Electron Transfer Products Research Articles

Electron transfer ion/ion reactions in a three-dimensional quadrupole ion trap: reactions of doubly and triply protonated peptides with SO2*-.

Ion-ion reactions between a variety of peptide cations (doubly and triply charged) and SO2 anions have been studied in a 3-D quadrupole ion trap, resulting in proton and electron transfer. Electron transfer dissociation (ETD) gives many c- and z-type fragments, resulting in extensive sequence coverage in the case of triply protonated peptides with SO2*-. For triply charged neurotensin, in which a direct comparison can be made between 3-D and linear ion trap results, abundances of ETD fragments relative to one another appear to be similar. Reactions of doubly protonated peptides with SO2*- give much less structural information from ETD than triply protonated peptides. Collision-induced dissociation (CID) of singly charged ions formed in reactions with SO2*- shows a combination of proton and electron transfer products. CID of the singly charged species gives more structural information than ETD of the doubly protonated peptide, but not as much information as ETD of the triply protonated peptide.

Structural and EPR characterisation of single electron and alkyl transfer products from reaction of dimethyl magnesium with bulky α-diimine ligands

Treatment of dimethylmagnesium with bulky alpha-diimine ligands provides either the biradical methyl-bridged complexes [(alpha-diimine-.)Mg+(mu-CH3)]2 via single electron transfer (SET), or the product of methyl transfer to an imine carbon atom depending upon conditions.

Multiwalled carbon nanotubes in donor–acceptor nanohybrids—towards long-lived electron transfer products

Novel multiwalled carbon nanotube/metalloporphyrin nanohybrids are devised and probed as versatile donor-acceptor hybrids.

Single Transition State Serves Two Mechanisms. Ab Initio Classical Trajectory Calculations of the Substitution−Electron Transfer Branching Ratio in CH2O•- + CH3Cl

The reaction of a formaldehyde radical anion with methyl chloride is an example of a reaction in which a single transition state serves two mechanisms: substitution at carbon (Sub(C)) and electron transfer (ET). This reaction has been studied by ab initio molecular dynamics at the HF/6-31G(d) level of theory. Initial conditions were sampled from thermal distributions at the transition state, and ca. 200 trajectories were calculated at each of four different temperatures. Some trajectories go directly to ET products, but most go to the Sub(C) valley. Analysis of the initial conditions did not reveal any definitive factors that consistently favored one channel over the others. About half of the molecules in the Sub(C) valley subsequently dissociate to ET products within the 800−1200-fs simulation time of the present calculations. These molecules showed unimolecular kinetics for dissociation consistent with a chemically activated species. The ratio of ET to Sub(C) products varied from 1.02 to 1.43 over the ...

Electrochemical microgravimetric study on microcrystalline particles of phenazine attached to gold electrodes

Phenazine solid crystals have been attached to gold electrodes and investigated by cyclic and potential step electrochemical quartz crystal microbalance (ECQM) measurements in the presence of aqueous acidic media. The freshly deposited phenazine layers exhibit a break-in phenomenon. The number of potential cycles required for the layer to be fully electroactive depends on its thickness and also on the nature and concentration of the supporting electrolyte as well as on the scan rate. After the break-in, a considerable amount of solvent molecules remains embedded in the surface layer. The protonated and unprotonated forms of phenazine, whose relative amounts depend on the pH of the contacting solutions, are reduced at different potentials; however, the stable product of the first electron transfer is the respective phenazylium salt. During the second reduction step 5,10-dihydrophenazine and charge-transfer complexes of different compositions are formed. Both the current and microgravimetric responses supplied evidences for the structural rearrangements of the solid phases that accompany the redox reactions. The large separation of the reduction and oxidation peaks relates to the additional energy needed to create the solid/solid interface between the reduced and unreduced or partially reduced forms. The chronoamperometric response shows the characteristics of nucleation and growth kinetics. The phase transformation proceeds with the release of hydration water, and the EQCM response is affected by the strain that develops as a consequence of the phase transformation.

Excited-State Properties of Flavin Radicals in Flavoproteins: Femtosecond Spectroscopy of DNA Photolyase, Glucose Oxidase, and Flavodoxin

In isolated, catalytically inactive, DNA photolyase from Escherichia coli (E. coli), the flavin adenine dinucleotide cofactor is in its neutral radical state FAD?. It can be activated by a unique light-induced reduction of the flavin, a process initiated by the formation of the excited-state FAD?*. As the photophysical properties of this state are essentially unknown, we performed a comparative characterization by femtosecond transient absorption spectroscopy of FAD?* in DNA photolyase from E. coli and glucose oxidase from Aspergillus niger, and of the excited neutral radical flavin mononucleotide (FMN?*) in flavodoxin from Desulfovibrio gigas. In contrast to photolyase, in glucose oxidase and flavodoxin no electron-transfer products are observed after selective excitation of the flavin radical. In glucose oxidase, FAD?* decays to the ground state in 59 ± 5 ps, close to the 80-ps intrinsic lifetime of the excited state in photolyase, and we discuss that the intrinsic lifetime of the excited state of flavin radical in protein environment is in the 50-80 ps range. FMN?* in flavodoxin decays much faster (2.3 ± 0.3 ps), possibly because of quenching by formation of a very short-lived (INF 0.7 ps) electron-transfer intermediate. Spectroscopically, the excited state of FAD? in photolyase displays a pronounced spectral feature that is absent in the other systems studied. Further characterization by polarized photoselection experiments identifies the feature as an additional induced absorption band at ~ 550 nm superimposed on the ground-state bleaching signal. In view of the unique U-shape configuration of FAD in photolyase, we suggest it to reflect a flavin-adenine charge-transfer interaction.

Low Quantum Yields of Relaxed Electron Transfer Products of Moderately Coupled Ruthenium(II)−Cobalt(III) Compounds on the Subpicosecond Laser Excitation

Photoinduced electron-transfer reactions of [(tpy)RuII(tpy-tpy)CoIII(tpy)]5+ (tpy = 2,2‘:6‘,2‘ ‘-terpyridine and tpy-tpy = 6‘,6‘ ‘-bis(2-pyridyl)-2,2‘:4‘,4‘ ‘:2‘ ‘,2‘ ‘‘-quarterpyridyne), [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+ (tpy-ph-tpy = 1,4-bis[2,2‘:6‘,2‘ ‘-terpyridine-4‘-yl]benzene), and [(bpy)2RuII(tpphz)CoIII(bpy)2]5+ (bpy = 2,2‘-bipyridine, and tpphz = tetrapyrido[3,2-a:2‘,3‘-c:3‘ ‘,2‘ ‘-h::2‘ ‘‘,3‘ ‘‘-j]phenazine) were studied in the range 140−298 K by means of subpicosecond transient absorption spectroscopy. 3MLCT(Ru) of [(tpy)RuII(L−L)CoIII(tpy)]5+ (L−L: tpy-tpy and tpy-ph-tpy) underwent intramolecular electron-transfer (EET) reaction to form [2RuIII(tpy)(L−L)2CoII(tpy)]5+ in a shorter time than 10 ps. Low quantum yields of the EET products (0.53 for tpy-tpy and 0.41 for tpy-ph-tpy in butyronitrile at 298 K) measured before the fast return electron-transfer were found as in the intermolecular electron-transfer quenching of 3MLCT(Ru) by [Co(tpy)2]3+. The quantum yield of [(tpy)RuIII(tpy-tpy)CoII(tpy)]5+ (Φ) decreases to 0.38 in a slowly reorganizing solvent of propylene carbonate and that of [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+ to 0.21 at 180 K. The reductions in the EET yields, 1 − Φ, can be ascribed to a fast transition of the nonrelaxed EET product to the lowest triplet d−d* state of [CoIII(tpy)2] moiety during the solvent reorganization. A tunneling transition of the nonrelaxed EET products to the lowest lying d−d excited-state of [CoIII(tpy)2] moiety takes place as a hole transfer, HT, from the dπ-orbital of Ru(III) to that of Co(II) with a configuration of dπ6dσ*. An electronic coupling of dπ(Ru)−dπ(Co) estimated from the intensity of inter-valence transition of [(tpy)RuIII(L−L)RuII(tpy)]5+ (Collin, J.-P.; Laine, P.; Launay, J.-P.; Sauvage, J.-P.; Sour, A. J. Chem. Soc., Chem. Commun. 1993, 434) is large enough to carry out the HT. The weak coupling of dπ(Ru)−dπ(Co) in the case of [(bpy)2Ru(tpphz)Co(bpy)2]5+ attenuates the HT rate to bring about a high yield of EET (0.8). The possibility that energy-transfer of 3MLCT(Ru) to the Co(III) moiety via an intermediate super-exchange coupling between dπ(Ru) and dπ(Co) occurs in competition with the EET of [(tpy)RuIII(tpy-ph-tpy)CoII(tpy)]5+ is also pursued.

Adsorption and precipitation during the redox transformations of phenazine

The adsorption/desorption and deposition/dissolution phenomena occurring during the electrochemical transformations of phenazine (Ph) at a gold electrode in aqueous acidic solutions have been studied by cyclic and potential step electrochemical quartz crystal microbalance measurements. Phenazine exhibits two successive one-electron reduction steps in acidic media. In dilute phenazine solutions, the product of the first electron transfer is phenazylium cation radical ( PhH 2 + ̇ ) which adsorbs in the form of a phenazylium salt ( PhH 2 + ̇ ClO 4 −, PhH 2 + ̇ Cl −) at the electrode surface. The perchlorate salt is highly soluble and the electroreduction takes place via a loosely adsorbed state. In chloride containing solutions, multilayer adsorption is observed. The second electron transfer in dilute phenazine solutions results in the formation of 5,10-dihydrophenazine (PhH 2) in HClO 4, which desorbs from the electrode surface. In HCl solutions, a substantial portion of the fully reduced product, which is a charged dimer, remains on the surface. In more concentrated phenazine solutions in the potential region of the second reduction wave, a deposition process can be observed, which is due to the formation of the quinhydrone-analogue, phenazinehydrine charge-transfer complex. The formation of the charge-transfer complex obeys a second-order kinetics, however, the rate of the film growth is influenced by the simultaneous dissolution process. The increase of the acid concentration enhances the dissolution, and may prevent the film formation. In dilute phenazine solutions, both redox waves are reversible and likewise the adsorption/desorption processes. In concentrated solutions, the reoxidation of the phenazinehydrine film results in a complicated voltammetric response related to a dissolution–redeposition–dissolution sequence.

Differences in the ROS-generating efficacy of various ultraviolet wavelengths in detached spinach leaves.

The capacity of ultraviolet (UV) radiation to generate reactive oxygen species (ROS) in spinach leaves was studied with fluorescent sensors. Leaf segments were exposed to short-term (15-30 min), narrow-band UV irradiation of various wavelengths between 290-390 nm corresponding to equal numbers of photons and, depending on the wavelength, 18-36 μmol m-2 s-1 quantum flux. This caused 5-30% loss of photosynthesis measured as loss of variable chlorophyll fluorescence. In accordance with previous results, photosynthesis was less affected by longer than by shorter wavelength UV. UV-irradiated leaves were probed for two major representatives of ROS, namely the superoxide radical anion, a product of electron transfer to oxygen, and singlet oxygen, which is usually formed in photosensitised reactions between oxygen and triplet dyes. Both ROS were detected, but the efficacy of UV in promoting their production was different. Formation of superoxide radicals was positively correlated with loss of photosynthesis; more radicals were observed in leaves exposed to shorter UV wavelengths than in leaves irradiated at 370-390 nm, suggesting a functionally damaged electron transport as a possible site of oxygen reduction. Singlet oxygen was, however, mainly inducible in leaves exposed to longer UV wavelengths and was barely induced by 290-300-nm irradiation. The mechanism of singlet oxygen generation by UV radiation in the leaf, reported for the first time, is not clear, but the process does not appear to be linked to the UV-induced damage of photosynthesis.

Dynamics of photoinduced electron-transfer processes in fullerene-based dyads: effects of varying the donor strength.

Two classes of fullerene-based donor-bridge-acceptor (D-B-A) systems containing donors of varying oxidation potentials have been synthesized. These systems include fullerenes linked to heteroaromatic donor groups (phenothiazine/phenoxazine) as well as substituted anilines (p-anisidine/p-toluidine). In contrast to the model compound, an efficient intramolecular electron transfer is observed from the fullerene singlet excited state in polar solvents. An increase in the rate constant and quantum yield of charge separation (kcs and phi cs) has been observed for both classes of dyads, with decrease in the oxidation potentials of the donor groups. This observation indicates that the rates of the forward electron transfer fall in the normal region of the Marcus curve. The long-lived charge separation enabled the characterization of electron transfer products, namely, the radical cation of the donor and radical anion of the pyrrolidinofullerene, by using nanosecond transient absorption spectroscopy. The small reorganization energy (lambda) of C60 coupled with large negative free energy changes (-delta G degree) for the back electron transfer places the back electron process in the inverted region of Marcus curve, thereby stabilizing the electron transfer products.

Solvent-dependent dynamics of the MQ*-->ReII excited-state electron transfer in [Re(MQ+)(CO)3(dmb)]2+.

The Re-->MQ(+) MLCT excited state of [Re(MQ(+))(CO)(3)(dmb)](2+) (MQ(+) = N-methyl-4,4'-bipyridinium, dmb = 4,4'-dimethyl-2,2'-bipyridine), which is populated upon 400-nm irradiation, was characterized by picosecond time-resolved IR and resonance Raman spectroscopy, which indicate large structural differences relative to the ground state. The Re-->MQ(+) MLCT excited state can be formulated as [Re(II)(MQ*)(CO)(3)(dmb)](2+). It decays to the ground state by a MQ*-->Re(II) back-electron transfer, whose time constant is moderately dependent on the molecular nature of the solvent, instead of its bulk parameters: formamides approximately DMSO approximately MeOH (1.2-2.2 ns) < THF, aliphatic nitriles (3.2-3.9 ns) << ethylene-glycol approximately 2-ethoxyethanol (4.2-4.8 ns) < pyridine (5.7 ns) < MeOCH(2)CH(2)OMe (6.9 ns) < PhCN (7.5 ns) < MeNO(2) (8.6 ns) <<< CH(2)Cl(2), ClCH(2)CH(2)Cl (25.9-28.9 ns). An approximate correlation was found between the back-reaction rate constant and the Gutmann donor number. Temperature dependence of the decay rate measured in CH(2)Cl(2), MeOH, and BuCN indicates that the inverted MQ*-->Re(II) back-electron transfer populates a manifold of higher vibrational levels of the ground state. The solvent dependence of the electron transfer rate is explained by solvent effects on inner reorganization energy and on frequencies of electron-accepting vibrations, by interactions between the positively charged MQ(+) pyridinium ring and solvent molecules in the electron-transfer product, that is the [Re(MQ(+))(CO)(3)(dmb)](2+) ground state.

Dinitrosulfodienes of the 2,5-dihydrothiophene-1,1-dioxide series in reactions with nucleophiles

Reactions of dinitrosulfo-dienes of the dihydrothiophene- 1,1 dioxide series, 3-methyl-4-nitro-2(1’-nitro-1’-aryl)methylene-2,5-dihydrothiophene- 1,1-dioxides, with N- and S- nucleophiles are presented. Interaction between N,N-dimethylaniline and aromatic thiols leads to the electrontransfer products. In the first case, stable molecular charge-transfer complexes are formed, while the S -nucleophiles lead to diaryl disulfides and products of reduction of the initial compounds. The reaction with the barely oxidizing sodium azide proceeds according to the second competitive route and results in the formation of products of nucleophilic vinylic substitution. Both pathways appear to be possible in the interaction with dodecanethiol, while the major process is SNVin.

Electron transfer and paramagnetic products of reactions of haloquinones with aliphatic amines

The reactions of chloranil (Cl4Q) and bromanil (Br4Q) with aliphatic amines in a DMF : H2O (5 : 1, vol/vol) mixture were studied. The radical anions of 2,5-didimethylamino-3,6-chloro-p-benzoquinone and 2,5-didimethylamino-3,6-bromo-p-benzoquinone were identified by ESR spectra. The reaction rate constant of the replacement of two chlorine atoms by the amino groups in the radical anion of Cl4Q at 288 K was estimated.

Temperature-Independent Rate of Electron-Transfer between a Cobalt(II) and a Ruthenium(III) of Doublet Electronic Configuration

A study of laser kinetic spectroscopy on [RuII(tpy)(BL)CoIII(tpy)]5+ (BL: 1,4-bis[2,2‘:6‘,2‘ ‘-terpyridine-4‘-yl]benzene (btbz) and 6‘,6‘ ‘-bis(2-pyridyl)- 2,2‘:4‘,4‘ ‘:2‘ ‘,2‘ ‘‘-quarter-pyridine (bpqp)) revealed rise-and-decay of a triplet Ru-to-ligand charge-transfer state, 3CT(Ru), and an electron-transfer product of [(tpy)RuIII(BL)CoII(tpy)]5+. 3CT(Ru) underwent electron-transfer with a rate constant of 0.3 × 1012 and 1 × 1012 s-1 to produce [(tpy)RuIII(BL)CoII(tpy)]5+ with a doublet electronic configuration dπ6dσ* of cobalt(II). The return electron transfer (RET) of [RuIII(tpy)(BL)CoII(tpy)]5+ (BL:btbz and bpqp) in acetonitrile occurred with a rate constant of 1 × 1010 and 6 × 1010 s-1 at 298 K, respectively, which are much faster than intramolecular RET of [(bpy)2Ru(BL)Co(bpy)2]5+ from cobalt(II) with a quartet electronic configuration of dπ5dσ*2. The magnitude of reorganization energy (0.95 ± 0.15 eV) and electronic coupling matrix element (0.8 and 2 meV) are evaluated from the independence of RE...

Diphenol radical cations and semiquinone radicals as direct products of the free electron transfer from catechol, resorcinol and hydroquinone to parent solvent radical cations

In the pulse radiolysis of solutions of catechol, resorcinol and hydroquinone in n-butylchloride, dihydroxybenzene radical cations (40%) as well as semiquinone radicals (60%) are observed as direct synchronously formed products of the electron transfer from the solvent parent ions to the solute. This is explained in terms of free electron transfer succeeding in nearly every encounter of the reactants, which in the case of the studied dihydroxybenzenes involves femtosecond molecular dynamics effects. The rotation of the C–OH bond causes cycling of the molecule through transient conformations also exhibiting different electron distributions. From the more chemical point of view, the diphenol radical cations represent the first and till now unknown intermediates of oxidative semiquinone radical formation.

Electron Transfer Quenching and Electrochemiluminescence Comparative Studies of the Systems Containing N-Methylpyridinium Cations and Ru(2,2‘-bipyridine)32+ or Ru(1,10-phenathroline)32+ Complexes

Quenching of the luminescent state of tris(2,2‘-bipyridine)ruthenium(II) and tris(1,10-phenanthroline)ruthenium(II) (Ru(bipy)32+ and Ru(phen)32+) complexes by a series of pyridinium ions has been studied in 0.1M (C2H5)4NPF6 acetonitrile solutions. 4-Acetyl, 4-cyano, 4-carbomethoxy, and 4-carbamido-1-methylpyridium cations R+ were employed as electron acceptor quenchers. The quenching rates were measured by using steady-state technique. Separation efficiencies of electron-transfer products were obtained by measurements of the transient absorbances of photogenerated N-methylpyridinium radicals R• using laser flash photolysis. The quenching rate constants and radical separation efficiencies were used for the calculation of the kinetic parameters for forward, reverse and back electron transfer. Opposite electron-transfer process, i.e., a generation of the excited states *Ru(bipy)32+ and *Ru(phen)32+ (in reactions Ru(bipy)33+ + R• or Ru(phen)33+ + R•, respectively), has been studied by means of the electrochem...

Monitoring of the Heterogroup Twisting Dynamics in Phenol Type Molecules via Different Characteristic Free-Electron-Transfer Products

Phenol radical cations as well as phenoxyl radicals were observed as direct products of free electron transfer from phenol type solute molecules to solvent parent radical cations generated by ionizing irradiation. It is shown that the finding of the two species in comparable amounts can be explained by a nuclear-structure dependent solute cation dissociation behavior: Quantum-chemical calculations indicate that for phenol as solute primarily its conformers with perpendicular C−OH axis orientation to the aromatic ring tend to prompt deprotonation after ionization. A quite similar behavior could be predicted also for the heteroanalogous thiophenols and selenophenols. Quite generally considerable changes in the electron distribution of the ground-state molecules with the twisting angle of the −OH, −SH and −SeH groups could be calculated, with the greatest differences between “parallel” and “perpendicular” conformations. On the assumption that the fast electron-transfer projects the equilibrium solute conformer distribution onto the solute cation conformer one it is demonstrated that the experimental findings are compatible with a simple solute-cation internal relaxation model. By applying the quantum-chemically calculated conformer interconversion barrier heights it can be understood that radical fraction among the direct products increases if phenols are substituted by thiophenols or those by selenophenols, as observed in experiment.

Adsorptive immobilization of cytochrome c on indium/tin oxide (ITO): electrochemical evidence for electron transfer-induced conformational changes

The adsorptive immobilization and electrochemistry of horse and yeast cytochrome c on indium/tin oxide (ITO) electrodes is reported. Near-monolayer coverage was achieved in pH 7 phosphate buffers of ionic strength equal to 10 and 50 mM, respectively, for the horse and yeast species. The layers exhibit very well-behaved voltammetry and are stable on the timescale of hours to days. Cyclic voltammetry revealed quasireversible behavior that is a product of both electron transfer (ET) kinetics and ET-induced conformational changes. A square scheme mechanism linking the redox states and the conformational states is proposed. Using a simple ET kinetic model that adequately describes the voltammetry at higher scan rates, a standard ET rate constant of 18 s −1 was determined for adsorbed horse cytochrome c. With decreasing scan rate, we observed a limiting peak separation of approximately 10 mV, an example of unusual quasireversibility (UQR) that we attribute to the effect of conformational changes. Finally we note that the intrinsic cytochrome c ET rate on ITO is some 6 orders of magnitude less than for gold.

Activation parameters for the competing electron transfer and SN2 pathways of the reaction of anthracene radical anion with cyclopropylmethyl bromide

The reaction between the electrogenerated radical anion of anthracene and alkyl halides has been studied in the temperature interval of −50 to 40 °C in N,N-dimethylformamide in order to describe in detail the competition between the electron transfer (ET) and SN2 pathways. For cyclopropylmethyl bromide the competition can be quantified directly on the basis of an analysis of the distribution of ring-opened versus ring-closed products. In general, the ET pathway is found to prevail although the SN2 pathway becomes of greater importance as the temperature is lowered. When the product analysis is combined with the measurement of the overall rate constant for the reaction the pertinent activation parameters can be extracted for both mechanisms. As expected ΔH‡SN2 is smaller than ΔH‡ET (by 7 kJ mol−1) because of the stronger bonding interaction present in the transition state of SN2. On the other hand, this effect is more than counterbalanced by the fact that ET is entropically favoured because of smaller geometrical restrictions (ΔS‡ET − ΔS‡SN2 = 37 J mol−1 K−1). The SN2 mechanism leads to substitution in the 9 position of anthracene while for the ET mechanism it takes place in all three positions of 1, 2 and 9. The overall distribution of products in the three positions is largely independent of temperature. This is due to the fact that the increased amount of SN2 products formed in the 9 position as the temperature is lowered is accompanied by an equally large decrease in the amount of ET products obtained in the very same position. Exactly the same constancy in the product distribution is seen for the reaction of anthracene radical anion with bromoethane, where both mechanisms also exist. For the reaction involving the sterically hindered 1-iodoadamantane, on the other hand, the SN2 pathway is precluded and in this case there is a profound decrease in the amount of 9-substitution as the temperature is lowered.

Photoinduced Electron Transfer between 1,2,5-Triphenylpyrrolidinofullerene Cluster Aggregates and Electron Donors

Fullerene derivative 1,2,5-triphenylpyrrolidinofullerene (TPPF) forms optically transparent clusters (mean diameter of ∼180 nm) in toluene/acetonitrile mixtures. The bimolecular rate constants for the quenching of a singlet excited state of TPPF clusters by various electron donors (substituted anilines and heteroaromatics) were found to be significantly higher than that of the corresponding monomeric analogue. The local concentration of the fullerene molecules is much higher in these clusters because the microheterogeneous environment facilitates trapping of donor molecules. Formation of long-lived electron-transfer products, following the photoexcitation of the TPPF cluster and various electron donors, was confirmed through flash photolysis studies. The TPPF cluster−donor assemblies when deposited as a thin film on a nanostructured SnO2 semiconductor film, act as a photosensitive electrode material. Light energy can be harvested using an intermolecular electron transfer between the TPPF cluster film and ...