Temperature Dependence Of Electron Transfer Research Articles

Thermal Analysis of Photoelectron Emission (PE) and X-ray Photoelectron Spectroscopy (XPS) Data for Iron Surfaces Scratched in Air, Water, and Liquid Organics

Little is known about the temperature dependence of electron transfer occurring at real metal surfaces. For iron surfaces scratched in seven environments, we report Arrhenius activation energies obtained from the data of photoelectron emission (PE) and X-ray photoelectron spectroscopy (XPS). The environments were air, benzene, cyclohexane, water, methanol, ethanol, and acetone. PE was measured using a modified Geiger counter during repeated temperature scans in the 25–339 °C range under 210-nm-wavelength light irradiation and during light wavelength scans in the range 300 to 200 nm at 25, 200, and 339 °C. The standard XPS measurement of Fe 2p, Fe 3p, O 1s, and C 1s spectra was conducted after wavelength scan. The total number of electrons counted in the XPS measurement of the core spectra, which was called XPS intensity, strongly depended on the environments. The PE quantum yields during the temperature scan increased with temperature, and its activation energies (ΔEaUp1) strongly depended on the environment, being in the range of 0.212 to 0.035 eV. The electron photoemission probability (αA) obtained from the PE during the wavelength scan increased with temperature, and its activation energies (ΔEαA) were almost independent of the environments, being in the range of 0.113–0.074 eV. The environment dependence of the PE behavior obtained from temperature and wavelength scans was closely related to that of the XPS characteristics, in particular, the XPS intensities of O 1s and the O2− component of the O 1s spectrum, the acid–base interaction between the environment molecule and Fe–OH, and the growth of non-stoichiometric FexO. Furthermore, the origin of the αA was attributed to the escape depth of hot electrons across the overlayer.

Size and Temperature Dependence of Electron Transfer between CdSe Quantum Dots and a TiO2 Nanobelt

Understanding charge transfer reactions between quantum dots (QD) and metal oxides is fundamental for improving photocatalytic, photovoltaic, and electronic devices. The complexity of these processes makes it difficult to find an optimum QD size with rapid charge injection and low recombination. We combine time-domain density functional theory with nonadiabatic molecular dynamics to investigate the size and temperature dependence of the experimentally studied electron transfer and charge recombination at CdSe QD–TiO2 nanobelt (NB) interfaces. The electron injection rate shows strong dependence on the QD size, increasing for small QDs. The rate exhibits Arrhenius temperature dependence, with the activation energy of the order of millielectronvolts. The charge recombination process occurs due to coupling of the electronic subsystem to vibrational modes of the TiO2 NB. Inelastic electron–phonon scattering happens on a picosecond time scale, with strong dependence on the QD size. Our simulations demonstrate t...

Kinetics and Low Temperature Studies of Electron Transfers in Films of Small (<2 nm) Au Monolayer Protected Clusters

This work examines the temperature dependence of electron transfer (ET) kinetics in solid-state films of mixed-valent states of monodisperse, small (<2 nm) Au monolayer protected clusters (MPCs). The mixed valent MPC films, coated on interdigitated array electrodes, are Au25(SR)18(0/1-), Au25(SR)18(1+/0), and Au144(SR)60(1+/0), where SR = hexanethiolate for Au144 and phenylethanethiolate for Au25. Near room temperature and for ca. 1:1 mol:mol mixed valencies, the bimolecular ET rate constants (assuming a cubic lattice model) are ~2 × 10(6) M(-1) s(-1) for Au25(SR)18(0/1-), ~3 × 10(5) M(-1) s(-1) for Au25(SR)18(1+/0), and ~1 × 10(8) M(-1) s(-1) for Au144(SR)60(1+/0). Their activation energy ET barriers are 0.38, 0.34, and 0.17 eV, respectively. At lowered temperatures (down to ca. 77 K), the thermally activated (Arrhenius) ET process dissipates revealing a tunneling mechanism in which the ET rates are independent of temperature but, among the different MPCs, fall in the same order of ET rate: Au144(+1/0) > Au25(0/1-) > Au25(1+/0).

Pulse Radiolysis Studies of Temperature Dependent Electron Transfers among Redox Centers in ba3-Cytochrome c Oxidase from Thermus thermophilus: Comparison of A- and B-Type Enzymes.

The functioning of cytochrome c oxidases involves orchestration of long-range electron transfer (ET) events among the four redox active metal centers. We report the temperature dependence of electron transfer from the CuAr site to the low-spin heme-(a)bo site, i.e., CuAr + heme-a(b)o → CuAo + heme-a(b)r in three structurally characterized enzymes: A-type aa3 from Paracoccus denitrificans (PDB code 3HB3) and bovine heart tissue (PDB code 2ZXW), and the B-type ba3 from T. thermophilus (PDB codes 1EHK and 1XME). k,T data sets were obtained with the use of pulse radiolysis as described previously. Semiclassical Marcus theory revealed that λ varies from 0.74 to 1.1 eV, Hab, varies from ∼2 × 10-5 eV (0.16 cm-1) to ∼24 × 10-5 eV (1.9 cm-1), and βD varies from 9.3 to 13.9. These parameters are consistent with diabatic electron tunneling. The II-Asp111Asn CuA mutation in cytochrome ba3 had no effect on the rate of this reaction whereas the II-Met160Leu CuA-mutation was slower by an amount corresponding to a decreased driving force of ∼0.06 eV. The structures support the presence of a common, electron-conducting "wire" between CuA and heme-a(b). The transfer of an electron from the low-spin heme to the high-spin heme, i.e., heme-a(b)r + heme-a3o → heme-a(b)o + heme-a3r, was not observed with the A-type enzymes in our experiments but was observed with the Thermus ba3; its Marcus parameters are λ = 1.5 eV, Hab = 26.6 × 10-5 eV (2.14 cm-1), and βD = 9.35, consistent also with diabatic electron tunneling between the two hemes. The II-Glu15Ala mutation of the K-channel structure, ∼ 24 Å between its CA and Fe-a3, was found to completely block heme-br to heme-a3o electron transfer. A structural mechanism is suggested to explain these observations.

Secondary Quinone in Photosystem II ofThermosynechococcus elongatus: Semiquinone−Iron EPR Signals and Temperature Dependence of Electron Transfer

The secondary quinone acceptor, Q(B), has been studied in photosystem II (PSII) isolated from Thermosynechococcus (T.) elongatus. Thermoluminescence indicated that Q(B) was present in this preparation. An EPR signal observed at low temperature at g = 1.9 was attributed to Fe2+ Q(B)- on the basis of the characteristic period-of-two variations in its intensity depending on the number of laser flashes given at 20 degrees C. When samples showing the Fe2+ Q(B)- signal were illuminated at 77 K, an EPR signal at g = 1.66 appeared with an amplitude proportional to that of the Fe2+ Q(B)- signal. This signal is attributed to the Q(A)- Fe2+ Q(B)- state. While these attributions have been made previously in PSII from other origins, they have remained relatively tentative since the characteristic period-of-two oscillations of Q(B) had not previously been observed. The flash experiments indicated that more than one exchangeable plastoquinone is associated with the isolated PSII. The g = 1.66 signal from the Q(A)- Fe2+ Q(B)- state was used to study the temperature dependence of electron transfer between the two quinones. Electron transfer occurred in half of the centers (after 30 s incubation) at -28 degrees C for Q(A)- to Q(B) but at -58 degrees C for Q(A)- to Q(B)-. This marked difference for the two electron transfer reactions indicates different types of rate-limiting reactions. In the better studied but homologous system, the purple bacterial reaction center, the Q(A)- to Q(B) step is limited by a gating process, while the Q(A)- to Q(B)- step is limited by protonation events. Similar reactions in PSII could give rise to the observed temperature dependence.

Dependence of Photosynthetic Electron-Transfer Kinetics on Temperature and Energy in a Density-Matrix Model

We use a multidimensional density-matrix model to explore the temperature dependence of electron transfer from the excited singlet state of the primary electron donor (P*) to the neighboring bacteriochlorophyll (B A ) in photosynthetic bacterial reaction centers. This reaction, which has the unusual property of increasing in rate with decreasing temperature, occurs too rapidly to be treated reliably by approaches that assume thermal equilibration of the vibrational levels of the reactant electronic state. In the density-matrix treatment, the frequencies and displacements of the vibrational modes that are coupled to electron transfer, and the microscopic time constants for transitions between different vibrational states, are obtained from molecular-dynamics simulations by the dispersed-polaron (spin-boson) approach. The electron-transfer dynamics are simulated by integrating the stochastic Liouville equation following excitation of the system with a short pulse of light. In this model, the increase in the rate with decreasing temperature depends strongly on the fact that electron transfer occurs more rapidly than vibrational thermalization. The high rate of electron transfer also affects the dependence of the kinetics on the energy difference between the reactant and product electronic states (ΔE°), making the optimal value of -AE° smaller than the reorganization energy. This interesting effect can be rationalized qualitatively in simple semiclassical and quantum mechanical models.

Temperature dependence of electron transfer in coupled quantum wells

We report on the temperature dependence of electron transfer between coupled quantum wells in a voltage tunable two-color quantum-well infrared photodetector (QWIP). The detection peak of this QWIP switches from 7.1 μm under positive bias to 8.6 μm under negative bias for temperatures T⩽40 K. For T⩾40 K, the 7.1 μm peak is present under both bias polarities and increases significantly with T while the 8.6 μm peak decreases correspondingly. We determine the temperature dependence of electron densities in the two QWs from the detector absorption spectra that are deduced using corrugated QWIPs and find that electron transfer is efficient only when thermionic emission is not significant.

Electron Spin Resonance Study of the Temperature Dependence of Electron Transfer in DNA: Competitive Processes of Tunneling, Protonation at Carbon, and Hopping

In this work, we employ electron spin resonance (ESR) spectroscopy to investigate the effects of temperature on excess electron and hole transfer through DNA. The competitive processes of tunneling...

Temperature Dependence of Photoinduced Electron Transfer within Self-Assembled Uroporphyrin−Cytochrome c Complexes

In this report the temperature dependence of photoinduced electron transfer between both singlet- and triplet-state uroporphyrin (1URO and 3URO, respectively) and the heme group of ferricytochrome c (Cc) has been examined. Analysis of the forward electron-transfer rate originating from 1URO as a function of temperature gives a reorganizational energy (λ) of 1.0 ± 0.02 eV with an associated electronic coupling term (HAB) of (3.7 ± 0.3) × 10-4 eV. In addition, the singlet lifetime data exhibit a temperature-dependent Gaussian distribution that narrows as the temperature increases. This suggests a conformational distribution of bound porphyrin with a narrow range of potential wells at lower temperatures. The corresponding temperature dependence of the electron transfer between 3URO/URO+ and the heme group of Cc gives a λ = 0.49 ± 0.05 eV/0.19 ± 0.03 eV and HAB ∼ 4 × 10-4 eV/1.4 × 10-4 eV, respectively. These observations are consistent with orientational dependent conformational states as well as possible ga...

Negative Enthalpies of Activation and Isokinetic Relationships in the Electron Transfer Quenching Reaction of Pd−Tetraphenylporphyrin by Aromatic Nitro Compounds and Quinones

The temperature dependence of electron transfer quenching of triplet excited neutral Pd−tetraphenylporphyrin is investigated in a series of isotropic solvents and in a nematic crystalline one. Aromatic nitro compounds and quinones of different redox potentials are used as quenchers so that the quenching reaction is exergonic to slightly endergonic. The reactions of quenchers with the largest negative ΔG0 are diffusion controlled and thus have a positive enthalpy of activation, the quenchers with moderately negative ΔG0 values quench in the activation controlled realm with negative activation enthalpies, and the weakest quenchers again show positive activation enthalpies. An isokinetic relationship is found only in isotropic solvents, the first case of the isokinetic temperature being well above the temperatures of the experiments. Reactions with negative activation enthalpy are generally considered to have a pre-equilibrium, but it is shown that an elementary reaction may exhibit a negative enthalpy of activation, too, if the reaction entropy is negative enough. It is conjectured that the negative enthalpies of activation observed in this work are due to an elementary reaction.

Solvent and temperature dependence of electron transfer in the inverted region

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSolvent and temperature dependence of electron transfer in the inverted regionPingyun Chen, Sandra L. Mecklenburg, and Thomas J. MeyerCite this: J. Phys. Chem. 1993, 97, 50, 13126–13131Publication Date (Print):December 1, 1993Publication History Published online1 May 2002Published inissue 1 December 1993https://doi.org/10.1021/j100152a015RIGHTS & PERMISSIONSArticle Views208Altmetric-Citations33LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (656 KB) Get e-Alerts

A molecular mechanism of conformational gating of electron transfer in photosynthetic reaction centra

A mechanism based on conformational control of electron transfer provides a reinterpretation of the observed temperature dependence of electron transfer from cytochrome c to the special pair of bacteriochlorophylls in the reaction center of several photosynthetic bacteria. More generally, it constitutes an alternative contribution to the temperature dependence of electron transfer reactions in biological systems compared to the vibronic coupling and parallel path mechanisms. Starting from the crystallographic structure of the reaction center of Rhodopseudomonas viridis, a detailed molecular mechanism of conformational gating of electron transfer is derived by studies of conformational states and electronic structure. The low temperature, unactivated, electron transfer is assigned to a direct, superexchange, mechanism involving intermediate orbitals on a bridging tyrosine. In the high temperature, activated, region, hydrogen bond formation and proton transfer between the bridging tyrosine and an asparagine residue, in a high energy conformational state of the reaction center, makes sequential electron transfer dominate the rate. This mechanism predicts the formation of a neutral tyrosine radical during electron transfer, which should be possible to detect by timeresolved spectroscopy. The mechanism also is of interest with respect to coupling of electron and proton transfer, gating of electron transfer in biological energy transduction, the role of bridging aromatic amino acids in long-range electron transfer, and molecular electronic devices.

An electron transfer switch in photosynthetic reaction centra

A detailed molecular mechanism of a conformational switch of electron transfer is derived from studies of conformational states and electronic structure in the reaction center of the photosynthetic bacterium Rhodopseudomonas viridis. This type of mechanism accounts for the observed temperature dependence of electron transfer from cytochrome c to the photoionized special pair of bacteriochlorophylls in the reaction center of several photosynthetic bacteria and thereby provides an alternative interpretation to the vibronic coupling and parallel path models. The mechanism also has interesting implications for electron gating in biological energy transduction, coupling of electron and proton transfer, the role of bridging aromatic amino acids in electron transfer, and molecular electronic devices.

Vibrational relaxation and coherence and primary electron transfer in photosynthetic reaction centers

Vibrational coherence recently observed by Vos et al. has been analyzed to determine the rate of vibrational relaxation involved in the primary electron transfer (ET) in reaction centers. A modified ET model is introduced to determine multi-mode effects and the temperature dependence of ET for a series of Rb. capsulatus mutants

Picosecond study of the temperature dependence of electron transfer in the excited singlet state of a polymethine dye

The temperature dependence of the photoinduced electron transfer from the polymethine dye 1,1′-dimethyl-3,3′-di- p—N-phenylacetanilino-6,6′-di-cyanoimidacarbocyanine iodide to methylviologen has been investigated in methanolic solution by means of picosecond fluorescence spectroscopy in the temperature range 278–328 K. At an acceptor concentration of 0.5 mol/ℓ an activation energy Δ G* = (24.1 ± 3.5) kJ/mol, and a value of the frequency factor A=4.7 × 10 14 ℓ mol −1 s −1 have been evaluated from the experimental data.

A mechanism of temperature dependent electron transfer reactions in biological systems

A mechanism contributing to the temperature dependence of electron transfer reactions in biological systems, based on conformational control of electron transfer, is considered. It is shown that such a mechanism can account for the observed temperature dependence of electron transfer between cytochrome c and the special pair of bacteriochlorophylls in the reaction center of several photosynthetic bacteria. It is also shown that a parallel path model, recently proposed as an alternative to the vibronic coupling analysis because of criticism against the latter, cannot explain the observations on some of these systems. In those systems for which a complete structure has been determined, a specific structural component is suggested to be involved in the conformational control of electron transfer. The reaction center of the photosynthetic bacterium Rhodopseudomonas viridis is shown to have a conformational state at an energy above the ground state close to the experimental high temperature activation energy of electron transfer and with an enhanced electron transfer capability compared to the ground state in accordance with the proposed mechanism. Suggestions for further experimental and theoretical investigations of the mechanism are provided.

Erratum: Free Energy and Temperature Dependence of Electron Transfer at the Metal-Electrolyte Interface

In the report "Free energy and temperature dependence of electron transfer at the metal-electrolyte interface" by C. E. D. Chidsey (22 Feb., p.919), the axis label in figure 3B should have been "Time (s)," not "Time (ms)." Also, the text following equation 8 should have read, "where C = [See equation in the PDF file]

Free Energy and Temperature Dependence of Electron Transfer at the Metal-Electrolyte Interface

The rate constant of the electron-transfer reaction between a gold electrode and an electroactive ferrocene group has been measured at a structurally well-defined metal-electrolyte interface at temperatures from 1 degrees to 47 degrees C and reaction free energies from -1.0 to +0.8 electron volts (eV). The ferrocene group was positioned a fixed distance from the gold surface by the self-assembly of a mixed thiol monolayer of (eta(5)C(5)H(5))Fe(eta(5)C(5)H(4))CO(2)(CH(2))(16)SH and CH(3)(CH(2))(15)SH. Rate constants from 1 per second (s(-1)) to 2 x 10(4) s(-1) in 1 molar HClO(4) are reasonably fit with a reorganization energy of 0.85 eV and a prefactor for electron tunneling of 7 x 10(4) s(-1) eV(-1). Such self-assembled monolayers can be used to systematically probe the dependence of electron-transfer rates on distance, medium, and spacer structure, and to provide an empirical basis for the construction of interfacial devices such as sensors and transducers that utilize macroscopically directional electron-transfer reactions.

Temperature dependence of electron transfer between bacteriopheophytin and ubiquinone in protonated and deuterated reaction centers of Rhodopseudomonas sphaeroides

The rate of the electron-transfer reaction between bacteriopheophytin and the first quinone in isolated reaction centers of Rhodopseudomonas sphaeroides has an unusual temperature dependence. The rate increases about threefold with decreasing temperature between 300 and 25 K, and decreases abruptly at temperatures below 25 K. Partial deuteration of the reaction centers alters the temperature dependence of the rate constant. Qualitative features of the temperature dependence can be understood in the context of a theory of nonadiabatic electron transfer (Sarai, 1980. Biochim. Biophys. Acta 589:71-83). We conclude that very low-energy (10-50 cm-1) processes, perhaps skeletal vibrations of the protein, are important to electron transfer. Higher-energy vibrations, possibly involving the pyrrolic N--H bonds of bacteriopheophytin, also are important in this process.

A possible new mechanism of temperature dependence of electron transfer in photosynthetic systems

A new theory for the electron transfer by the non-adiabatic process is formulated taking into account the origin shift and the frequency change of the vibration. The resultant formulas are quite similar to those of Jortner (Jortner, J. (1976) J. Chem. Phys. 64, 4860–4867) except that the free energy gap ΔG is used instead of the energy gap ΔE. By applying this theory to the photosynthetic electron transfer, the role of the remarkable temperature dependence of the electron transfer from cytochrome to P + in Chromatium vinosum and the experimental data were reproduced very well using a small value of the coupling strength in contrast with the previous theory. This implies that proteins play a role to exclude many of the solvent molecules from the region of the electron transfer reaction between the donor and acceptor molecules. The negative activation process in the back electron transfer from Q − A to P +, the very slow back electron transfer from I − to P + and the solvent isotope effect on the cytochrome oxidation are also successfully explained by this new theory. It is shown that even a qualitative conclusion as to the molecular parameters obtained from the temperature dependence of the electron transfer is different between the present theory and that of Jortner.