Hole Hopping Mechanism Research Articles

Capturing a bis-Fe(IV) State in Methylosinus trichosporium OB3b MbnH.

The diheme bacterial cytochrome c peroxidase (bCcP)/MauG superfamily is a diverse set of enzymes that remains largely uncharacterized. One recently discovered member, MbnH, converts a tryptophan residue in its substrate protein, MbnP, to kynurenine. Here we show that upon reaction with H2O2, MbnH forms a bis-Fe(IV) intermediate, a state previously detected in just two other enzymes, MauG and BthA. Using absorption, Mössbauer, and electron paramagnetic resonance (EPR) spectroscopies coupled with kinetic analysis, we characterized the bis-Fe(IV) state of MbnH and determined that this intermediate decays back to the diferric state in the absence of MbnP substrate. In the absence of MbnP substrate, MbnH can also detoxify H2O2 to prevent oxidative self damage, unlike MauG, which has long been viewed as the prototype for bis-Fe(IV) forming enzymes. MbnH performs a different reaction from MauG, while the role of BthA remains unclear. All three enzymes can form a bis-Fe(IV) intermediate but within distinct kinetic regimes. The study of MbnH significantly expands our knowledge of enzymes that form this species. Computational and structural analyses indicate that electron transfer between the two heme groups in MbnH and between MbnH and the target tryptophan in MbnP likely occurs via a hole-hopping mechanism involving intervening tryptophan residues. These findings set the stage for discovery of additional functional and mechanistic diversity within the bCcP/MauG superfamily.

Towards direct enzyme wiring: a theoretical investigation of charge carrier transfer mechanisms between glucose oxidase and organic semiconductors.

In this work, a general theoretical and numerical approach based on semiconductor theory, which could be applied to a study of direct enzyme wiring, has been discussed. Marcus-Hush theory was applied to evaluate the potential transfer of charge carriers (holes and electrons) between glucose oxidase (GOx) and organic semiconductors. Two mechanisms of multistep hopping of charge to/from the oxidised/reduced flavin-based moiety through residues of aromatic amino acids located in GOx and long range charge direct tunnelling from the cofactor to the organic semiconductor surface have been proposed and evaluated. It was determined that the hole-hopping mechanism is possible and proceeds at a low ionization potential of the organic semiconductor. The calculations reveal that hopping of electrons is blocked, but direct electron tunnelling between the cofactor and the organic semiconductor is still probable. The most optimal conditions and tunable characteristics of GOx-based biosensors such as the ionization potential, electron affinity of organic semiconductors and distance between the enzyme and surface were estimated for the first time.

Mobility versus Alignment of a Semiconducting π-Extended Discotic Liquid-Crystalline Triindole.

The p-type semiconducting properties of a triphenylene-fused triindole mesogen, have been studied by applying two complementary methods which have different alignment requirements. The attachment of only three flexible alkyl chains to the nitrogen atoms of this π-extended core is sufficient to induce columnar mesomorphism. High hole mobility values (0.65 cm2 V-1 s-1) have been estimated by space-charge limited current (SCLC) measurements in a diode-like structure which are easily prepared from the melt, rendering this material a good candidate for OPVs and OLEDs devices. The mobility predicted theoretically via a hole-hopping mechanism is in very good agreement with the experimental values determined at the SCLC regime. On the other hand the hole mobility determined on solution processed thin film transistors (OFETs) is significantly lower, which can be rationalized by the high tendency of these large molecules to align on surfaces with their extended π-conjugated core parallel to the substrate as demonstrated by SERS. Despite the differences obtained with the two methods, the acceptable performance found on OFETs fabricated by simple drop-casting processing of such an enlarged aromatic core is remarkable and suggests facile hopping between neighboring molecular columns owing to the large conducting/isolating ratio found in this discotic compound.

Oxidation triggers extensive conjugation and unusual stabilization of two di-heme dication diradical intermediates: role of bridging group for electronic communication.

MauG is a diheme enzyme that utilizes two covalently bound c-type hemes to catalyse the biosynthesis of the protein-derived cofactor tryptophan tryptophylquinone. The two hemes are physically separated by 14.5 Å and a hole-hopping mechanism is proposed in which a tryptophan residue located between the hemes undergoes reversible oxidation and reduction to increase the effective electronic coupling element and enhance the rate of reversible electron transfer between the hemes in bis-Fe(iv) MauG. The present work describes the structure and spectroscopic investigation of 2e-oxidations of the synthetic diheme analogs in which two heme centers are covalently connected through a conjugated ethylene bridge that leads to the stabilization of two unusual trans conformations (U and P' forms) with different and distinct spectroscopic and geometric features. Unlike in MauG, where the two oxidizing equivalents are distributed within the diheme system giving rise to the bis-Fe(iv) redox state, the synthetic analog stabilizes two ferric hemes, each coupled with a porphyrin cation radical, a scenario resembling the binuclear dication diradical complex. Interestingly, charge resonance-transition phenomena are observed here both in 1e and 2e-oxidised species from the same system, which are also clearly distinguishable by their relative position and intensity. Detailed UV-vis-NIR, X-ray, Mössbauer, EPR and 1H NMR spectroscopic investigations as well as variable temperature magnetic studies have unraveled strong electronic communications between two porphyrin π-cation radicals through the bridging ethylene group. The extensive π-conjugation also allows antiferromagnetic coupling between iron(iii) centers and porphyrin radical spins of both rings. DFT calculations revealed extended π-conjugation and H-bonding interaction as the major factors in controlling the stability of the conformers.

Dielectric Relaxation of Rare Earth Ordered Double Perovskite Oxide Ba2ErTaO6

The electrical properties of rare-earth based ordered double perovskite oxide barium erbium tantalate, Ba2ErTaO6 synthesized by solid-state reaction method are investigated. The x-ray diffraction pattern of the sample shows cubic Fm3m phase at room temperature with ordering of the B cations. Fourier transform infrared spectrum shows two primary phonon modes of the sample at around 350 cm−1 and 600 cm−1. The dielectric relaxation of the sample is investigated in the frequency range from 50 Hz to 1.1 MHz and in the temperature range from 303 K to 673 K. Electric modulus and electrical impedance data are fitted to the Cole–Cole equation. The frequency dependent conductivity spectra follow the power law. Summerfield scaling is used to explain the conduction mechanism. The scaling behavior of the imaginary part of the impedance spectra suggests that the relaxation shows the same mechanism at various temperatures. The complex impedance plane plots show that the relaxation (conduction) mechanism in this material is mainly due to grain boundary effect for all temperatures and grain effect for low temperature. The relaxation frequency corresponding to dielectric loss is found to obey Arrhenius law with activation energy of 0.50 eV. The values of activation energy indicate that the dielectric relaxation and the conduction mechanism are due to adiabatic small polaronic hole hopping mechanism.

Electrical and dielectric properties of nanocrystalline LaCrO3

The X-ray diffraction pattern of nanocrystalline LaCrO3 synthesized by combustion method reveals the perovskite phase of the material. The synthesized material has an average crystallite size of 24 nm. The characterization techniques such as X-ray diffraction studies, scanning electron microscopy, transmission electron microscopy etc., were employed to study the average particle size, phase and composition of the material. The effects of constituent phase variation on the dielectric, resistivity and impedance properties are examined. The dielectric dispersion with frequency has been explained on the basis of an electron–hole hopping mechanism, this is responsible for conduction and polarization. From the ac conductivity study, it is confirmed that the conduction in the present material is of small polaron type. Such materials may be useful to fabricate the interconnecting materials for the solar cell, gas sensors, gas transducers, optoelectronic devices, catalyst etc.

Change of Mott variable range to small polaronic hole hopping conduction mechanism and formation of Schottky barriers in Nd0.9Sr0.1FeO3

In this work, single phase Nd0.9Sr0.1FeO3 electroceramic material has been synthesized by solid state reaction method. Phase purity and microstructure were analyzed by X-ray diffraction and SEM. By employing impedance spectroscopy, ac electrical properties have been measured in wide temperature (77–358 K) and frequency (1–1.5 × 107 Hz) range. To correlate the electrical properties an equivalent circuit (RgQg)(RgbQgb)(RcQc) has been used at lower temperatures which gradually changed to (Rgb)(RcQc) at higher temperatures. Around 218 K, the conduction mechanism changed from variable range hole hopping to small polaronic hole hopping mechanism which was also confirmed by ac conductivity and magnetization measurements. The localization length calculated for grains and grain boundaries are 1.17 Å and 1.06 Å, respectively. Hall measurements pointed out that majority of the charge carriers are p-type. The dc bias measurements confirmed the presence of non-ohmic electrode contacts and obeyed the Mott-Schottky law. The observed permittivity of grains is from 8 to 10, whereas the high permittivity at elevated temperatures may be due to the formation of different Schottky barriers, as a result of ceramic interfaces, ceramic surfaces, and electrode effects.

Tryptophan-mediated charge-resonance stabilization in the bis-Fe(IV) redox state of MauG.

The diheme enzyme MauG catalyzes posttranslational modifications of a methylamine dehydrogenase precursor protein to generate a tryptophan tryptophylquinone cofactor. The MauG-catalyzed reaction proceeds via a bis-Fe(IV) intermediate in which one heme is present as Fe(IV)=O and the other as Fe(IV) with axial histidine and tyrosine ligation. Herein, a unique near-infrared absorption feature exhibited specifically in bis-Fe(IV) MauG is described, and evidence is presented that it results from a charge-resonance-transition phenomenon. As the two hemes are physically separated by 14.5 Å, a hole-hopping mechanism is proposed in which a tryptophan residue located between the hemes is reversibly oxidized and reduced to increase the effective electronic coupling element and enhance the rate of reversible electron transfer between the hemes in bis-Fe(IV) MauG. Analysis of the MauG structure reveals that electron transfer via this mechanism is rapid enough to enable a charge-resonance stabilization of the bis-Fe(IV) state without direct contact between the hemes. The finding of the charge-resonance-transition phenomenon explains why the bis-Fe(IV) intermediate is stabilized in MauG and does not permanently oxidize its own aromatic residues.

Small polaronic hole hopping mechanism and Maxwell-Wagner relaxation in NdFeO3

In the modern micro-electronics, transition metal oxides due to their colossal values of dielectric permittivity possess huge potential for the development of capacitive energy storage devices. In the present work, the dielectric permittivity and the effects of temperature and frequency on the electrical transport properties of polycrystalline NdFeO3, prepared by solid state reaction method, are discussed. Room temperature Mossbauer spectrum confirms the phase purity, octahedral environment for Fe ion, and high spin state of Fe3+ ion. From the impedance spectroscopic measurements, three relaxation processes are observed, which are related to grains, grain boundaries (gbs), and electrode-semiconductor contact in the measured temperature and frequency ranges. Decrease in resistances and relaxation times of the grains and grain boundaries with temperature confirms the involvement of thermally activated conduction mechanisms. Same type of charge carriers (i.e., small polaron hole hopping) have been found responsible for conduction and relaxation processes through the grain and grain boundaries. The huge value of the dielectric constant (∼8 × 103) at high temperature and low frequency is correlated to the Maxwell-Wagner relaxation due to electrode-sample contact.

Characterization of electron tunneling and hole hopping reactions between different forms of MauG and methylamine dehydrogenase within a natural protein complex.

Respiration, photosynthesis, and metabolism require the transfer of electrons through and between proteins over relatively long distances. It is critical that this electron transfer (ET) occur with specificity to avoid cellular damage, and at a rate that is sufficient to support the biological activity. A multistep hole hopping mechanism could, in principle, enhance the efficiency of long-range ET through proteins as it does in organic semiconductors. To explore this possibility, two different ET reactions that occur over the same distance within the protein complex of the diheme enzyme MauG and different forms of methylamine dehydrogenase (MADH) were subjected to kinetic and thermodynamic analysis. An ET mechanism of single-step direct electron tunneling from diferrous MauG to the quinone form of MADH is consistent with the data. In contrast, the biosynthetic ET from preMADH, which contains incompletely synthesized tryptophan tryptophylquinone, to the bis-Fe(IV) form of MauG is best described by a two-step hole hopping mechanism. Experimentally determined ET distances matched the distances determined from the crystal structure that would be expected for single-step tunneling and multistep hopping. Experimentally determined relative values of electronic coupling (H(AB)) for the two reactions correlated well with the relative H(AB) values predicted from computational analysis of the structure. The rate of the hopping-mediated ET reaction is also 10-fold greater than that of the single-step tunneling reaction despite a smaller overall driving force for the hopping-mediated ET reaction. These data provide insight into how the intervening protein matrix and redox potentials of the electron donor and acceptor determine whether the ET reaction proceeds via single-step tunneling or multistep hopping.

Microwave assisted synthesis and characterization of Co doped Cu ferrite nanoparticles

Co doped Cu nanocrystalline ferrites with composition Cu1−xCoxFe2O4 (x=0.0⩽x⩽0.5) were successfully synthesized through microwave assisted sol–gel method. Structural and electrical properties were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), dielectric and impedance spectroscopy measurements. XRD analysis revealed the single phase cubic spinel structure of all samples with Fd3m space group. No secondary phase was detected confirming the substitution of Cu ions by Co ions in the Cu ferrite matrix. The average crystallite size calculated from XRD data was found to decrease from 28 to 10nm with the increase in Co doping. Room temperature dielectric and impedance spectroscopy measurements showed that the dielectric constant, loss tangent and ac conductivity increases up to 3% Co doping and thereafter, these parameters decrease. The analysis of complex impedance data showed two semicircles, one corresponding to the grain and other corresponding to grain boundary volume which suggests that the conduction mechanism takes place predominantly through grain boundary volume as well as grains in the studied samples. The observed behavior of the samples has been explained on the basis of an electron–hole hopping mechanism in the light of Maxwell–Wagner two layer models and Koop’s phenomenological theory, which is responsible for conduction and polarization.

Theoretical Studies on Dicyanoanthracenes as Organic Semiconductor Materials: Reorganization Energy

‡Internal reorganization energy due to the structural relaxation in hole or electron hopping mechanism is one of the measurements of key indices in designing an organic thin film transistor (OTFT) for flexible display devices. In this study, the reorganization energies of dicyanoanthracenes for the hole and electron transfer were estimated by adiabatic potential energy surface and normal mode analysis method in order to examine the effect on the energies for the positional variation of the cyano substituents in the anthracene as a protocol of acenes to design an organic field effect transistor. The reorganization energy for the hole transfer was reduced considerably upon cyanation of anthracene, especially at the 9,10positions of anthracene, and the origin of the reduction was interpreted in terms of understanding the coupling of vibrational modes to the hole transfer.

Carrier Generation Process on Photoconductive Polymer Films as Studied by Magnetic Field Effects on the Charge-Transfer Fluorescence and Photocurrent

We have studied the magnetic field effects (MFEs) on the charge-transfer fluorescence and transient photocurrent of a 1,2,4,5-tetracyanobenzene-doped poly(N-vinylcarbazole) film, which reflect the recombination and escape yields of the carriers, respectively. The recombination yield dependence of the external magnetic field (B) clearly shows two types of the MFEs, growth with increasing B due to the hyperfine mechanism (HFM) and a negative dip due to the level-crossing mechanism (LCM). On the other hand, the escape yield indicates complementary MFEs with a sharp decrease in yield with increasing B and then a positive dip. Simultaneous observation of the HFM- and LCM-MFEs proves the stepwise hole-hopping mechanism rather the long-range hole-jumping one. The quantitative analysis of the recombination and escape MFEs is performed using the stochastic Liouville equations (SLE) for a one-dimensional lattice model in which the stepwise hole hops take place between the nearest neighbor carbazole units with spin conservation. The SLE analysis provides the recombination and hole transfer rate constants of 7.0 x 10(7) and 4.5 x 10(8) s(-1), respectively. The boundary site number for the ion pairs in the one-dimensional model is estimated by the best fit to the experimental results. The interionic distance of the boundary ion pair in the one-dimensional model including eight sites agrees with the thermalization distance in the Onsager model. Hence, it is concluded that the elementary processes in the Onsager model applied to molecular amorphous solids are the stepwise hole hops rather than a long-range hole jump.

Charge Transport Mechanism in Native Deoxyribonucleic Acid

The frequency dependent electrical conductivity below 1 MHz was studied on wet-spun, macroscopically oriented, calf thymus DNA bulk samples. Temperature dependence of the quasistatic electrical conductivity measured perpendicular to the long DNA axis is discussed within the framework of activated variable range hopping mechanism. It is shown that the frequency dependence of the electrical conductivity in the frequency range of approximately 10-3 Hz–1015 Hz agrees well with predictions of the hole hopping mechanism. Temperature dependence of the electrical conductivity at various frequencies exhibits a crossover at lower temperatures to almost temperature independent mechanism.

Conductivity anomaly in CuInGaO 4 and CuIn 2Ga 2O 7 ceramics

The compounds CuInGaO 4 and CuIn 2Ga 2O 7 crystallize in the ( LnFeO 3) n FeO layer structure. In this paper, the electrical properties of CuInGaO 4 and CuIn 2Ga 2O 7 ceramics prepared by solid-state synthesis were examined. Both compounds exhibited a dramatic change in physical properties across a second-order phase transition at 226 K identified by calorimetry experiments. Above this temperature, the conductivity increased over four orders of magnitude and the relative permittivity increased and became strongly dispersive. These observations are explained on the basis of a hole-hopping mechanism in the Cu layers within the ( LnFeO 3) n FeO crystal structure.

Two redox-active beta-carotene molecules in photosystem II.

Photosystem II (PS II) contains secondary electron-transfer paths involving cytochrome b(559) (Cyt b(559)), chlorophyll (Chl), and beta-carotene (Car) that are active under conditions when oxygen evolution is blocked such as in inhibited samples or at low temperature. Intermediates of the secondary electron-transfer pathways of PS II core complexes from Synechocystis PCC 6803 and Synechococcus sp. and spinach PS II membranes have been investigated using low temperature near-IR spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. We present evidence that two spectroscopically distinct redox-active carotenoids are formed upon low-temperature illumination. The Car(+) near-IR absorption peak varies in wavelength and width as a function of illumination temperature. Also, the rate of decay during dark incubation of the Car(+) peak varies as a function of wavelength. Factor analysis indicates that there are two spectral forms of Car(+) (Car(A)(+) has an absorbance maximum of 982 nm, and Car(B)(+) has an absorbance maximum of 1027 nm) that decay at different rates. In Synechocystis PS II, we observe a shift of the Car(+) peak to shorter wavelength when oxidized tyrosine D (Y(D)*) is present in the sample that is explained by an electrostatic interaction between Y(D)* and a nearby beta-carotene that disfavors oxidation of Car(B). The sequence of electron-transfer reactions in the secondary electron-transfer pathways of PS II is discussed in terms of a hole-hopping mechanism to attain the equilibrated state of the charge separation at low temperatures.

Adiabatic and non-adiabatic small polaron hopping in molecularly doped polymers

New hole drift mobility measurements for the molecularly doped polymer DEH ( p-diethylaminobenzaldehyde-diphenyl hydrazone) in polycarbonate and literature data for TPD (N,N′-diphenyl-N,N′-bis (3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine) in polycarbonate are analyzed by a new procedure which separates the functional dependence of the mobility on temperature and molecular spacing ϱ. Our analysis indicates that the dependence of the mobility on ϱ arises from different sources. In DEH: polycarbonate it is due to an overlap integral, as normally expected. In TPD: polycarbonate it is due entirely to the dependence of the activation energy on p, a significant clue to the underlying microscopic hole hopping mechanism. These data are consistent with the small polaron hopping theory in two regimes, non-adiabatic hopping in DEH:polycarbonate and adiabatic hopping in TPD: polycarbonate.