Art van der Est

Nanosecond electron transfer kinetics in photosystem I as obtained from transient EPR at room temperature

Transient EPR spectra of photosystem I (PS I) in spinach chloroplasts and PS I particles prepared from Synechococcus are presented. Two consecutive spectra are observed after the laser pulse. The decay time of the first spectrum is equal to the rise time of the second spectrum. The two spectra represent sequential charge‐separated states in the electron transfer chain and the time constant of the electron transfer step between them is found to be t 1/e = 260 ± 20 ns for the Synechococcus PS I particles as well as for the spinach chloroplasts. The first spectrum is assigned to the P+ 700 A− 1 pair, where A− 1 is the second electron acceptor, probably a quinone‐like molecule. The second spectrum covers the region of the P+ 700 signal and may be part of the spectrum due to the coupled radical pair P+ 700 (FeS), where (FeS)− is the first iron‐sulfur center along the electron transfer chain.

Photoinduced Charge Separation in a Ferrocene−Aluminum(III) Porphyrin−Fullerene Supramolecular Triad†

Light-induced electron transfer is investigated in a ferrocene-aluminum(III) porphyrin-fullerene supramolecular triad (FcAlPorC(60)) and the constituent dyads (AlPorC(60) and FcAlPorPh). The fullerene unit (C(60)) is bound axially to the aluminum(III) porphyrin (AlPor) via a benzoate spacer, and ferrocene (Fc) is attached via an amide linkage to one of the four phenyl groups in the meso positions of the porphyrin ring. The absorption spectra and voltammetry data of the complexes suggest that the ground state electronic structures of the Fc, AlPor, and C(60) entities are not significantly perturbed in the dyads and triad. Time-resolved optical and transient electron paramagnetic resonance (EPR) data show that photoexcitation of the AlPorC(60) dyad results in efficient electron transfer from the excited singlet state of the porphyrin to fullerene, producing the charge-separated state AlPor(•+)-C(60)(•-). The fluorescence and transient EPR data also suggest that some energy transfer from the porphyrin to fullerene may occur. The lifetime of the radical pair AlPor(•+)-C(60)(•-) measured by transient absorbance spectroscopy is found to be 39 ns in o-dichlorobenzene at room temperature. At 200 K, transient EPR experiments place a lower limit of 5 μs on the radical pair lifetime. In the triad, the data suggest that excitation of the porphyrin gives rise to the charge-separated state Fc(•+)-AlPor-C(60)(•-) in two electron transfer steps. Photocurrent measurements demonstrate that both dyads and the triad have good photovoltaic performance. However, when Fc is appended to AlPorC(60), the expected improvement of the radical pair lifetime and the photovoltaic characteristics is not observed.

Time-resolved EPR of the radical pair P865+.QA−. in bacterial reaction centers. Observations of transient nutations, quantum beats and envelope modulation effects

Abstract The time evolution of the spin-polarized transient EPR signals of the radical pair state P 865 +. Q A −. is studied in perdeuterated reaction centers of Rhodobacter sphaeroides R-26 in which the non-heme iron has been replaced by zinc. The transients show modulations of the signal amplitude on several time scales. Fast quantum beat oscillations with frequencies around 15 MHz occur within the first 100 ns after the light excitation because the radical pair is generated in a coherent superposition of its eigenstates. Transient nutations also occur as a result of the precession of the magnetization about the applied microwave field B 1 . At the maximum B 1 field available (0.125 mT) a nutation frequency of ≈ 3.5 MHz is obtained. Small variations in the nutation frequency throughout the spectrum occur as a result of the dipolar coupling in the radical pair. Additional oscillations in the frequency ranges 1.5 to 2.0 MHz and 2.5 to 3.0 MHz are also observed which are attributed to the interaction between the nuclear spins and the unpaired electron spins. Analogies and differences to well-known nuclear modulation phenomena observed in pulsed EPR experiments are discussed.

Transient EPR spectroscopy of the charge separated state P+Q− in photosynthetic reaction centers. Comparison of Zn-substituted Rhodobacter sphaeroides R-26 and Photosystem I

Abstract Transient EPR spectra measured at 24 GHz and room temperature are compared for: (i) bacterial reaction centers (bRC) of Rhodobacter sphaeroides R-26 in which the non-heme iron has been replaced by zinc, (ii) Photosystem I (PS I) particles from Synechocystis 6803 and (iii) PS I in perdeuterated Synechococcus lividus algae. A comparison of Zn-bRC spectra at 9 GHz and 24 GHz in liquid and frozen solution is also presented. The spectra are assigned to the charge separated state P+·Q−· (P = primary chlorophyll donor, Q = primary quinone acceptor) and show no evidence of motional narrowing thus confirming the condition of slow molecular motion of the RC complex on the microsecond time scale of the experiment. The spectra are interpreted on the basis of the correlated coupled radical pair concept and are dependent upon the relative orientation of the two radical ions as well as their magnetic interaction parameters which are available from independent experiments. For PS I, simulations using independently measured g-tensors confirm that the phylloquinone x-axis (along the C=O bonds) lies roughly parallel to the axis connecting P+· and Q−· (dipole axis). For the Zn-bRCs the spectra show that these two axes make an angle of ∼ 60° with each other in agreement with the two independent sets of atomic coordinates which are available for the ground state structure of Rhodobacter sphaeroides R-26. The two sets of coordinates differ in the orientation of Q which results in large changes in the simulated spectra, primarily because of a shift in the effective g-factor along the dipolar axis. Starting from one of the two sets of atomic coordinates, it is shown that a rotation of Q−· through 12° about its x-axis results in a change in the sign of the polarization of the X-band spectrum. The orientation of Q−· (and P+·) in the charge separated state can thus be determined with a high degree of accuracy by transient EPR, if the magnetic interaction tensors are sufficiently well known from independent measurements.

Nanosecond electron transfer kinetics in photosystem I following substitution of quinones for vitamin K1 as studied by time resolved EPR

Room temperature transient EPR spectra of photosystem I (PS I) particles from Synechocystis 6803 are presented. Native PS I samples and preparations depleted in the A1‐acceptor site by solvent extraction and then reconstituted with the quinones (Q) vitamin K1 (VK1), duroquinone (DQ and DQd12) and naphthoquinone (NQ) have been studied. Sequential electron transfer to P+ 700A− 1 (FeS) and P+ 700A1 (FeS)− is recovered only with VK1. With DQ and NQ electron transfer is restored to form the radical pair P+ 700Q− as specified by a characteristic electron spin polarization (ESP)‐pattern, but further electron transfer is either slowed down or blocked. A qualitative analysis of the K‐band spectrum suggests that the orientation of reconstituted NQ in PS I is different from the native acceptor A1 = VK1.

Light-induced spin polarization in type I photosynthetic reaction centres

The use of light-induced spin polarization to study the structure and function of type I reaction centres is reviewed. The absorption of light by these systems generates a series of sequential radical pairs, which exhibit spin polarization as a result of the correlation of the unpaired electron spins. A description of how the polarization patterns can be used to deduce the relative orientation of the radicals is given and the most important structural results from such studies on photosystem I (PS I) are summarized. Quinone exchange experiments which demonstrate the influence of protein-cofactor interactions on the polarization patterns are discussed. The results show that there are significant differences between the binding sites of the primary quinone acceptors in PS I and purple bacterial reaction centres and suggest that pi-pi interactions probably play a more important role in PS I. Studies using spin-polarized EPR transients and spectra to investigate the electron transfer pathway and kinetics are also reviewed. The results from PS I, green-sulphur bacteria and Heliobacteria are compared and the controversy surrounding the role of a quinone in the electron transfer in the latter two systems is discussed.

A Coordination Compound of Ge0 Stabilized by a Diiminopyridine Ligand

Reduction of the cationic Ge(II) complex [dimpyrGeCl][GeCl3] (dimpyr=2,6-(ArN=CMe)2NC5H3, Ar=2,6-iPr2C6H3) with potassium graphite in benzene affords an air sensitive, dark green compound of Ge(0), [dimpyrGe], which is stabilized by a bis(imino)pyridine platform. This compound is the first example of a complex of a zero-valent Group 14 element that does not contain a carbene or carbenoid ligand. This species has a singlet ground state. DFT studies revealed partial delocalization of one of the Ge lone pairs over the π*(C=N) orbitals of the imines. This delocalization results in a partial multiple-bond character between the Ge atom and imine nitrogen atoms, a fact supported by the X-ray crystallography and IR spectroscopy data.

Characterization of divalent and trivalent species generated in the chemical and electrochemical oxidation of a dimeric pincer complex of nickel.

The electrolytic and chemical oxidation of the dimeric pincer complex [κ(P),κ(C),κ(N),μ(N)-(2,6-(i-Pr(2)POC(6)H(3)CH(2)NBn)Ni](2) (1; Bn = CH(2)Ph) has been investigated by various analytic techniques. Cyclic voltammetry measurements have shown that 1 undergoes a quasi-reversible, one electron, Ni-based redox process (ΔE(0)(1/2) = -0.07 V vs Cp(2)Fe/[Cp(2)Fe](+)), and spectroelectrochemical measurements conducted on the product of the electrolytic oxidation, [1](+•), have shown multiple low-energy electronic transitions in the range of 10,000-15,000 cm(-1). Computational studies using Density Functional Theory (B3LYP) have corroborated the experimentally obtained structure of 1, provided the electronic structure description, and helped interpret the experimentally obtained absorption spectra for 1 and [1](+·). These calculations indicate that the radical cation [1](+·) is a dimeric, mixed-valent species (class III) wherein most of the spin density is delocalized over the two nickel centers (Ni(+2.5)(2)N(2)), but some spin density is also present over the two nitrogen atoms (Ni(2+)(2)N(2)·). Examination of alternative structures for open shell species generated from 1 has shown that the spin density distribution is highly sensitive toward changes in the ligand environment of the Ni ions. NMR, UV-vis, electron paramagnetic resonance (EPR), and single crystal X-ray diffraction analyses have shown that chemical oxidation of 1 with N-Bromosuccinimide (NBS) follows a complex process that gives multiple products, including the monomeric trivalent species κ(P),κ(C),κ(N)-{2,6-(i-Pr(2)PO)(C(6)H(3))(CH═NBn)}NiBr(2) (2). These studies also indicate that oxidation of 1 with 1 equiv of NBS gives an unstable, paramagnetic intermediate that decomposes to a number of divalent species, including succinimide and the monomeric divalent complexes κ(P),κ(C),κ(N)-{2,6-(i-Pr(2)PO)(C(6)H(3))(CH═NBn)}NiBr (3) and κ(P),κ(C),κ(N)-{2,6-(i-Pr(2)PO)(C(6)H(3))(CH(2)N(H)Bn)}NiBr(2) (4); a second equivalent of NBS then oxidizes 3 and 4 to 2 and other unidentified products. The divalent complex 3 was synthesized independently and shown to react with NBS or bromine to form its trivalent homologue 2. The new complexes 2 and 3 have been characterized fully.

Sequential charge separation in two axially linked phenothiazine-aluminum(III) porphyrin-fullerene triads.

New supramolecular triads (PTZpy→AlPor-C(60), TPTZpy→AlPor-C(60)), containing aluminum(III) porphyrin (AlPor), fullerene (C(60)), and phenothiazine (phenothiazine = PTZ, 2-methylthiophenothaizine = TPTZ) have been constructed. In these triads the fullerene and phenothiazine units are bound axially to opposite faces of the porphyrin plane via covalent and coordination bonds, respectively. The ground- and excited-state properties of the triads and reference dyads are studied using steady-state and time-resolved spectroscopic techniques. The time-resolved data show that photoexcitation results in charge separation from the excited singlet state of the porphyrin to the C(60) unit, generating (Donor)py→AlPor(•+)-C(60)(•-), Donor = PTZ and TPTZ. A subsequent hole shift from the porphyrin to phenothiazine generates the charge-separated state (Donor)(•+)py→AlPor-C(60)(•-). The lifetime of the charge separation exhibits a modest increase from 39 ns in the absence of the donor to 100 ns in PTZpy→AlPor-C(60) and 83 ns in TPTZpy→AlPor-C(60). These lifetimes are discussed in terms of the electronic coupling between phenothiazine, the porphyrin, and C(60).

Transient EPR of weakly coupled spin-correlated radical pairs in photosynthetic reaction centres: increase spectral resolution from nutation analysis

Abstract Transient EPR signals at high microwave power are calculated for a weakly coupled, spin-correlated radical pair and compared to exponential results obtained for photosystem I in fully deuterated Synechococcus lividus. It is shown that the observed phase effects in the transient nutations result from the coupling between the spins in the radical pair P + 700 Q − . The spectral variation of the nutation frequency leds to an effective increase in the spectral resolution and is used to show that the principal x axis of the g tensor og Q − is roughly parallel to the symmetry axis of the pair dipole coupling tensor, a conclusion reached previously from the simulation of the spin-polarized spectrum.