Transient EPR Spectroscopy Research Articles

Transient and pulsed EPR spectroscopy on the radical pair state P 865 +. Q A −. to study light-induced changes in bacterial reaction centers

The radical pair state P 865 +. Q A −. (P865: primary donor, QA: quinone acceptor) in Zn-substituted bacterial reaction centers is investigated using transient and pulsed EPR spectroscopy. For photoexcited samples not frozen in the dark but under continuous illumination, a prolonged lifetime of this radical pair state is observed in agreement with previous studies using time resolved optical spectroscopy. The transient EPR spectra revealed neither a different orientation of the quinone acceptor anion nor a change of itsg-anisotropy in the sample frozen in the charge separated state as compared with that frozen in the dark. The latter finding indicates a similar hydrogen bonding situation for Q A −. in both samples. Changes observed in the transient EPR spectra are interpreted as result of contributions from spin-polarized Q A −. which was generated in part of the sample while freezing under illumination. From the out-of-phase echo modulation pattern observed in the pulsed EPR measurements, it follows that the distance between P 865 +. and Q A −. is the same in dark frozen samples and in those frozen under continuous illuminaton. This is in contrast to the model suggested by Kleinfeld D., Okamura M.Y., Feher G.: Biochemistry23, 5780 (1984), in which an increased distance and a larger distribution of distances was suggested for samples frozen under illumination. The prolonged lifetime of the radical pair P 865 +. Q A −. is discussed in terms of differences in the relaxation behaviour of the protein.

Influence of iron-removal procedures on sequential electron transfer in photosynthetic bacterial reaction centers studied by transient EPR spectroscopy.

Electron spin polarized electron paramagentic resonance (ESP EPR) spectra were obtained with deuterated iron-removed photosynthetic bacterial reaction centers (RCs) to specifically investigate the effect of the rate of primary charge separation, metal-site occupancy, and H-subunit content on the observed P865+QA- charge-separated state. Fe-removed and Zn-substituted RCs from Rb. sphaeroides R-26 were prepared by refined procedures, and specific electron transfer rates (kQ) from the intermediate acceptor H- to the primary acceptor QA of (200 ps)-1 vs (3-6 ns)-1 were observed. Correlation of the transient EPR and optical results shows that the observed slow kQ rate in Fe-removed RCs is H-subunit-independent, and, in some cases, independent of Fe-site occupancy as Zn2+ substitution does not ensure retention of the native kQ. In addition, shifts in the optical spectrum of P865 and differences in the high-field region of the Q-band ESP spectrum for Fe-removed RCs with slow kQ indicate possible structural changes near P865. The experimental X-band and Q-band spin-polarized EPR spectra for deuterated Fe-removed RCs where kQ is at least 15-fold slower at room temperature than the (200 ps)-1 rate observed for native Fe-containing RCs have different relative amplitudes and small g-value shifts compared to the spectra of Zn-RCs which have a kQ unchanged from native RCs. These differences reflect the trends in polarization predicted from the sequential electron transfer polarization (SETP) model [Morris et al. (1995) J. Phys. Chem. 99, 3854-3866; Tang et al. (1996) Chem. Phys. Lett. 253, 293-298]. Thus, SETP modeling of these highly resolved ESP spectra obtained with well-characterized proteins will provide definitive information about any light-induced structural changes of P865, H, and QA that occur upon formation of the P865+QA- charge-separated state.

Differences in the binding of the primary quinone receptor in Photosystem I and reaction centres of Rhodobacter sphaeroides-R26 studied with transient EPR spectroscopy

Abstract The binding of the primary quinone acceptor, Q, in Photosystem I (PS I) and reaction centres (RC's) of Rhodobacter Sphaeroide-R26 in which, the non-heme iron has been replaced by zinc (Zn-bRC's) is studied using transient EPR spectroscopy. In PS I, Q is phylloquinone (vitamin K1, VK1) and is referred to as A1. In Zn-bRC's, it is ubiquinone-10 (UQ10) and called QA. Native samples of the two RC's as well as those in which A1 and QA have been replaced by perdeuterated napthoquinone (NQ-d6) and duroquinone (DQ-d12) are compared. The spin polarized K-band (24 GHz) spectra of the charge separated state P+.Q−. (P = primary chlorophyll donor) in Zn-bRC's show that substitution of QA, with NQ-d6 and DQ-d12 does not have a measurable effect on the quinone orientation in the QA site. In contrast, large differences in the orientation of VK1, NQ-d6 and DQ-d12 in the A1 site in PS I are found. In addition, all three quinones in PS I are oriented differently than QA in Zn-bRC's. Further, the x and y principal values of the g-tensors of VK1−., NQ−. and DQ−. in PS I are shown to be significantly larger than in frozen alcohol and Zn-bRC's. It is suggested that the differences in the orientation and a g-values of the quinones in the two RC's arise from a weaker binding to the protein in PS I.

ChemInform Abstract: Transient EPR Spectroscopy of Short‐Lived Triplet States and Radicals

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Nonradiative deactivation and triplet states in donor-aryl-acceptor compounds (dialkylaminobenzonitriles)

Intersystem crossing rates and triplet state properties of TICT (twisted intramolecular charge transfer) model compounds (dimethylaminobenzonitrile — DMABN — and its methyl substituted derivatives, 3-cyanoindoline, 3-cyanobenzquinuclidine — CBQ — and benzonitrile) were studied by various experimental and theoretical methods (emission spectroscopy, transient absorption techniques, EPR spectroscopy and semiempirical quantum chemical calculations) with the aim to obtain a quantitative deactivation pattern of this class of compounds in solvents of different polarity. The lowest triplet state, populated after excitation, is generally, a non-charge-transfer state, i.e. the 3A 1 of planar DMABN, irrespective of solvent polarity and compound flexibility, with the only exception of the rigidly perpendicular CBQ, which relaxes to the 3A 1 state of the benzonitrile moiety. In non-polar solvents intersystem crossing increases with increasing charge transfer character of the excited singlet; in ethanol, the intersystem crossing rate is about the same for all compounds studied, with the exception of benzonitrile, and in general significantly lower than in a non-polar environment. From the mechanistic point of view, intersystem crossing from 1TICT can be described as a thermal charge recombination process in the normal Marcus region. With increasing solvent polarity, the decreasing energy gap between these two states then entails a decreasing intersystem crossing rate and an increasing 1TICT lifetime, as experimentally observed. In hydroxylic solvents, internal conversion to the ground state is the second dominant deactivation pathway of 1TICT states; it shows a significant deuterium isotope effect.

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

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.

Time resolved EPR of triplet excitons in Phenazine-TCNQ charge transfer crystal

Transient nutation EPR spectroscopy has been applied to study the dynamical properties of the excited triplets in the Phenazine-Tetracyanoquinodimenthane 1: 1 CT crystal. Measurements have been carried out with the magnetic field set along the principal axes of the ZFS tensor. Spin-spin and spin-lattice relaxation times have been determined at different temperatures together with the decay rate constants from the triplet sublevels which are found to be highly spin selective. The temperature dependence of the initial optical electron polarization carried by the triplet has been also analyzed. It is shown that the single fission and the intersystem crossing caused by spin-orbit coupling are both responsible for the generation of the triplet in this crystal, the former prevailing at room temperature. Our results are in agreement with previous investigations on the same crystal.

The application of time resolved EPR to the study of photoreactive molecules in solid and liquid crystalline environments

The use of transient EPR spectroscopy for the study of photoreactions in solid and liquid crystalline is demonstrated. The method is applied to an intramolecular photoreaction in 2-(4-methylpentyl)-7-nonanoylfluorene (MPNOF) and to an intermolecular photoreaction in 4-cyano-4′-octylbiphenyl (8CB) doped with phenazine. It is shown that both MPNOF and phenazine undergo photoreactions via their first excited triplet state. Orientational ordering in the solid and liquid crystalline phase is studied via the excited triplet powder spectrum. Spectra of precursor and products can be identified and separated by their kinetics. The possible nature of the products of reaction is discussed.