示范中心开发了一款以"颜色背后的化学"为主题的用于科普的实物展架。它以颜色相关化学理论为逻辑主线,通过展示化学物质的颜色,系统地科普物质背后的颜色成因。本文主要介绍了此款化学实物展架的设计理念、展示内容及其应用场景。     

Synthesis of isomeric angularly annealed dinaphthoporphyrin systems: examination of the relative positioning and orientation of ring fusion as factors influencing the porphyrin chromophore

Porphyrins built up from two naphtho[1,2-c]pyrrole subunits and two beta-substituted pyrroles can produce five isomeric dinaphthoporphyrin systems. To gain insights into the effects of ring fusion on extended porphyrin chromophores, all five of these systems were synthesized in isomerically pure form. In four of these syntheses, dihydronaphthopyrroles were used to introduce one or both of the naphthalene subunits, and dehydrogenation with DDQ in refluxing toluene later produced the fully conjugated systems. Naphthopyrroles were also prepared by reacting isocyanoacetate esters with 1-nitronaphthalene in the presence of a phosphazene base. These compounds proved to be less stable than their dihydronaphthopyrrolic counterparts but could still be utilized in these synthetic studies. Three isomeric adj-dinaphthoporphyrin systems were prepared using the MacDonald "2 + 2" condensation or by the cyclization of a,c-biladiene intermediates with copper(II) chloride or AgIO(3)-Zn(OAc)(2). A dinaphthoporphyrin with two naphthalene units pointing toward one another could only be obtained in low yields due to a combination of stability and steric factors, but the other two adj-difused systems were isolated in good overall yields. However, the final dehydrogenation step occurred in moderate yields (50-60%) and could only be performed when the porphyrins bore propionate ester side chains that produced sufficient solubility in organic solvents. The two related opp-dinaphthoporphyrins were synthesized by a "head-to-tail" self-condensation of a dipyrrylmethane aldehyde, or a "3 + 1" synthesis using a tripyrrane intermediate bearing two fused dihydronaphthalene moieties, in excellent yields. In both cases, a final dehydrogenation step was required, but the opp-dinaphthoporphyrins were consistently formed in virtually quantitative yields. The opp-dinaphthoporphyrin series gave UV-vis spectra with relatively strong Soret bands at 425 nm, and the visible region was dominated by an unusually strong Q-band III. The adj-dinaphthoporphyrins produced broader less intense Soret bands and four well-defined Q-bands, including a relatively strong absorption at 645 nm. However, the relative orientation of the naphthalene rings had no significant effects on these spectra. On the other hand, the dications produced in TFA-chloroform solutions showed more discrimination between the individual porphyrin systems, and the metallo derivatives also displayed significant variations in their electronic absorption spectra.     

Structure and dynamics of photogenerated triplet radical ion pairs in DNA hairpin conjugates with anthraquinone end caps

A series of DNA hairpins (AqGn) possessing a tethered anthraquinone (Aq) end-capping group were synthesized in which the distance between the Aq and a guanine-cytosine (G-C) base pair was systematically varied by changing the number (n - 1) of adenine-thymine (A-T) base pairs between them. The photophysics and photochemistry of these hairpins were investigated using nanosecond transient absorption and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Upon photoexcitation, (1*)Aq undergoes rapid intersystem crossing to yield (3*)Aq, which is capable of oxidizing purine nucleobases resulting in the formation of (3)(Aq(-?)Gn(+?)). All (3)(Aq(-?)Gn(+?)) radical ion pairs exhibit asymmetric TREPR spectra with an electron spin polarization phase pattern of absorption and enhanced emission (A/E) due to their different triplet spin sublevel populations, which are derived from the corresponding non-Boltzmann spin sublevel populations of the (3*)Aq precursor. The TREPR spectra of the (3)(Aq(-?)Gn(+?)) radical ion pairs depend strongly on their spin-spin dipolar interaction and weakly on their spin-spin exchange coupling. The anisotropy of (3)(Aq(-?)Gn(+?)) makes it possible to determine that the π systems of Aq(-?) and G(+?) within the radical ion pair are parallel to one another. Charge recombination of the long-lived (3)(Aq(-?)Gn(+?)) radical ion pair displays an unusual bimodal distance dependence that results from a change in the rate-determining step for charge recombination from radical pair intersystem crossing for n < 4 to coherent superexchange for n > 4.     

A normal mode-based geometric simulation approach for exploring biologically relevant conformational transitions in proteins

A three-step approach for multiscale modeling of protein conformational changes is presented that incorporates information about preferred directions of protein motions into a geometric simulation algorithm. The first two steps are based on a rigid cluster normal-mode analysis (RCNMA). Low-frequency normal modes are used in the third step (NMSim) to extend the recently introduced idea of constrained geometric simulations of diffusive motions in proteins by biasing backbone motions of the protein, whereas side-chain motions are biased toward favorable rotamer states. The generated structures are iteratively corrected regarding steric clashes and stereochemical constraint violations. The approach allows performing three simulation types: unbiased exploration of conformational space; pathway generation by a targeted simulation; and radius of gyration-guided simulation. When applied to a data set of proteins with experimentally observed conformational changes, conformational variabilities are reproduced very well for 4 out of 5 proteins that show domain motions, with correlation coefficients r > 0.70 and as high as r = 0.92 in the case of adenylate kinase. In 7 out of 8 cases, NMSim simulations starting from unbound structures are able to sample conformations that are similar (root-mean-square deviation = 1.0-3.1 ?) to ligand bound conformations. An NMSim generated pathway of conformational change of adenylate kinase correctly describes the sequence of domain closing. The NMSim approach is a computationally efficient alternative to molecular dynamics simulations for conformational sampling of proteins. The generated conformations and pathways of conformational transitions can serve as input to docking approaches or as starting points for more sophisticated sampling techniques.     

Mono-cyclopentadienyl complexes of lanthanum: synthesis and characterization of anilido derivatives

Use of the bulky cyclopentadienyl ligand [η⁵-C₅H₂(SiMe₃)₃-1,2,4]⁻ (Cp?) allows for the isolation of monomeric, mono-ring lanthanide species. As previously reported, (Cp?)K reacts with LaI₃(THF)₄ (THF=tetrahydrofuran) in THF/pyridine to form the mono-ring complex (Cp?)LaI₂(py)₃ (1) (py=pyridine); a minor product of this reaction is the bis-ring species (Cp?)₂LaI(py) (2). The solid state structure of 2 reveals a monomeric compound containing a pseudo-tetrahedral metal center exhibiting no unusual intramolecular contacts. Addition of one equiv of KNHAr (Ar=2,6-ⁱPr₂C₆H₃) to complex 1 in THF generates the mono-anilido compound (Cp?)LaI(NHAr)(THF)₂ (3), which may be converted to the more stable pyridine adduct (Cp?)LaI(NHAr)(py)₂ (4) by the addition of pyridine to 3. An X-ray crystal structure of 3 indicated a trigonal bipyramidal metal center with the anilido group oriented trans to the iodide atom (N1-La1-I1=123.1(3)°). A structural study on the bis-pyridine adduct 4 revealed a similar C_s-symmetric structure with a slightly increased N_anilido-La-I angle of 132.1(2)°. Addition of KNHAr to the di-iodo bipyridine adduct (Cp?)LaI₂(bipy)(py) (5), in which the two iodide atoms are cis-disposed, yields the mono-anilido complex (Cp?)LaI(NHAr)(bipy)(py) (6) (bipy=2,2^′-bipyridine); this compound may also be prepared by the addition of bipy to (Cp?)LaI(NHAr)(py)₂ (4). An X-ray diffraction study shows that the lanthanum center in 6 is octahedrally coordinated by a Cp? ring, an iodide, an anilido group, a pyridine molecule and two nitrogens of a bipy molecule. In this case, the anilido moiety and the iodide ligand are arranged in a cis fashion (N_anilido-La-I=111.2(2)°), resulting in a complex with C₁ symmetry. Both (Cp?)LaI(NHAr)(py)₂ (4) and (Cp?)LaI(NHAr)(bipy)(py) (6) are inactive as catalysts for the hydroamination/cyclization of 2-amino-hex-5-ene.