Intramolecular Electron Transfer Mechanism Research Articles

Electrochemical Studies of Intramolecular Electron Transfer in Laccase from Trametes versicolor

AbstractThis paper describes electrochemical behavior of laccase from the fungus Trametes versicolor. The issues related to discrimination of the redox potentials corresponding to copper centers T1 and T2/T3 in the active site and possible mechanism of intramolecular electron transfer have been discussed. The electron‐transfer rate constant for laccase immobilized on carbon electrode is 3.4 s−1. The bioelectrocatalytic activity of the enzyme was studied in the presence of 1,4‐hydroquinone (HQ). The kinetics of HQ oxidation is very fast (KM=3.8 μM). However, the catalytic activity of laccase in the presence of high concentration of HQ decreases drastically. It is suggested that the T2/T3 copper center is able to accept electrons from HQ molecules directly via intramolecular channel.

Intramolecular Electron Transfer in the Dihaem Cytochrome c Peroxidase of Pseudomonas aeruginosa

Mutant forms of the enzyme cytochrome c peroxidase from Pseudomonas aeruginosa, in which the peroxidatic haem ligand (H71) and putative haem-bridging amino acid (W94) have been mutated, were produced in an E. coli expression system as a means of investigating possible mechanisms of intramolecular electron transfer within the enzyme. EPR spectroscopy indicated the presence of a high-spin, presumably five-coordinate, peroxidatic haem site in the H71G and H71G/W94A mutants, whilst the W94A mutant apparently retained the normal six-coordinate haem structures. In turnover experiments, these mutants show 55, 4, and <1% activity, respectively, as compared to the wild-type enzyme. The W94A mutant shows essentially no activity in turnover experiments. Circular dichroism spectroscopy indicates no measurable difference in the secondary structure of the H71G mutant from that of the native enzyme, whilst some small differences are observed for the double mutant. Treatment of the oxidised mutant proteins with hydrogen peroxide, in the absence of preactivation or exogenous reductants, yields products that suggest the formation of a tryptophan radical species in the case of the H71 mutant and the production of a porphyrin radical in the case of the double mutant. These results are discussed in terms of the intramolecular electron transfer in this enzyme.

Intramolecular Electron Transfer between Tyrosyl Radical and Cysteine Residue Inhibits Tyrosine Nitration and Induces Thiyl Radical Formation in Model Peptides Treated with Myeloperoxidase, H2O2, and NO2-: EPR SPIN TRAPPING STUDIES

We investigated the effects of a cysteine residue on tyrosine nitration in several model peptides treated with myeloperoxidase (MPO), H(2)O(2), and nitrite anion (NO(2)(-)) and with horseradish peroxidase and H(2)O(2). Sequences of model peptides were acetyl-Tyr-Cys-amide (YC), acetyl-Tyr-Ala-Cys-amide (YAC), acetyl-Tyr-Ala-Ala-Cys-amide (YAAC), and acetyl-Tyr-Ala-Ala-Ala-Ala-Cys-amide (YAAAAC). Results indicate that nitration and oxidation products of tyrosyl residue in YC and other model peptides were barely detectable. A major product detected was the corresponding disulfide (e.g. YCysCysY). Spin trapping experiments with 5,5'-dimethyl-1-pyrroline N-oxide (DMPO) revealed thiyl adduct (e.g. DMPO-SCys-Tyr) formation from peptides (e.g. YC) treated with MPO/H(2)O(2) and MPO/H(2)O(2)/NO(2)(-). The steady-state concentrations of DMPO-thiyl adducts decreased with increasing chain length of model peptides. Blocking the sulfydryl group in YC with methylmethanethiosulfonate (that formed YCSSCH(3)) totally inhibited thiyl radical formation as did substitution of Tyr with Phe (i.e. FC) in the presence of MPO/H(2)O(2)/NO(2)(-). However, increased tyrosine nitration, tyrosine dimerization, and tyrosyl radical formation were detected in the MPO/H(2)O(2)/NO(2)(-)/YCSSCH(3) system. Increased formation of S-nitrosated YC (YCysNO) was detected in the MPO/H(2)O(2)/(*)NO system. We conclude that a rapid intramolecular electron transfer reaction between the tyrosyl radical and the Cys residue impedes tyrosine nitration and induces corresponding thiyl radical and nitrosocysteine product. Implications of this novel intramolecular electron transfer mechanism in protein nitration and nitrosation are discussed.

Molecular Basis of Intramolecular Electron Transfer in Sulfite-oxidizing Enzymes Is Revealed by High Resolution Structure of a Heterodimeric Complex of the Catalytic Molybdopterin Subunit and a c-Type Cytochrome Subunit

Sulfite-oxidizing molybdoenzymes convert the highly reactive and therefore toxic sulfite to sulfate and have been identified in insects, animals, plants, and bacteria. Although the well studied enzymes from higher animals serve to detoxify sulfite that arises from the catabolism of sulfur-containing amino acids, the bacterial enzymes have a central role in converting sulfite formed during dissimilatory oxidation of reduced sulfur compounds. Here we describe the structure of the Starkeya novella sulfite dehydrogenase, a heterodimeric complex of the catalytic molybdopterin subunit and a c-type cytochrome subunit, that reveals the molecular mechanism of intramolecular electron transfer in sulfite-oxidizing enzymes. The close approach of the two redox centers in the protein complex (Mo-Fe distance 16.6 A) allows for rapid electron transfer via tunnelling or aided by the protein environment. The high resolution structure of the complex has allowed the identification of potential through-bond pathways for electron transfer including a direct link via Arg-55A and/or an aromatic-mediated pathway. A potential site of electron transfer to an external acceptor cytochrome c was also identified on the SorB subunit on the opposite side to the interaction with the catalytic SorA subunit.

Porphyrin-anthraquinone dyads: Synthesis, spectroscopy and photochemistry

Free-base (H2L2), copper(II) (CuL2) and zinc(II) (ZnL2) derivatives of a porphyrin-anthraquinone conjugate with an azomethine group separating the two photoactive subunits have been synthesized and characterized by mass (FAB), IR, UV-visible, 1H NMR and ESR spectroscopic techniques and also by cyclic and differential pulse voltammetric methods. Analysis of the data reveals that the spectral and electrochemical properties of the individual chromophoric entities are retained and that there is no specific π-π interaction between the porphyrin and anthraquinone subunits. H2L2 and ZnL2 are shown to exhibit substantial quenching (88–97%) of the porphyrin fluorescence compared to their corresponding monomeric analogues. An intramolecular electron-transfer mechanism is proposed for the substantial decrease in fluorescence in both derivatives. The fluorescence decays of porphyrin-anthraquinone conjugates are fit to 2/3 exponentials and indicate that multiple orientations of the porphyrin and anthraquinone groups contribute to the electron-transfer event. These results are in good agreement with steady-state fluorescence results. From the time-resolved fluorescence data, the electron-transfer rate constants are calculated, indicating kET values in the range of 1·1 × 109 to 9·9 × 1010 s−1 that are dependent upon the solvent.

Photomagnetism in Cyano-Bridged Hexanuclear Clusters [MnII(bpy)2]4[MIV(CN)8]2·xH2O (M = Mo, x = 14, and M = W, x = 9)

The magnetic and photomagnetic properties of two cyano-bridged, [Mn(bpy)2]4[Mo(CN)8]2·14H2O (1) and [Mn(bpy)2]4[W(CN)8]2·9H2O (2) (bpy = 2,2‘-bipyridyl), hexanuclear clusters (hereafter named MnII4MIV2) have been investigated. Before irradiation both compounds behave as paramagnets in almost the whole temperature range due to the presence of magnetic MnII centers separated by the diamagnetic MIV spacers in the molecules. After several hours of irradiation (337−356, 406−415, and 480 nm) in a superconducting quantum interference device at 10 K, the magnetic signal at 10 K has increased by a factor of almost 30% compared to that of the initial magnetic state. The magnetic characterization of the irradiated samples indicates the formation of a photoproduct exhibiting antiferromagnetic interactions. The X-ray photoelectron spectroscopy (XPS) data of the irradiated samples show the presence of MnII, MnI, MIV, and MV centers. The formation of MnI and MV centers is interpreted in terms of the reductive quenching of the excited state through an intramolecular electron-transfer mechanism in the MnII4MIV2 clusters. After irradiation at room temperature in the presence of air, both compounds behave as a paramagnet, but with a lower magnetic response than that of the initial state. The XPS data show the presence of MIV, MVI, and only MnII centers. The photochemical pathway in the presence of O2 is discussed in relation to the ligand-field photochemistry of octacyanometalates(IV), leading ultimately to the formation of [MVI(CN)4(O2)] and low-spin [MnII(bpy)2(CN)2] species in the solid state.

Participation of chromophore pairs in photoinduced intramolecular electron transfer for a naphthalimide spermine conjugate

Photophysical and electron transfer properties of the conjugate of 1,8-naphthalimide and spermine (BNS) and related chromophore system including, N-propyl-1,8-naphthalimide (NAP), have been investigated. Absorption and emission spectra for the BNS-spermine derivative revealed a ground state intramolecular complexation of the two naphthalimide chromophores, particularly for aqueous media. Fluorescence quenching studies of BNS using an electron donor, 1,4-diazabicyclo[2,2,2]octane (DABCO), yielded quenching rate constants near that anticipated for a diffusion-controlled process. Transients observed on laser flash photolysis ( λ exc=355 nm) corresponding to the naphthalimide radical anion ( λ max=410 nm) and the triplet state of BNS ( λ max=470 nm) have been identified. Observation of a lower fluorescence quantum yield (in comparison to NAP) and the appearance of the radical anion intermediate on laser flash photolysis in the absence of any external electron donor (e.g., for BNS in its free base form) suggests a mechanism of intramolecular electron transfer involving either (a) interaction of two naphthalimide chromophores or (b) interaction of naphthalimide groups and a dialkyl amine moiety.

Redox Chemistry and Functionalities of Conjugated Ferrocene Systems

AbstractFor Abstract see ChemInform Abstract in Full Text.

A single-molecule probe based on intramolecular electron transfer.

Detecting structure, dynamics, and chemical reactions at the single-molecule level represents the ultimate degree of sensitivity for sensing and imaging. There is a tremendous need to develop new molecular systems and methodology for single-molecule-based sensing. This work presents for the first time the single-molecule spectroscopy of a new molecular probe which uses an intramolecular electron transfer mechanism to detect binding, local structure, and interfacial processes. Moreover, we show how information about the interaction of these probes with their environment is obtained from an analysis of the intensity, duration and time-varying behavior of the single-molecule fluorescence.

Quimiluminescência de peróxidos orgânicos: geração de estados eletronicamente excitados na decomposição de 1,2-dioxetanos

In this review article, we give a general introduction on the mechanisms involved in organic chemiluminescence, where three basic models for excited state formation are presented. The chemiluminescence properties of 1,2-dioxetanes - four membered ring peroxides - are briefly outlined in the second part. In the main part, the mechanisms involved in the decomposition of 1,2-dioxetanes and analogous peroxides are discussed: (i) the unimolecular decomposition of 1,2-dioxetanes; (ii) the electron transfer catalyzed decomposition of peroxides by an intermolecular CIEEL (Chemically Initiated Electron Exchange Luminescence) mechanism; (iii) 1,2-dioxetane decomposition catalyzed by an intramolecular electron transfer mechanism (intramolecular CIEEL). Special emphasis is given to the latter subject, where recent examples with potential analytical applications are presented.

Photoinduced Electron-Transfer within Free Base and Zinc Porphyrin Containing Poly(Amide) Dendrimers

The synthesis and photophysical characterization of a series of free base and zinc porphyrin containing, Newkome-type dendrimers terminated with anthraquinone groups (FbP-Ga-AQn and ZnP-Ga-AQn) and ethyl groups (FbP-Ga-Etn and ZnP-Ga-Etn) are described. These dendrimers were designed for use as mimics of the photosynthetic reaction center. Red-shifts in the absorption spectra, particularly in the anthraquinone-terminated series, were interpreted as resulting from backfolding of the dendrimer branches. Dendrimers FbP-Ga-AQn were shown to exhibit substantial quenching (58−75%) of the porphyrin fluorescence as measured against the analogous ethyl-terminated dendrimers (FbP-Ga-Etn) in steady-state fluorescence experiments. The zinc porphyrin containing dendrimers ZnP-Ga-AQn exhibited nearly complete quenching (96−99.5%) of the porphyrin fluorescence. An intramolecular electron-transfer mechanism is proposed for the substantial decrease in fluorescence in both series of dendrimers. Time-resolved fluorescence experiments for FbP-Ga-AQn were fit to 2−3 exponentials and indicated that multiple orientations of the porphyrin and anthraquinone groups contribute to the electron-transfer event. These results were in good agreement with the steady-state fluorescence results. From the time-resolved fluorescence data, the electron-transfer rate constants were calculated, indicating kET values in the range of 3.77 × 107 s-1 to 2.28 × 108 s-1 that were dependent upon both dendrimer generation number and solvent. Similar experiments on ZnP-Ga-AQn also indicated that multiple zinc porphyrin anthraquinone conformations were likely responsible for the electron-transfer. Dramatic differences between the steady-state and time-resolved fluorescence data in the zinc porphyrin dendrimers were interpreted in terms of ligation of the terminal anthraquinone groups with the zinc porphyrin that results in either a nonemissive state or an ultrafast electron-transfer.

The Thermodynamics, Kinetics, and Molecular Mechanism of Intramolecular Electron Transfer in Human Ceruloplasmin

Ceruloplasmin is a multicopper oxidase that contains three type 1 Cu sites and a type 2/type 3 trinuclear Cu cluster. All other multicopper oxidases contain only one type 1 Cu and a trinuclear cluster. In oxidized human ceruloplasmin, one type 1 Cu site is reduced and cannot be oxidized, at least in part due at to a high reduction potential, which is not catalytically relevant. Here we have examined the thermodynamics and kinetics of electron transfer among the five redox-active Cu sites to obtain insight into the molecular mechanism of intramolecular electron transfer and the function of the additional, redox-active T1 Cu site. The redox potentials of the Cu sites of human ceruloplasmin were determined by reductive and poised potential titrations. In pH 7.0 phosphate buffer, the potentials of the type 1, type 2, and type 3 Cu sites are 448, 491, and 415 mV, respectively. Cl- binds to the trinuclear cluster and significantly increases the potentials of the type 2 and type 3 Cu, indicating that Cl- is a ph...

Picosecond Mechanism of Metal-Ion-Sensitive Fluorescence of Phenylimidazoanthraquinone with Azacrown

Dynamic mechanism of metal-ion-sensitive fluorescence of phenylimidazoanthraquinone with azacrown (PIAQ−AC) was investigated by time-resolved fluorescence and absorption spectra upon excitation with a femtosecond laser. PIAQ−AC showed marked fluorescence quenching, while the fluorescence intensity was greatly enhanced upon binding of either calcium or barium ions. The transient absorption of PIAQ−AC ascribed to the excited singlet decayed with a lifetime of 5.8 ps in the absence of calcium ions. The lifetime of the excited singlet increased to 200 ps upon binding of calcium ions to the azacrown. The transient absorption spectrum of the phenylimidazoanthraquinone anion radical was observed in the absence of metal ions. These results clearly indicate that the fluorescence was switched on and off by an intramolecular electron-transfer mechanism controlled with metal ions.

Photochemical Synthesis of s-Triazolo[3,4-b]benzothiazole and Mechanistic Studies on Benzothiazole Formation

A general photochemical synthesis of s-triazolo[3,4-b]benzothiazoles from 4,5-disubstituted 1,2,4-triazole-3-thione is described. Steady photolysis, quantum yield determination, and laser flash photolysis experiments have been carried out. An intramolecular electron transfer mechanism has been proposed.

Tris(2,2′-bipyridyl) ruthernium(II)-mediated photoinduced electron transfer of engineered cytochrome P450 1A2

In this study, it is shown that an electron from photoreduced tris(2,2′-bipyridyl)Ru 2+ ion reaches the haem iron of engineered wild-type cytochrome P450 1AA2 (P450 1A2) with an electron transfer rate of 6.04×10 −3 min −1. The electron transfer rate, 4.05×10 −1 min −1, of a His 163Glu mutant, which has a redox potential 40 mV lower than that of the wild type, is more than sixfold faster than that of the wild type. The photoinduced electron transfer rates of the present system are strongly influenced by detergents, cholic acid and Emulgen 913. We discuss the intermolecular and intramolecular electron transfer mechanism of the P450 1A2 system based on the kinetic data.

Marked detergents effects on safranine T-mediated photo-induced electron transfer in cytochrome P-450 1A2

Cytochrome P-450 accepts electrons from electron transfer proteins to facilitate monooxidation reactions. It is suggested that basic amino acids such as Lys and Arg on the P-450 molecular surface interact with acidic amino acids such as Gin and Asp of the electron transfer protein. Safranine T is a basic compound which mediates electron transfer with illumination. It was found with flash photolysis that an electron from photo-reduced safranine T quickly reaches the heme iron of cytochrome P-450 1A2 (P-450 1A2). The photo-induced reduction kinetics of P-450 IA2 were analyzed by the Runge-Kutta method on the second order assumption. The electron-transfer rate constant from safranine T to P-450 1A2 was 2.1 ∗d 10 6 M −1s −1. The rate constant was remarkably increased up to 3.1 ∗d 10 8 M −1 s −1 by adding cholic acid, while that was drastically reduced down to 3.5 ∗d 104 M −1 s −1 by adding Emulg 913. The electron-transfer rate of a His 163-Glu mutant, which has a 40 mV lower redox potential than that of the wild type, was the same as that of the wild type in the absence of the detergents, although the reduced fraction of the mutant was 30% lower than that of the wild type. The electron-transfer rate of the mutant also changed significantly by adding the detergents in the same way as the wild type. Based on these results, together with optical absorbance and fluorescence data, we discuss the inter- and intramolecular electron-transfer mechanism of P-450 1A2.

Solvent Gating of Intramolecular Electron Transfer

The rates for ionic photodissociation of malachite green leucocyanide to form cyanide ion and a malachite green carbonium ion were measured as a function of solvent and temperature. The observed rates in mixtures of polar and nonpolar solvents all had an activation energy of about 1 kcal/mol for a wide range of dielectric constants. This dissociative intramolecular electron transfer (DIET) is unusual because it is the first example where solvent configurational entropy changes are required to enable a large amplitude molecular distortion leading to a nonadiabatic electron transfer and ionic dissociation. This solvent gated intramolecular electron-transfer mechanism is supported by analysis of the preexponential and activation energy trends in dipolar aprotic solven mixtures and alcohol solvents. The large amplitude motion is not separately measurable due to the slow gating rates, but viscosity effects on both the preexponential and the activation energy are analyzed to demonstrate consistency with a barrierless diffusion model having a structural dependence on electron-transfer rate. The rate has an inverse dependence on viscosity raised to the 0.53 power. 36 refs., 6 figs., 4 tabs.

Photoinduced intramolecular electron transfer mechanism for photochemical dissociation of para-substituted benzyl 9,10-dimethoxyanthracene-2-sulfonates

The photochemical behavior of para-substituted benzyl 9,10-dimethoxyanthracene-2-sulfonates with varying electron-withdrawing ability of substituents has been investigated by both steady-state and laser flash photolysis techniques.

Anodic amide oxidations in the presence of electron-rich phenyl rings: evidence for an intramolecular electron-transfer mechanism

The anodic oxidations of amides in the presence of mono-, di-, and trialkoxyphenyl rings were examined. For example, the anodic oxidations of ((3-methoxyphenyl)acetyl)pyrrolidine, ((2-methoxyphenyl)acetyl)pyrrolidine, ((3-methoxy-4-(pivaloyloxy)phenyl)acetyl)pyrrolidine, and ((3,5-dimethoxy-4-(pivaloyloxy)phenyl)acetyl)pyrrolidine all led to selective methoxylation of the pyrrolidine ring

Synthesis and reactivity of some diaryliodonium salts for photoresist application.

Diaryliodonium and triarylsulfonium salts with non-nucleophilic metal halide anions are well known photoinitiators for cationic polymerization as well as photosensitive super acid generators, and play an important role in the chemical amplificated resists. Generally, photosensitizer is required to enhance the spectral response of resists or polymerization systems into the longer wavelength region, due to the low absorptivity of the simple diphenyliodonium salts at above 300nm. In order to circumvent the need for a second component, auxochromic or other groups were incorporated into the iodonium salt molecule which can increase or shift the absorptivity to longer wavelengths. Therefore, a number of new unsymmetrical iodonium salts of general structure(I) were prepared by us by modifying a method already reported. The photoactivity of these iodonium salts were investigated by gel time study. The results indicated that the iodonium salts incorporated with auxochromic groups exhibited higher photoactivity than that of diphenyliodonium salt. The 2-phenylsulfur methhyl 2′, 4′dimethyl diphenyliodonium salt showed the highest activity and an intramolecular electron transfer mechanism was proposed to explain the unexpectedly high activity.