Fast Charge Separation Research Articles

Pathways of energy transformation in antenna reaction center complexes of Heliobacillus mobilis.

The conversion of excitation energy in the antenna reaction center complex of Heliobacillus mobilis was investigated at 10 K as well as at 275 K by means of time-resolved absorbance difference spectroscopy of isolated membranes in the (sub)picosecond time range. Selective excitation of the primary electron acceptor, chlorophyll (Chl) a 670, and of the different spectral pools of bacteriochlorophyll (BChl) g (BChl g 778, BChl g 793, and BChl g 808) was applied. At 10 K, excitation at 770 or 793 nm resulted on the one hand in rapid energy transfer to BChl g 808 and on the other hand in fast charge separation from excited BChl g 793 ( approximately 1 ps). Once the excitations were on BChl g 808, the bleaching band shifted gradually to the red, from 806 to 813 nm, and charge separation from excited BChl g 808 occurred by a very slow process ( approximately 500 ps). The main purpose of our experiments was to answer the question whether an "alternative" pathway for charge separation exists upon excitation of Chl a 670. Our measurements showed that the amount of oxidized primary donor (P798(+)) relative to that of excited BChl g produced by excitation of Chl a 670 was considerably larger than upon direct excitation of BChl g. This indicates the existence of an alternative pathway for charge separation that does not involve excited antenna BChl g. This effect occurred at 10 K as well as at 275 K. The mechanism for this process is discussed in relation to different trapping models; it is concluded that charge separation occurs directly from excited Chl a 670.

Dynamics of Photoinduced Charge Separation and Charge Recombination in Synthetic DNA Hairpins with Stilbenedicarboxamide Linkers

The dynamics of photoinduced charge separation and charge recombination in synthetic DNA hairpins have been investigated by means of femtosecond and nanosecond transient spectroscopy. The hairpins consist of a stilbene linker connecting two complementary 6-mer or 7-mer oligonucleotide strands. Base pairing between these strands results in formation of hairpins in which the stilbene is approximately parallel to the adjacent base pair. The singlet stilbene is selectively quenched by guanine, but not by the other nucleobases, via an electron-transfer mechanism in which the stilbene singlet state is the electron acceptor and guanine is the electron donor. In a hairpin containing only A:T base pairs, no quenching occurs and the restricted geometry results in a long stilbene lifetime and high fluorescence quantum yield. In families of hairpins which contain a single G:C base pair at varying locations in the hairpin stem, the stilbene fluorescence lifetime and quantum yield decrease as the stilbene−guanine distance decreases. Transient absorption spectroscopy is used to monitor the disappearance of the stilbene singlet and the formation and decay of the stilbene anion radical. Analysis of these data provides the rate constants for charge separation and charge recombination. Both processes show an exponential decrease in rate constant with increasing stilbene−guanine distance. Thus, electron transfer is concluded to occur via a single-step superexchange mechanism with a distance dependence β = 0.7 Å-1 for charge separation and 0.9 Å-1 for charge recombination. The rate constants for charge separation and charge recombination via polyA vs polyT strands are remarkably similar, slightly larger values being observed for polyA strands. The dynamics of electron transfer in hairpins containing two adjacent G:C base pairs have also been investigated. When the guanines are in different strands, the second guanine has little effect on the efficiency or dynamics of electron transfer. However, when the guanines are in the same strand, somewhat faster charge separation and slower charge recombination are observed than in the case of hairpins with a single G:C base pair. Thus, the GG step functions as a shallow hole trap. The relationship of these results to other theoretical and experimental studies of electron transfer in DNA is discussed.

Dispersive charge separation and conformational cooling of P +H A− in reaction centers of Rb. sphaeroides R26: a spontaneous emission study

The decay of the excited state of the primary donor ( 1 P ∗ ) of reaction centers (RC) of Rb. sphaeroides R26 has been monitored via its spontaneous emission. Although there is general agreement that (i) a prominent drop of prompt emission within a few picoseconds results from fast charge separation of the majority of RCs and that (ii) a ∼15 ns delayed emission (amplitude ∼5·10 −5) is due to recombination of P +H A − (H A denoting bacteriopheophytin) when electron transport to the secondary quinone acceptor (Q A) is blocked, the nature of (iii) intermediate fluorescence components with time constants ranging from ∼100 ps to several nanoseconds has been unclear. In this paper these components are studied by manipulating the lifetime of P +H A − via the presence or absence of Q A. Reconstitution of Q A to Q A-free RCs leads to a reduction of this lifetime from 15 ns to 100–200 ps, thus eliminating any delayed emission at later times. In such preparations small components from prompt emission extend up to a nanosecond exhibiting dispersive charge-separation kinetics, which we attribute to an energetic dispersion of the primary radical pair P +B A − (B A is a bacteriochlorophyll). The high-energy wing of this energetic distribution is responsible for charge separation slower than 100 ps, which we find in a fraction of ∼0.3% of the RCs at 280 K. Since at low temperatures correspondingly smaller activation barriers are sufficient for such a retardation, this fraction increases to ∼3% at 85 K. In Q A-free RCs with a long P +H A − lifetime prompt emission still dominates at early times. However, its contribution is smaller than 50% of the total emission at about 600 ps at 280 K and 1 ns at 85 K, respectively. The amplitudes of this delayed emission are time dependent and reveal a relaxation of the average free energy between 1 P ∗ and P +H A − changing from Δ G 0≈−0.21 eV at about 70 ps to Δ G 0≈−0.29 eV after 50 ns. This relaxation is attributed to the slow protein response to charge separation (conformational cooling) and can be described by a Kohlrausch relaxation function with a time constant of 4.21 ns and a stretching exponent of 0.456.

Geminate Pair Recombination in Sensitized Polymers (Monte-Carlo simulations)

Dynamics of the geminate charge pair recombination in the sensitized poly-N-epoxypropylcarbazole films was described by the Monte-Carlo simulations. The model was based on the assumption of a very fast charge separation over several carbazolyls and further slow recombination governed by the diffusive motion of the hole on an energetically and structurally irregular lattice. Both diagonal and off-diagonal disorder are taken into account by analyzing the experimental data. It is concluded that for the ply-N-epoxypropylcarbazole polymer sensitized with trinitrofluorenone the initial separations of the charge pairs are distributed exponentially with the mean value of 5 lattice constants and the widths of the diagonal and off-diagonal disorders are 0.04eV and 0.3 times the lattice constant, respectively.

Charge Separation and Recombination in a Photoconducting Polymer with Electron Donor−Acceptor Complexes

The dynamics of electron−hole (e−h) pairs created by photoexcitation of charge-transfer (CT) states in films and solutions of oligomeric poly-N-epoxypropylcarbazole (PEPCz) doped with electron-accepting trinitrofluorenone were studied by investigating polarized transient absorption kinetics and spectra with picosecond and femtosecond time resolution. We observed very fast formation of the carbazolyl radical cation in PEPCz films, within the 100 fs excitation pulse. Transient anisotropy, which was calculated from polarized kinetics at the time delays of 100 fs and longer, is about 5 times lower in the films than in the solution of a monomeric analogue. This very fast loss of the excitation polarization memory in the films is attributed to hole transfer from the parent carbazolyl (Cz) moiety to the neighboring Czs. An asymptotic decay of the anisotropy is observed on the picosecond time scale, indicating that the fast charge separation is completed within a few picoseconds. The geminate recombination of e−h pairs in PEPCz films is nonexponential and extends into the nanosecond time domain. A dependence of the primary e−h separation efficiency on the excitation photon energy is concluded from recombination dynamics. After photoexcitation, limited hole transfer also occurs in PEPCz solution on the short oligomer chain, but recombination is much faster than in the films.

Energetics and Mechanism of Primary Charge Separation in Bacterial Photosynthesis. A Comparative Study on Reaction Centers of Rhodobacter sphaeroides and Chloroflexus aurantiacus

The high efficiency of charge separation in photosynthetic reaction centers arises from the interplay of energetics, electronic couplings, and reorganization energies relevant for the fast charge separation and slow recombination processes. All these parameters can be determined unambiguously only from magnetic-field-dependent measurements of the recombination dynamics of the intermediate radical pair P+HA- and the lifetime of the recombination product 3P*. Results obtained on QA-depleted reaction centers of Chloroflexus aurantiacus are compared with those for the well-characterized reaction centers of Rhodobacter sphaeroides. In contrast to Rb. sphaeroides, the magnetic field dependence of the triplet yield in Cf. aurantiacus has a pronounced resonance structure, allowing the direct determination of the exchange interaction of P+HA-, J = 21 G. The recombination rate kT is slightly larger for Cf. aurantiacus and shows a different temperature dependence. All these differences can be explained by the free energy of P+HA-, found to be larger by 0.04 eV in Cf. aurantiacus compared to Rb. sphaeroides. We propose that this different energy arises largely from the different amino acid at position L104, which is glutamic acid in the case of Rb. sphaeroides and glutamine in the case of Cf. aurantiacus. The electronic couplings and the reorganization energies, on the other hand, are very similar in both reaction centers. Implications for the mechanism of primary charge separation are discussed. The pronounced nonexponential kinetics of charge separation in Cf. aurantiacus is explained by the energetic inhomogeneity of the primary radical pair P+BA-.

Patch-clamp study on flash-induced secondary electrogenic transport in the thylakoid membrane. Interpretation in terms of a Q-cycle

Photo-electrical signals are studied using a patch-clamp method on single chloroplasts of Peperomia metallica in the so-called ‘whole-thylakoid’ configuration. The thylakoid transmembrane potential after a single-turnover flash shows multiphasic kinetics which can be described as the sum of two components: (1) Reaction Rl/RC associated with fast primary charge separation in the reaction centers of photosystems I and II and (2) Reaction R1/Q, a slow rising component most likely associated with secondary electrogenic steps within the cyt. b 6 f complex. The latter assignment is supported by the inhibition of the slow rising component by 2.5 μM DBMIB. The dark relaxation time of both RI/RC and R1/Q associated with the field dissipative back flow of ions was independently and unambiguously obtained from the discharge RC-time after current-injection. Reaction Rl/Q shows sigmoid rise kinetics which is modelled by two populations of cyt. b 6 f complexes incorporating a 2-step consecutive reaction scheme. The average time constants associated with the two steps are 13 ms and 28 ms and might be related to plastoquinol oxidation at the lumenal interface and plastoquinol formation at the stromal interface, respectively. The ratio of charges displaced by Rl/Q and Rl/RC (Q/RC-ratio) varied between 0.15 and 0.95 depending, amongst others, on pre-treatment with flashes.

Porphyrin-Quinone Compounds with a Spacer of Diacetylene Unit

Abstract Porphyrin-quinone model compounds with a diacetylene spacer were synthesized. Electronic state of the porphyrin ring was found to be altered due to the direct introduction of the diacetylene group into the meso position, and very fast charge separation was observed in these model compounds.

Electric Field Effects on Steady State and Time Resolved Fluorescence from Photosynthetic Reaction Centers

Abstract A promising approach to identifying the primary electron acceptor and thereby the mechanism of primary charge separation in photosynthetic reaction centers (RCs) is the orientational determination of the dipole moment of the primary radical pair. This can be achieved by studying the angular dependence of the primary charge separation rate in an external electric field as reflected in the Dichroic Excitation spectrum of the electric Field modulated Yield of the prompt fluorescence (DELFY). For RCs from Rb.sphaeroides R-26 steady state, low temperature DELFY experiments point (within an angle of 5°) to the orientation of the dipole moment of P+H− A. Time resolved fluorescence measurements revealed that the major contribution to the steady state fluorescence quantum yield and to the electric field effect thereon originates from a slow component with a lifetime of ≃ 300 ps at 80K. This fluorescence component is by two orders of magnitude slower than the primary charge separation rate measured in absorption and might originate from a small (≃3%) subset of RCs characterized by slow, unistep charge separation. Recent transient absorption measurements in electric fields revealed a significant reduction of the quantum yield of P+H− A formation within 30ps demanding a fast loss channel. In order to achieve dynamic competition with fast charge separation in the majority of RCs a more efficient loss channel than internal conversion is required, such as fast parking or trapping of excitation energy. From such a field modulated trapping state with the ability to fluoresce one would expect the observed loss of P+H− A quantum yield accompanied by a slow fluorescence component, which should be stronger than the measured one, however. This model would imply direct charge separation from 1P* to P+H− A in one step in the majority of RCs. Electric field induced charge separation to the B-branch forming P+B− B would better comply with the data, in case its energy is high enough to produce a quadratic field dependence of the delayed recombination fluorescence.

A view of fast charge separation in the bacterial photosynthetic reaction centre involving weak environmental damping of a populated auxiliary chlorophyll state

The initial fast electrical charge separation in the bacterial photosynthetic reaction centre involves electron transfer (ET) from the photo-excited special pair chlorophyll to pheophytin. The ET is assisted by the auxiliary chlorophyll monomer which may be populated in the sub-picosecond time range. We provide a new theoretical description of this process which rests on three-level coherent ET theory, incorporating environmental damping and partial vibrational relaxation in the intermediate state of auxiliary chlorophyll population. If damping is not too strong, this view offers a clue to the ultrafast, activationless and possibly coherent character of both steps, also in the limit when the electronic coupling between the three molecular entities is weak.

Formation and decay of doubly charged ions in n‐alkanes from methane to triacontane

AbstractCoincidence techniques were used to study dissociative double ionization of selected n‐alkanes from methane to triacontane (C30H62) and of the hexane isomers. Following photoionization at 40.8 eV, both covalent and coulombic dissociations of the molecular dications take place. The main decay route of doubly charged alkanes larger than butane is fast charge separation followed by secondary dissociation of energetic singly charged primary ions. A simulation based on quasi‐equilibrium theory and the spectra of the isomers confirm this breakdown mechanism for hexane.

The fragmentation of doubly charged methanol

Doubly charged methanol ions formed by photoionization at vacuum ultraviolet wavelengths decay mainly by fast charge separation along a large number of pathways. The major reactions involve sequential, not concerted processes, with little hydrogen scrambling. The species CH 2OH 2+ and CHOH 2+ are observed as relatively long-lived intermediates with mean lifetimes between 10 and 100 ns.

Intramolecular electron transfer in peripherally metallated tetraphenyl porphyrin derivatives containing molybdenum mononitrosyl redox centres

Picosecond laser spectroscopy measurements on the novel complexes [Mo(NO)(L)Cl(Etpp)][Etpp = 5-(E-substituted phenyl-10,15,20-triphenylporphyrin; L = hydrotris(3,5-dimethylpyrazol-1-yl)borate, E =para-O, meta-O, ortho-O; para-NH] indicate that the observed fluorescence quenching in these compounds is due to fast (τ < 30 ps) intramolecular charge separation to produce transient species with lifetimes in the range 120–290 ps.

Electron transfer in porphyrin—quinone cyclophanes

We have measured the fluorescence properties of five cyclophanes containing, in a vertical face-to-face arrangement, a porphyrin as electron donor and one of several substituted quinones as electron acceptor with varying acceptor strength in a series of solvents covering a wide range of polarity. The results of time-resolved fluorescence decay measurements are interpreted in terms of a very fast intramolecular charge separation within 1–10 ps with driving forces decreasing from −Δ G cs = 0.7 to about 0 eV and a charge recombination in the order of 10–200 ps.

Picosecond charge separation and triplet formation in a closely spaced photosynthetic model system

Intramolecular electron transfer has been investigated for three diphenyletioporphyrins covalently linked to an anthraquinone via a sulfonyloxy bridge at the ortho, meta or para substitution position of the phenyl groups of the porphyrin. The ortho-linked porphyrin is strongly folded and exhibits extremely fast charge separation (CS) and recombination (CR) between porphyrin and anthraquinone ( k CS > 3 × 10 11 s −1 and k CR = 5.9 × 10 10 s −1 in CH 2Cl 2). The less-folded meta- and para-linked porphyrins have smaller electron transfer rate constants. For the ortho-linked compound in toluene a biphasic charge separation is observed. The triplet state is formed in high yield via fast intersystem crossing in the charge-separated state.

Influence of stray capacitance and sample resistance on the kinetics of fast photovoltages from oriented purple membranes

Flash-induced photovoltages were measured with metal electrodes in two experimental systems of purple membranes oriented by an electric field. One system consisted of a suspension of purple membranes cooled to 80 K. The photovoltage evoked by a xenon flash lamp displayed a single phase with a fast rise and a slow RC-decay. The signal shape is consistent with a fast charge separation occurring before the decay of the K-intermediate. The other system consisted of purple membranes embedded and stabilized in polyacrylamide gel. At room temperature, the photovoltage, evoked by a 10 ns laser flash, displayed a negative phase in the submicrosecond range and a slower positive one. The shape of the signals were altered in a complex manner by the stray capacitance and the ionic strength. The rise-time of the negative phase was approx. 14 and approx. 40 ns at ionic strengths of 10 and 1 mM, respectively. The initial peak amplitudes of the photovoltage from both experimental systems depended on the external capacitance in an inverse manner, indicating that both experimental systems were not impedance-matched. The evaluation of kinetic data of molecular reactions from measurements of the photovoltage is discussed.

On the rise time of the R1-component of the "early receptor potential": evidence for a fast light-induced charge separation in rhodopsin.

The rising phase of the R1-component of the early receptor potential from isolated cattle retinas was measured with high time resolution. When the measuring capacitance was 133 pF, a latency of about 200 ns was observed. A rise time of about 0.8 mus at 0 degrees C and 1.6 mus at 37 degrees C (extrapolated to ideal measuring conditions) was found. The negative temperature dependence indicates that the rise is not directly related to the production and decay of photolysis products of rhodopsin since the latter have positive temperature coefficients. An increase of the external measuring capacitance caused a slower rise time. The analysis of this effect allowed the determination of the source impedance of the R1-component. The experimental results can be described with a model in which it is assumed that a fast charge separation (ns or ps) takes place in the outer segment of a photoreceptor cell, and spreads passively to the inner segment via the resistance of the interconnecting cilium. The "inner" relaxation could be circumvented by using isolated rod outer segments which lack the passive inner segments, i.e., a rise time of 90 ns could be measured when isolated rod outer segments were attached to Millipore filters. The results suggest that the molecular event leading to the R1-component is an early charge separation which may be as fast as the cis-trans isomerization of the retinal chromophore.

Theory of thermal and photoassisted electron tunneling

A simple quantum mechanical theory of electron tunneling appropriate for biological electron transfers is developed. The theory predicts discrepancies between optical and thermal tunneling data and clarifies how fast charge separation is produced in photosynthesis. Barrier heights and intermolecular separations are determined for a variety of biomolecular electron transfer experiments.