Impedance of faradeic process with partial charge transfer

The electrochemical impedance of charge-transfer reaction involving the stages of adsorption and desorption of intermediate partially charged substance is analyzed theoretically in the frequency-potential plane in order to study the possibility for measuring the partial charge transfer. The simplest case of adsorption two-electron transfer, namely, the Langmuir adsorption with liner dependence of reaction rate on the coverage, is considered. It is assumed that the partial charge transfer is independent of the electrode potential.     

Influence of polarity of medium on the nature of charge transfer complex,kinetics, and mechanism of reaction with a one-electron transfer in a tetracyanoethylene-n-alkyl-substituted aniline system

The influence of the polarity of the medium (B, ) on the thermodynamic characteristics of the formation of a charge transfer complex (CTC) and one electron transfer from a donor D to and acceptor A was studied in N-alkyl-substituted aniline (D) — tetracyanoethylene (A) systems. It was shown that in systems with CTC in which [B]/[D] 1, the limiting step in the reaction with one electron transfer in weakly polar solvents (<20) is the formation of molecular (AD), and in strongly polar sovents (>20), of ionic (AD) CTC. A correlation was established between the spectral (VCT) characteristics of the formation of the CTC and the kinetic parameters (k_eff) of one electron transfer reaction and degrees of charge transfer in molecular and ionic CTC were determined.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 2, pp. 167–173, March–April, 1989.     

Thermodynamic and kinetic characteristics of one-electron transfer reactions in N-alkylaniline-tetracyanoethylene charge transfer complex systems

One-electron transfer reactions in systems containing an N,N-dialkylaniline (D) and tetracyanoethylene (A) in methylene chloride proceed through the formation of AD and AD₂ charge transfer complexes. The tetracyanoethylene -system and nitrogen unshared electron pair of the N,N-dialkylaniline participate in the formation of the charge transfer complex. The equilibrium constants for complex formation K and rates of formation of the TCE^– radical-anions k were determined for this reaction. The sign of the regression coefficient in the correlation between the thermodynamic (K) and kinetic indices (k) for the one-electron transfer reactions in charge transfer complex systems indicates the formation of shortlived intermediates between the starting compounds and final Reaction products (in the case of increasing K(AD) values with increasing k(TCE^–)) or of several longlived intermediate complexes (in the case of increasing K(AD) values with decreasing k(TCE^–).Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 23, No. 6, pp. 692–699, November–December, 1987.     

Transmembrane charge transfer in photosynthetic reaction centers: some similarities and distinctions

This mini review presents a general comparison of structural and functional peculiarities of three types of photosynthetic reaction centers (RCs)--photosystem (PS) II, RC from purple bacteria (bRC) and PS I. The nature and mechanisms of the primary electron transfer reactions, as well as specific features of the charge transfer reactions at the donor and acceptor sides of RCs are considered. Comparison of photosynthetic RCs shows general similarity between the core central parts of all three types, between the acceptor sides of bRC and PS II, and between the donor sides of bRC and PS I. In the latter case, the similarity covers thermodynamic, kinetic and dielectric properties, which determine the resemblance of mechanisms of electrogenic reduction of the photooxidized primary donors. Significant distinctions between the donor and acceptor sides of PS I and PS II are also discussed. The results recently obtained in our laboratory indicate in favor of the following sequence of the primary and secondary electron transfer reactions: in PS II (bRC): Р(680)(Р(870)) → Chl(D1)(В(А)) → Phe(bPhe) → Q(A); and in PS I: Р(700) → А(0А)/A(0B) → Q(A)/Q(B).     

Photosynthetic models with maximum entropy production in irreversible charge transfer steps

Steady-state bacterial photosynthesis is modelled as cyclic chemical reaction and is examined with respect to overall efficiency, power transfer efficiency, and entropy production. A nonlinear flux–force relationship is assumed. The simplest two-state kinetic model bears complete analogy with the performance of an ideal (zero ohmic resistance of the P–N junction) solar cell. In both cases power transfer to external load is much higher than the 50% allowed by the impedance matching theorem for the linear flux–force relationship. When maximum entropy production is required in the transition with a load, one obtains high optimal photochemical yield of 97% and power transfer efficiency of 91%. In more complex photosynthetic models, entropy production is maximized in all irreversible electron/proton (non-slip) transitions in an iterative procedure. The resulting steady-state is stable with respect to an extremely wide range of initial values for forward rate constants. Optimal proton current increases proportionally to light intensity and decreases with an increase in the proton-motive force (the backpressure effect). Optimal affinity transfer efficiency is very high and nearly perfectly constant for different light absorption rates and for different electrochemical proton gradients. Optimal overall efficiency (of solar into proton-motive power) ranges from 10% (bacteriorhodopsin) to 19% (chlorophyll-based bacterial photosynthesis). Optimal time constants in a photocycle span a wide range from nanoseconds to milliseconds, just as corresponding experimental constants do. We conclude that photosynthetic proton pumps operate close to the maximum entropy production mode, connecting biological to thermodynamic evolution in a coupled self-amplifying process.     

Caldariomyces fumago chloroperoxidase catalyzes the oxidative dehalogenation of chlorophenols by a mechanism involving two one-electron steps

We have employed rapid scan stopped-flow spectroscopy to examine whether the mechanism of oxidative dehalogenation catalyzed by C. fumago chloroperoxidase (CCPO) involves two consecutive one-electron steps or a single two-electron oxidation. First, we optimized the formation of CCPO compound I (CCPO-I) [Fe(IV)=O/porphyrin radical] and CCPO compound II (CCPO-II) [Fe(IV)=O] for use in double mixing rapid scan stopped-flow experiments. Reaction of CCPO-I with 2,4,6-trichlorophenol (TCP) quickly yielded CCPO-II. Reaction of CCPO-II, a one-electron oxidant, with TCP rapidly regenerated the ferric resting state of the enzyme. The rates of the reaction of both CCPO-I and -II with TCP are first-order with respect to [TCP]. In the absence of organic substrate, CCPO-I is slowly reduced to CCPO-II and then the ferric state. The ability of both CCPO-I and -II to carry out the oxidative dehalogenation reaction is consistent with a mechanism involving two consecutive one-electron oxidations. In contrast, reaction of CCPO-I with thioanisole generated the ferric enzyme with no evidence of CCPO-II, consistent with a single two-electron oxidation by insertion of an oxygen atom. The relative stability of CCPO-I and -II has allowed us to differentiate between one- and two-electron substrate oxidations using rapid scan stopped-flow techniques.     

司帕沙星与茜素红荷移反应的研究

采用紫外分光光度法研究了π电子受体茜素红与电子给予体司帕沙星的荷移反应。确定了反应条件,建立了一种快速简便灵敏准确测定司帕沙星的荷移分光光度法。结果表明,司帕沙星与茜素红在(4+6)乙醇-水介质中,室温条件下即可形成1∶1稳定的荷移络合物,该络合物的最大吸收波长为530 nm,表观摩尔吸光系数为4.8×103L.mol-1.cm-1,司帕沙星药物质量浓度在6~160mg/L范围内服从比尔定律,r=0.9991。当该法用于片剂中司帕沙星的测定时,回收率为99.75%~100.00%,相对标准偏差为1.4%(n=10)。     

On the surface steps of a heterogeneous catalytic reaction

Based on a study of the properties of intermediate complexes in the reactions of low-temperature ammonia oxidation and nitrogen oxide reduction in the presence and absence of oxygen on various catalysts, it was hypothesized that the sequence of steps in the formation of reaction products on chemically different catalysts can be the same if the initial adsorption forms of reactants are identical. In this case, variations in catalysts or reaction conditions affect the ratio between reaction rates.     

阿替洛尔与紫色素的荷移反应研究

采用光度法研究了电子给予体阿替洛尔与电子接受体紫色素之间的荷移反应,据此建立了荷移光度法测定阿替洛尔含量的方法。实验表明,阿替洛尔与紫色素在乙醇介质中,室温条件下即可形成稳定的1∶1型荷移络合物,该络合物的最大吸收波长为542nm,表观摩尔吸光系数为6.17×103L.mo-l1.cm-1。阿替洛尔的质量浓度在0~35 mg.L-1范围内符合比尔定律,相关系数为0.9993。当阿替洛尔的质量浓度为20 mg.L-1时,测定结果的相对标准偏差(n=6)为0.78%。采用本法测定片剂中阿替洛尔的含量,平均回收率在98.9%~101.6%之间。     

The reaction mechanism of xanthine oxidase: evidence for two-electron chemistry rather than sequential one-electron steps

Current research on xanthine oxidase has favored a mechanism involving base-catalyzed proton abstraction from a Mo-OH group, allowing nucleophilic attack on the substrate and hydride transfer from the substrate to Mo=S group in the active site. During the course of this reaction mechanism, the molybdenum redox cycles from MoVI to MoIV, with reoxidation of the MoIV speices to form the EPR active MoV intermediate. However, it has also been suggested that the reaction occurs in two subsequent one-electron steps. We have determined kinetic parameters kred and kred/Kd for a variety of plausible substrates as well as the one-electron reduction potentials for these substrates. Our data indicate no correlation between these kinetic parameters and their one-electron reduction potentials, as would be expected if the enzyme were using two subsequent one-electron reduction steps. Our results provide additional support to current evidence for the favored two-electron reduction mechanism.     

Two-step one-electron transfer reaction of chromium(III) complex containing levodopa and uridine with N-bromosuccinimide

[Cr^III(LD)(Urd)(H₂O)₄](NO₃)₂?·?3H₂O (LD?=?Levodopa; Urd?=?uridine) was prepared and characterized. The product of the oxidation reaction was examined using HPLC. Kinetics of the oxidation of [Cr^III(LD)(Urd)(H₂O)₄]²⁺ with N-bromosuccinimide (NBS) in an aqueous solution was studied spectrophotometrically, with 1.0–5.0?×?10^?4?mol?dm^?3 complex, 0.5–5.0?×?10^?2?mol?dm^?3 NBS, 0.2–0.3?mol?dm^?3 ionic strength (I), and 30–50°C. The reaction is first order with respect to [Cr^III] and [NBS], decreases as pH increases in the range 5.46–6.54 and increases with the addition of sodium dodecyl sulfate (SDS, 0.0–1.0?×?10^?3?mol?dm^?3). Activation parameters including enthalpy, ΔH*, and entropy, ΔS*, were calculated. The experimental rate law is consistent with a mechanism in which the protonated species is more reactive than its conjugate base. It is assumed that the two-step one-electron transfer takes place via an inner-sphere mechanism. A mechanism for this reaction is proposed and supported by an excellent isokinetic relationship between ΔH* and ΔS* for some Cr^III complexes. Formation of [Cr^III(LD)(Urd)(H₂O)₄]²⁺ in vivo probably occurs with patients who administer the anti-Parkinson drug (Levodopa), since Cr^III is a natural food element. This work provides an opportunity to identify the nature of such interactions in vivo similar to that in vitro.     

荷移分光光度法测定酒石酸美托洛尔

研究了酒石酸美托洛尔与紫色素之间的荷移反应。确定了反应条件,建立了一种测定酒石酸美托洛尔的荷移分光光度法。酒石酸美托洛尔与紫色素在乙醇介质中,室温条件下即可形成稳定的1∶1型荷移络合物,该络合物的最大吸收波长为540 nm,表观摩尔吸光率为6.83×103L.mol-1.cm-1。酒石酸美托洛尔质量浓度在10~80 mg/L范围内服从比耳定律,相关系数为0.9991。相对标准偏差为1.3%(n=11),回收率为99.3%~102.7%。     

青霉素V钾片与茜素红荷移反应的研究

研究了电子给体青霉素V钾与电子受体茜素红之间的电荷转移反应,确定了反应的最佳条件。实验结果表明：青霉素V钾与茜素红在异丙醇介质中,于室温下即可生成稳定的1∶2络合物,该络合物在535 nm处有最大吸收,表观摩尔吸光系数为2.67&#215;103L.mol^-1.cm^-1,青霉素V钾质量浓度在0.30-240 mg/L范围内符合比耳定律,其线性回归方程为A=6.87ρ（mg/mL）＋0.0904,相关系数R=0.9975,相对标准偏差为0.52%（n=11）。方法可用于测定药物制剂中青霉素V钾,其回收率在98.7%以上。     

Stepwise charge transfer complexation of some pyrimidines with sigma-acceptor iodine involving a new unconventional acceptor

Interactions of some pyrimidine derivatives, 4-amino-2,6-dimethylpyrimidine, kyanmethin, (4AP), 2-amino-4,6-dimethylpyrimidine (2AP), 2-aminopyrimidine (AP), 2-amino-4-methylpyrimidine (AMP), 2-amino-4-methoxy-6-methylpyrimidine (AMMP), and 4-amino-5-chloro-2,6-dimethylpyrimidine (ACDP) as electron donors, with iodine (I(2)), as a typical sigma-electron acceptor, have been studied. Electronic absorption spectra of these interactions in several organic solvents of different polarities have performed instant appearance of clear charge transfer (CT) bands. Formation constants (KCT), molar absorption coefficients (epsilonCT) and thermodynamic properties, DeltaH, DeltaS, and DeltaG, of these interactions have been determined and discussed. Electronic absorption spectra of the solutions of the synthesized pyrimidines-iodine, P-I2, CT complexes have shown the characteristic bands of the triiodide ion, I3*. UV/vis spectral tracking of these interactions have shown that by lapse of time the first formed CT complex, P-I2, is transformed to the corresponding triiodide complex, P(+)I.I3*, then, the later interacts as a new unconventional acceptor and it forms a CT complex of the form (P).(P+I.I3*). Elemental analyses of these solid complexes have indicated the stoichiometric ratio 2:2, or formally 1:1, P:I2.     

First experimental evaluation of partial charge transfer during anion adsorption

The partial charge transfer during anion adsorption cannot be measured in electrochemical experiments or reliably estimated by ab initio quantum-chemical and DFT calculations. However, it is directly involved in the adsorbate charge that is experimentally accessible by in situ contact electric resistance technique. We present the first quantitative evaluation of charge transfer during halides adsorption on silver from aqueous solutions in dependence on the electrode potential. Partial charge transfer from adsorbed bromide and iodide ions to the silver electrode increases with the positive potential shift and may reach complete charge transfer resulting in adatom creation. Chloride adsorption on silver occurs with much lower charge transfer, but on some stainless superalloys it proceeds with substantial charge transfer owing to their specific electronic structure.     

特非那定的荷移分光光度法测定

研究了特非那定与甲基红的电荷转移反应,建立了快速测定特非那定的可见分光光度法.结果表明,电荷转移络合物的络合比为1∶1,最大吸收波长是430nm,表观摩尔吸光系数为1.15×104 L·mol-1·cm-1.特非那定质量浓度在0.4~48mg/L范围内服从比耳定律,相关系数为0.998 9.当特非那定浓度为20mg/L时,6次测定结果的相对标准偏差为1.40%.本方法用于测定药物中特非那定的含量,加标回收率为97.8%和101.2%.     

Positron annihilation studies of some charge transfer molecular complexes

Positron annihilation lifetimes were measured for some solid charge transfer (CT) molecular complexes of quinoline compounds (2,6-dimethylquinoline, 6-methoxyquinoline, quinoline, 6-methylquinoline, 3-bromoquinoline and 2-chloro-4-methylquinoline) as electron donor and picric acid as an electron acceptor. The infrared spectra (IR) of the solid complexes clearly indicated the formation of the hydrogen-bonding CT-complexes.

The annihilation spectra were analyzed into two lifetime components using PATFIT program. The values of the average and bulk lifetimes divide the complexes into two groups according to the non-bonding ionization potential of the donor (electron donating power) and the molecular weight of the complexes. Also, it is found that the ionization potential of the donors and molecular weight of the complexes have a conspicuous effect on the average and bulk lifetime values. The bulk lifetime values of the complexes are consistent with the formation of stable hydrogen-bonding CT-complexes as inferred from the IR-spectral data.