Mechanisms of electron-transfer oxidation of NADH analogues and chemiluminescence. Detection of the keto and enol radical cations

The radical cation of an NADH analogue (BNAH: 1-benzyl-1,4-dihydronicotinamide) has been successfully detected as the transient absorption and ESR spectra in the thermal electron transfer from BNAH to Fe(bpy)(3)(3+) (bpy = 2,2'-bipyridine) and Ru(bpy)(3)(3+). The ESR spectra of the radical cations of BNAH and the dideuterated compound (BNAH-4,4'-d(2)) indicate that the observed radical cation is the keto form rather than the enol form in the tautomerization. The deprotonation rate and the kinetic isotope effects of the keto form of BNAH(*)(+) were determined from the kinetic analysis of the electron-transfer reactions. In the case of electron transfer from BNAH to Ru(bpy)(3)(3+), the chemiluminescence due to Ru(bpy)(3)(2+) was observed in the second electron-transfer step from BNA(*), produced by the deprotonation of the keto form of BNAH(*)(+), to Ru(bpy)(3)(3+). The observation of chemiluminescence due to Ru(bpy)(3)(2+) provides compelling evidence that the Marcus inverted region is observed even for such an intermolecular electron-transfer reaction. When BNAH is replaced by 4-tert-butylated BNAH (4-t-BuBNAH), no chemiluminescence due to Ru(bpy)(3)(2+) has been observed in the electron transfer from 4-t-BuBNAH to Ru(bpy)(3)(3+). This is ascribed to the facile C-C bond cleavage in 4-t-BuBNAH(*)(+). In the laser flash photolysis of a deaerated MeCN solution of BNAH and CHBr(3), the transient absorption spectrum of the enol form of BNAH(*)(+) was detected instead of the keto form of BNAH(*)(+), and the enol form was tautomerized to the keto form. The rate of intramolecular proton transfer in the enol form to produce the keto form of BNAH(*)(+) was determined from the decay of the absorption band due to the enol form and the rise in the absorption band due to the keto form. The kinetic isotope effects were observed for the intramolecular proton-transfer process in the keto form to produce the enol form.     

Transition metal complexes coordinated by an NAD(P)H model compound and their enhanced hydride-donating abilities in the presence of a base

The ruthenium(II) and rhenium(I) complexes containing an NAD(P)H model compound, 1-benzyl-1,4-dihydronicotinamide (BNAH), as ligand, [Ru(tpy)(bpy)(BNAH)]2+ (1 a) and [Re(bpy)(CO)3(BNAH)]+ (1 b), were quantitatively produced by the reaction of the corresponding metal hydrido complexes with BNA(+) (1-benzylnicotinamidium cation). In the presence of base with pK(a) = 8.9, 1 a and 1 b have much greater reducing power than "free" BNAH. The oxidation potentials of 1 a in the absence and the presence of triethylamine were 0.55 V and -0.04 V, respectively, versus Ag/AgNO(3), whereas that of "free" BNAH was 0.30 V. Spectroscopic results clearly showed that the base extracts a proton from the carbamoyl group on 1 a and 1 b to give the deprotonated BNAH coordinating to the transition-metal complexes [Ru(tpy)(bpy)(BNAH-H+)]+ (3 a) and [Re(bpy)(CO)3(BNAH-H+)] (3 b); this deprotonation underlies the enhancement in reducing ability. The deprotonated forms 3 a and 3 b can efficiently reduce other NAD(P) models to give the corresponding 1,4-dihydro form, resulting in the deprotonated BNA+ being coordinated to the metal complexes [Ru(tpy)(bpy)(BNA(+)-H+)]2+ (2 a) and [Re(bpy)(CO)3(BNA+-H+)]+ (2 b); "free" BNAH and the protonated adducts 1 a and 1 b cannot act in this way. X-ray crystallography was performed on the PF6- salt of 2 a, and showed that the deprotonated nitrogen atom on the carbamoyl group coordinates to the ruthenium(II) metal center with a bond length of 2.086(3) Angstroms. Infrared spectral data suggested that the deprotonated carbamoyl group on the reduced forms 3 a and 3 b is converted to the imido group, and that the oxygen atom coordinates to the metal center.     

NAD(P)H辅酶模型还原碳正离子反应的研究: 动力学、热力学及反应机理

本文对NAD(P)H模型物BNAH同xanthylium正离子反应的动力学进行了较为详尽的测定。同位素效应,自由基抑制剂对反应的影响以及对不同机理模式中各基元步骤的热力学趋动力的研究均表明,反应由决速的电子转移引发,随后通过快速的氢原子转移而形成产物。动力学活化参数的分析指出,电子转移前反应底物间首先形成一个预配合平衡。本文还对BNAH还原9-苯基xanthylium正离子和三苯甲烷正离子的机理进行了较合理的估测。     

Electron-transfer and acid-base properties of a two-electron oxidized form of quaterpyrrole that acts as both an electron donor and an acceptor

Electron-transfer interconversion between the four-electron oxidized form of a quaterpyrrole (abbreviated as P4 for four pyrroles) and the two-electron oxidized form (P4H2) as well as between P4H2 and its fully reduced form (P4H4) bearing analogous substituents in the alpha- and beta-pyrrolic positions was studied by means of cyclic voltammetry and UV-visible spectroelectrochemistry combined with ESR and laser flash photolysis measurements. The two-electron oxidized form, P4H2, acts as both an electron donor and an electron acceptor. The radical cation (P4H2*+) and radical anion (P4H2*-) are both produced by photoinduced electron transfer from dimeric 1-benzyl-1,4-dihydronicotinamide to P4H2, whereas the cation radical form of the compound is also produced by electron-transfer oxidation of P4H2 with [Ru(bpy)3]3+. The ESR spectra of P4H2*+ and P4H2*- were recorded at low temperature and exhibit spin delocalization over all four pyrrole units. Thus, the two-electron oxidized form of the quaterpyrrole (P4H2) displays redox and electronic features analogous to those seen in the case of porphyrins and may be considered as a simple, open-chain model of this well-studied tetrapyrrolic macrocycle. The dynamics of deprotonation from P4H2*+ and disproportionation of P4H2 were examined by laser flash photolysis measurements of photoinduced electron-transfer oxidation and reduction of P4H2, respectively.     

在光动力光疗中类卟啉稀土配合物光敏作用原初反应的特征: 产生自由基I型机制的ESR研究

本文用ESR方法研究了类卟啉稀土配合物[(CO2H-APPC)Gd]Cl2的光敏反应。用4-hydro-tetramethylpiperidine-N-oxide radical(4-hydro-TEMPO)作探针, 通过对其消自旋的作用, 证实[(CO2H-APPC)Gd]Cl2光敏反应中有阳离子自由基[(CO2H-APPC)Gd]^+产生, 加入还原剂可促使[(CO2H-APPC)Gd]^+生成。经由5, 5-Dimethyl-1-pyrrolineN-oxide(DMPO)对超氧阴离子(O2^-)和羟基自由基(.OH)的自旋捕捉及对该自旋加合物[DMPO-O2^-]和[DMPO-OH]的ESR测定, 证实有O2^-和.OH产生, 并用SOD清除O2^-和甲酸钠清除.OH的实验, 进一步证实O2^-和.OH的产生。上述结果说明[(CO2H-APPC)Gd]Cl2光敏反应存在着产生[(CO2H-APPC)Gd]^+和活性氧自由基的I型机制。     

Electrogenerated chemiluminescence 69: the tris(2,2'-bipyridine)ruthenium(II), (Ru(bpy)3(2+))/tri-n-propylamine (TPrA) system revisited-a new route involving TPrA*+ cation radicals

The reaction occurring on electrooxidation of Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine) and tri-n-propylamine (TPrA) leads to the production of Ru(bpy)(3)(2+) and light emission. The accepted mechanism of this widely used reaction involves the reaction of Ru(bpy)(3)(3+) and a reduced species derived from the free radical of the TPrA. However, this mechanism does not account for many of the observed features of this reaction. A new route involving the intermediacy of TPrA cation radicals (TPrA(*+)) in the generation of Ru(bpy)(3)(2+) was established, based on results of scanning electrochemical microscopy (SECM)-electrogenerated chemiluminescence (ECL) experiments, as well as cyclic voltammetry simulations. A half-life of approximately 0.2 ms was estimated for TPrA(*+) in neutral aqueous solution. Direct evidence for TPrA(*+) in this medium was obtained via flow cell electron spin resonance (ESR) experiments at approximately 20 degrees C. The ESR spectra of the TPrA(*+) species consisted of a relatively intense and sharp septet with a splitting of approximately 20 G and a g value of 2.0038.     

Oxidations of NADH analogues by cis-[RuIV(bpy)2(py)(O)]2+ occur by hydrogen-atom transfer rather than by hydride transfer

Oxidations of the NADH analogues 10-methyl-9,10-dihydroacridine (AcrH2) and N-benzyl 1,4-dihydronicotinamide (BNAH) by cis-[RuIV(bpy)2(py)(O)]2+ (RuIVO2+) have been studied to probe the preferences for hydrogen-atom transfer vs hydride transfer mechanisms for the C-H bond oxidation. 1H NMR spectra of completed reactions of AcrH2 and RuIVO2+, after more than approximately 20 min, reveal the predominant products to be 10-methylacridone (AcrO) and cis-[RuII(bpy)2(py)(MeCN)]2+. Over the first few seconds of the reaction, however, as monitored by stopped-flow optical spectroscopy, the 10-methylacridinium cation (AcrH+) is observed. AcrH+ is the product of net hydride removal from AcrH2, but hydride transfer cannot be the dominant pathway because AcrH+ is formed in only 40-50% yield and its subsequent oxidation to AcrO is relatively slow. Kinetic studies show that the reaction is first order in both RuIVO2+ and AcrH2, with k = (5.7 +/- 0.3) x 10(3) M(-1) s(-1) at 25 degrees C, DeltaH(double dagger) = 5.3 +/- 0.3 kcal mol(-1) and DeltaS(double dagger) = -23 +/- 1 cal mol(-1) K(-1). A large kinetic isotope effect is observed, kAcrH2/kAcrD2 = 12 +/- 1. The kinetics of this reaction are significantly affected by O2. The rate constants for the oxidations of AcrH2 and BNAH correlate well with those for a series of hydrocarbon C-H bond oxidations by RuIVO2+. The data indicate a mechanism of initial hydrogen-atom abstraction. The acridinyl radical, AcrH*, then rapidly reacts by electron transfer (to give AcrH+) or by C-O bond formation (leading to AcrO). Thermochemical analyses show that H* and H- transfer from AcrH2 to RuIVO2+ are comparably exoergic: DeltaG degrees = -10 +/- 2 kcal mol(-1) (H*) and -6 +/- 5 kcal mol(-1) (H-). That a hydrogen-atom transfer is preferred kinetically suggests that this mechanism has an equal or lower intrinsic barrier than a hydride transfer pathway.     

Binuclear 1,2,4,5-tetraimino-3,6-diketocyclohexane bis[bis(bipyridine)ruthenium(II)] redox series

The reaction of the [Ru(bpy)2(MeOH)2]2+ cation (bpy = 2,2'-bipyridine) with 1,2,4,5-tetraaminobenzene in the presence of trace water and oxygen yields the cation [(bpy)2Ru(1,2,4,5-tetraimino-3,5-diketocyclohexane)Ru(bpy)2]4+. This binuclear species undergoes ligand-based reductions, giving the 3+ and 2+ charged species. The X-ray structure, electrochemistry, ZINDO calculations, and NMR, ESR, UV/vis, and IR spectra were analyzed where possible, giving an electronic model of the binuclear species and some of its redox products. The X-ray structure reveals the [(bpy)2Ru] fragments symmetrically disposed across the 1,2,4,5-tetraimino-3,5-diketocyclohexane bridge in a molecule with Cs symmetry.     

Mechanistic borderline between one-step hydrogen transfer and sequential transfers of electron and proton in reactions of NADH analogues with triplet excited states of tetrazines and Ru(bpy)(3)2+

Efficient energy transfer from Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine, denotes the excited state) to 3,6-disubstituted tetrazines [R(2)Tz: R = Ph (Ph(2)Tz), 2-chlorophenyl [(ClPh)(2)Tz], 2-pyridyl (Py(2)Tz)] occurs to yield the triplet excited states of tetrazines ((3)R(2)Tz(*)), which have longer lifetimes and higher oxidizing ability as compared with those of Ru(bpy)(3)(2+). The dynamics of hydrogen-transfer reactions from NADH (dihydronicotinamide adenine dinucleotide) analogues has been examined in detail using (3)R(2)Tz(*) by laser flash photolysis measurements. Whether formal hydrogen transfer from NADH analogues to (3)R(2)Tz(*) proceeds via a one-step process or sequential electron and proton transfer processes is changed by a subtle difference in the electron donor ability and the deprotonation reactivity of the radical cations of NADH analogues as well as the electron-acceptor ability of (3)R(2)Tz(*) and the protonation reactivity of R(2)Tz(*)(-). In the case of (3)Ph(2)Tz(*), which is a weaker electron acceptor than the other tetrazine derivatives [(ClPh)(2)Tz; Py(2)Tz], direct one-step hydrogen transfer occurs from 10-methyl-9,10-dihydroacridine (AcrH(2)) to (3)Ph(2)Tz(*) without formation of the radical cation (AcrH(2)(*)(+)). The rate constant of the direct hydrogen transfer from AcrH(2) to (3)Ph(2)Tz(*) is larger than that expected from the Gibbs energy relation for the rate constants of electron transfer from various electron donors to (3)Ph(2)Tz(*), exhibiting the primary deuterium kinetic isotope effect. On the other hand, hydrogen transfer from 9-isopropyl-10-methyl-9,10-dihydroacridine (AcrHPr(i)) and 1-benzyl-1,4-dihydronicotinamide (BNAH) to (3)R(2)Tz(*) occurs via sequential electron and proton transfer processes, when both the radical cations and deprotonated radicals of NADH analogues are detected by the laser flash photolysis measurements.     

An endor and ESR study of the radical cation of N,N′-bis-(4-fluorophenyl)-4,4′-bipyridylium dichloride

The ESR and ENDOR spectra of the radical cation of N,N′-bis-(4-fluoro-phenyl)-4-4′-bipyridylium dichloride (fluorophenylquat FPQ) in methanol was studied over a temperature range from +_40° to ?90°. The ENDOR technique was used to obtain accurately the splitting constans for a highly complicated ESR spectrum and computer simulation showed excellent agreement. Fluorine ENDOR resonance was clearly observed with a line width similar to that of the protons. On decreasing the temperature the concentration of the radical cation decreases until at ?90° the ESR intensity was very small. This process is reversible and concentration studies indicate that the radical cation is in equilibrium with a diamagnetic dimer species. The thermodynamic parameters ΔH°, ΔG° and ΔS° for the process are reported.     

Temperature change of OD ESR and freon matrix ESR of the radical cations of 9,10-octalin and cyclohexene

The optical detection (OD) ESR and Freon matrix ESR spectra were observed for the radical cations of 9,10-octalin and cyclohexene from 77 to 300 K. The observed spectral change was attributed to ring inversion in both radical cations based on MO calculations and an analysis by the modified Bloch equation. The activation energy for the inversion of the radical cation of octalin was calculated as 18.8 kJ mol⁻¹.     

In situ electrochemical ESR and voltammetric studies on the anodic oxidation of para-haloanilines in acetonitrile

An in situ electrochemical electron spin resonance (ESR) study on the electro-oxidation of para-chloroaniline, para-bromoaniline, and para-iodoaniline dissolved in acetonitrile at gold electrodes is reported. ESR spectra obtained using a tubular flow cell reveal the presence of a paramagnetic dimer product derived from para-aminodiphenylamine, during oxidative electrolysis, suggesting the coupling of reactive electrogenerated radical cations with neutral parent haloaniline molecules. The ESR signal intensity behaves in a manner expected for a radical species reacting with second-order kinetics, suggesting the paramagnetic dimer is, itself, unstable. The theory describing the ESR signal intensity flow rate behavior for this reaction mechanism is developed for the tubular arrangement and used to extract mechanistic and kinetic data from the experimental results for the cases of para-chloroaniline and para-bromoaniline. Further mechanistic aspects, including proton and halide ion expulsion during dimerization, are explored voltammetrically and with the aid of digital simulations using Digisim. Comparison of the ESR signal and voltammetric measurements suggests that an additional mechanism operates which does not lead to paramagnetic products. Additionally, the in situ electrolysis of N,N-dimethyl-para-bromoaniline is reported to generate the stable radical cation of N,N,N',N'-tetramethylbenzidine, and a mechanism of electro-oxidation is, thus, proposed.     

MARY-detected ESR spectra of radical ions in liquid solutions for systems with crossing Zeeman levels

The effect of Zeeman levels crossing in spin-correlated radical ion pairs (naphthalene)⁺/(hexafluorobenzene)⁻ was monitored as the influence of an external magnetic field on the solution fluorescence under X-irradiation (MARY spectrum). The spectra obtained exhibit a line in the field, equal to triple the hfi coupling constant of hexafluorobenzene radical anion, its linewidth being determined by the unresolved hyperfine structure of the naphthalene cation. The theory predicts that the presence of weak hfi in the partner cation splits this line by the projections of the cation total nuclear momentum M_Z according to nuclear statistics. Thus, a direct correspondence between the MARY and ESR spectra of radical cations allows registration of the ESR spectrum without microwave pumping.     

共轭聚合物中稳定自由基的ESR研究

通过ＥＳＲ跟踪测试，研究了３种胺类化合物与聚（Ｎ，Ｎ－二甲基苯胺）（ＰＤＭＡ）中离域正离子自由基相线作用的过程，发现胺类化合物能使ＰＤＭＡ的ＥＳＲ谱形发生不同程度的劈裂，信号强度随时间的变化与胺的结构有关，说明正离子自由基与亲核试剂作用后发生了定域化转变，通过进一步比较发现ＰＤＭＡ自由基的亲电活性不如聚苯上的正离子的自由基。     

L-tryptophan radical cation electron spin resonance studies: connecting solution-derived hyperfine coupling constants with protein spectral interpretations

Fast-flow electron spin resonance (ESR) spectroscopy has been used to detect a free radical formed from the reaction of l-tryptophan with Ce (4+) in an acidic aqueous environment. Computer simulations of the ESR spectra from l-tryptophan and several isotopically modified forms strongly support the conclusion that the l-tryptophan radical cation has been detected by ESR for the first time. The hyperfine coupling constants (HFCs) determined from the well-resolved isotropic ESR spectra support experimental and computational efforts to understand l-tryptophan's role in protein catalysis of oxidation-reduction processes. l-Tryptophan HFCs facilitated the simulation of fast-flow ESR spectra of free radicals from two related compounds, tryptamine and 3-methylindole. Analysis of these three compounds' beta-methylene hydrogen HFC data along with equivalent l-tyrosine data has led to a new computational method that can distinguish between these two amino acid free radicals in proteins without dependence on isotope labeling, electron-nuclear double resonance, or high-field ESR. This approach also produces geometric parameters (dihedral angles for the beta-methylene hydrogens) that should facilitate protein site assignment of observed l-tryptophan radicals as has been done for l-tyrosine radicals.     

Electron-transfer oxidation properties of unsaturated fatty acids and mechanistic insight into lipoxygenases

Rate constants of photoinduced electron-transfer oxidation of unsaturated fatty acids with a series of singlet excited states of oxidants in acetonitrile at 298 K were examined and the resulting electron-transfer rate constants (k(et)) were evaluated in light of the free energy relationship of electron transfer to determine the one-electron oxidation potentials (E(ox)) of unsaturated fatty acids and the intrinsic barrier of electron transfer. The k(et) values of linoleic acid with a series of oxidants are the same as the corresponding k(et) values of methyl linoleate, linolenic acid, and arachidonic acid, leading to the same E(ox) value of linoleic acid, methyl linoleate, linolenic acid, and arachidonic acid (1.76 V vs SCE), which is significantly lower than that of oleic acid (2.03 V vs SCE) as indicated by the smaller k(et) values of oleic acid than those of other unsaturated fatty acids. The radical cation of linoleic acid produced in photoinduced electron transfer from linoleic acid to the singlet excited state of 10-methylacridinium ion as well as that of 9,10-dicyanoanthracene was detected by laser flash photolysis experiments. The apparent rate constant of deprotonation of the radical cation of linoleic acid was determined as 8.1 x 10(3) s(-1). In the presence of oxygen, the addition of oxygen to the deprotonated radical produces the peroxyl radical, which has successfully been detected by ESR. No thermal electron transfer or proton-coupled electron transfer has occurred from linoleic acid to a strong one-electron oxidant, Ru(bpy)3(3+) (bpy = 2,2'-bipyridine) or Fe(bpy)3(3+). The present results on the electron-transfer and proton-transfer properties of unsaturated fatty acids provide valuable mechanistic insight into lipoxygenases to clarify the proton-coupled electron-transfer process in the catalytic function.     

付氏反应中自由基的鉴定及其生成机理

本文通过一系列卤代烃与苯的付氏反应的ESR谱,研究了自由基物种的生成机理:它们起因于Lewis酸对由Scholl反应所生成的9,10-二取代蒽进行单电子氧化,并建立了一种生成9,10-二取代蒽自由基正离子的简单方法。首次报道了一个新自由基正离子1,2,3,4,5,6,7,8-八氘蒽的ESR谱。     

The Radical Cation of Cyclobutene and its Photoproduct. Preliminary communication

The hitherto unknown radical cation of cyclobutene ( 2 ) has been generated in a CFCl₃ matrix by γ rays at 77 K. The coupling constants, as determined from the ESR spectrum of 2 ⁺, are 2.80 and 1.11 mT for the four CH₂ and the two CH = protons, respectively. Photo-induced ring opening of 2 ⁺ yields a radical cation which exhibits the same ESR and ENDOR spectra as those observed upon direct ionization of s-trans-buta-1, 3-diene (s-trans -1 ). The radical cation s-trans -1 ⁺, should, therefore, be the final product of this conversion.     

Matrix dependence of the conformations of the 1,4-diethylbenzene radical cation at low temperatures

1,4-Diethylbenzene radical cation generated in six different halocarbon matrices by ⁶⁰Co γ-ray irradiation has been studied by ESR at low temperatures. The spectra exhibit drastic variations in different matrices, showing that the conformation of the radical cation is largely dependent on the matrix.     

Incorporation of 2-arylhexa-1,5-diene into pentasil zeolite: A distorted 1-arylcyclohexane-1,4-diyl radical cation at room temperature

Incorporation into a redox-active pentasil zeolite [(Na,H)-ZSM-5] converted 2-arylhexa-1,5-dienes (9a-c; aryl = phenyl, tolyl, anisyl) into 1-arylcyclohexane-1,4-diyl radical cations, 10a-c*+. The ESR spectra of 10a-c*+ (six lines, g = 2.0026; a = 9.0 G) indicated the presence of five essentially equivalent nuclei, indicating limited delocalization of spin and charge into the phenyl group. Sequestered in the pores of ZSM-5, the three species 10a-c*+ are stable at room temperature, in striking contrast to the parent radical cation in cryogenic matrices: cyclohexane-1,4-diyl radical cation is converted to cyclohexene radical cation above 90 K. The structures of radical cation 10a*+ (X = H) and of the unsubstituted parent were probed by density functional theory (DFT) and ab initio calculations.