Light-induced Electron Paramagnetic Resonance Signal Research Articles

Electron paramagnetic resonance of VN–VGa complex in BGaN

A metastable photoinduced electron paramagnetic resonance (EPR) signal at low temperatures is reported for GaN alloyed with boron (i.e., BxGa1−xN) epitaxial layers grown at temperatures ranging from 840 to 1090° C. An isotropic EPR line with g = 2.004 is observed with an intensity depending on the growth temperature for all the samples with boron content between 0.73% and 2.51%. The temperature dependence of EPR intensities is compared with the results of high-resolution photoinduced transient spectroscopy. This enables a link between particular traps and the EPR signal. The activation energies of these traps are consistent with the theoretical position of the VN–VGa complex. Thermal annihilation of the EPR signal with 30 meV activation energy corresponds to shallow donor ionization. A model explaining the light-induced EPR signal that involves redistribution of electrons between deep and shallow donors, mediated by photoionization to the conduction band, is proposed.

Long lasting phosphorescence of Eu2+-activated MO-B2O3 glasses

In this letter, Eu2+-activated MO-B2O3 (M=Ca, Sr, and Ba) glasses are prepared in reductive atmosphere and their long lasting phosphorescence properties are studied. The intensity of the phosphorescence for the glasses is Sr>Ca>Ba. The light-induced electron paramagnetic resonance signal is observed after the irradiation of an ultraviolet lamp, which is due to the electron trapped by oxygen vacancy. The energy depth of the glasses is calculated using Hoogenstraaten's method.

Identification of the deep level defects in AlN single crystals by electron paramagnetic resonance

Electron paramagnetic resonance (EPR) at 9.4 and 35 GHz were studied on two types of AlN single crystals, grown by a sublimation sandwich method. These investigations revealed the presence of transition metals impurities in the first sample: Fe2+ (S=2) and some paramagnetic centers with S=3/2, we suggest Cr3+ or Ni3+ as the possible candidates. The EPR spectra of Fe2+ were observed up to the room temperature. After sample illumination at 5 K with light (wavelength shorter 700 nm) strong EPR signal with a g factor of shallow donors (SDs) and slightly anisotropic linewidth appears. This light-induced EPR signal, once excited at low temperature, still persists after switching off the light and is about constant up to 30 K then it drops quickly. SDs show a negative correlation energy U and oxygen in the N position (ON) is the most probable model. EPR spectra of deep-donor center which was assumed to be the nitrogen vacancy VN have been observed in the second sample. The x-ray irradiation leads to considerable enhancement of deep donor’s (VN) signals intensity. The annealing resulted in recombination thermoluminescence and the deep donor (VN) energy level was estimated to be about 0.5 eV. The models of shallow (ON) and deep (VN) donor centers were supported by comprehensive hyperfine structure analysis.

B-Branch Electron Transfer in the Photosynthetic Reaction Center of a Rhodobacter sphaeroides Quadruple Mutant. Q- and W-Band Electron Paramagnetic Resonance Studies of Triplet and Radical-Pair Cofactor States

The directionality of light-induced charge transfer in bacterial photosynthetic reaction centers (RCs) with respect to their A and B cofactor branches is still poorly understood on the electronic level. A prominent example is primary electron transfer in the RCs from the purple bacterium Rb. sphaeroides. Site-directed mutants with specific alterations of the cofactor binding sites with respect to the native system can deliver useful information toward a better understanding of the directionality enigma. Here we report on electron paramagnetic resonance (EPR) studies of the LDHW quadruple mutant, HL(M182)/GD(M203)/LH(M214)/AW(M260), which contains crucial mutations in the electron-transfer pathway. The directionality of the charge separation process was studied under light- or dark-freezing conditions first directly by 95 GHz (W-band) high-field EPR spectroscopy examining the charge-separated radical pairs (P865•+ Q(B)•−) of the primary donor P865, a bacteriochlorophyll dimer, and the terminal acceptor, QB, a ubiquinone-10. Second, it was studied indirectly by 34 GHz (Q-band) EPR examining the triplet states of the primary donor ((3)P865) that occur as a byproduct of the photoreaction. At 10 K, the triplet state has been found to derive mainly from an intersystem crossing mechanism, indicating the absence of charge-separated radical-pair states with a lifetime longer than 10 ns. B-branch charge separation and formation of the triplet-state (3)P865 via a radical-pair mechanism can be induced with low yield at 10 K by direct excitation of the bacteriopheophytins in the B-branch at 537 nm. At this wavelength, charge separation most probably proceeds via hole transfer from bacteriopheophytin to the primary donor. The triplet state of the primary donor is found to be quenched by the carotenoid cofactor present in the RC. The light-induced transient EPR signal of P•+ Q(B)•− is formed in a minor fraction of RCs (<1%) for RCs frozen in the dark. In contrast, about 70% of RCs illuminated upon freezing are trapped in the long-lived (τ > 104 s) charge-separated-state P•+ Q(B)•−. The temperature dependence of the EPR signals from P•+ Q(B)•− points to two factors responsible for the forward electron transfer to the terminal acceptor QB and for the charge-recombination reaction. The first factor involves a significant protein conformational change to initiate P•+ Q(B)•− charge separation, presumably by moving the quinone from the distal to the proximal position relative to the iron. The second factor includes protein relaxation, which governs the charge-recombination process along the B-branch pathway of the LDHW mutant.

EPR studies on defects in sol–gel derived alumina films

We have studied the defect structures and mechanism for the luminescence of non-doped, Tb 3+-doped and Tb 3+–Zn 2+ codoped alumina films treated at various temperatures from 300 to 800 °C. The electron paramagnetic resonance (EPR) spectra observed before UV-irradiation are attributable to the non-bridging oxygen radicals; Al–O–O at treatment temperatures of 300–500 °C and Al–O at 800 °C. The UV-light irradiation of the non-doped alumina films gives the EPR spectra with the 27Al hyperfine splitting. There exists a good correlation between the treatment temperature dependence on the light-induced EPR signal intensity of the trapped electron and the NMR peak intensity due to the five-coordinated aluminum ion. We concluded that the electron is captured at the oxygen vacancy connected with the five-coordinated aluminum. The broad EPR spectra due to the ground state of the Tb 3+ ion were observed in the Tb 3+-doped and Tb 3+–Zn 2+ codoped alumina films. The Tb 3+ concentration dependence of the EPR signal intensity due to the non-bridging oxygens suggests that Tb 3+ prefers to form Al–O–Tb bonds and prevent to form a cluster of rare earth ions itself. The light-induced EPR signal of the electron trapped at oxygen vacancy next the five-coordinated aluminum was also observed for the Tb 3+–Zn 2+ codoped alumina film, which produced a long-lasting luminescence. The photogenerated trapped electron remained for several minutes under the room temperature. These experimental facts indicate that the five-coordinated aluminum ions act as the trapping sites to produce the long-lasting luminescence.

Detergent-Induced Reversible Denaturation of the Photosystem II Reaction Center: Implications for Pigment-Protein Interactions

Incubation of the D1-D2-cytochrome b559 complex with Triton X-100 modified the protein secondary structure, caused significant spectral modifications, and reduced the formation of light-induced spin-polarized triplet electron paramagnetic resonance (EPR) signal. After 24 h of incubation, the absorption spectrum shifted from 675.5 to 671.5 nm and the fluorescence spectrum shifted from 682 to 672 nm. These shifts were accompanied by an increase in the chlorophyll fluorescence yield and by decreases in the intensity of the circular dichroism in the red region and the secondary electron transport activity. The intensity of the light-induced triplet EPR signal was also markedly reduced in the same experimental conditions. Substitution of dodecyl beta-maltoside for Triton X-100 reversed all the above-mentioned parameters to the values exhibited by the native D1-D2-Cyt b559 complex, including the characteristic triplet EPR signal. We concluded that all observed changes were due to the destruction of P680 with Triton X-100 and to the reestablishment of P680 in the presence of dodecyl beta-maltoside. The easier but certainly not the only possible explanation to all these phenomena is to consider a dimeric structure for P680, at least in its ground state, where interactions take place within the two dimeric chromophores and with the apoprotein. Such a dimeric structure would be very sensitive to small modifications of the P680 domain, which convert the dimer absorbing at 680 into two chlorophyll monomers absorbing near 670 nm. The dodecyl beta-maltoside reestablished the structure of the native P680 domain.

The YF161D1 mutant of Synechocystis 6803 exhibits an EPR signal from a light-induced photosystem II radical.

The currently accepted model for the location of the redox-active tyrosines, D and Z, in photosystem II suggests that they are symmetrically located on the D1 and D2 polypeptides, which are believed to form the heterodimer core of the reaction center. Z, the electron conduit from the manganese catalytic site to the primary chlorophyll donor, has been identified with tyrosine-161 of D1. The YF161D1 mutant of Synechocystis 6803 [Debus, R. J., Barry, B. A., Sithole, I., Babcock, G. T., & McIntosh, L. (1988b) Biochemistry 27, 9071-9074; Metz, J. G., Nixon, P. J., Rogner, M., Brudvig, G. W., & Diner, B. A. (1989) Biochemistry 28, 6960-6969], in which this tyrosine has been changed to a phenylalanine, should have no light-induced EPR (electron paramagnetic resonance) signal from a tyrosine radical. This negative result has indeed been obtained in analysis of one of two independently constructed mutants through the use of a non-oxygen-evolving core preparation (Metz et al., 1989). Here, we present an analysis of a YF161D1 mutant through the use of a photosystem II purification procedure that gives oxygen-evolving particles from wild-type Synechocystis cultures. In our mutant preparation, a light-induced EPR signal from a photosystem II radical is observed under conditions in which, in a wild-type preparation, we can accumulate an EPR signal from Z+. This EPR signal has a different lineshape from that of the Z+ tyrosine radical, and spin quantitation shows that this radical can be produced in up to 60% of the mutant reaction centers. The EPR lineshape of this radical suggests that photosystem II reaction centers of the YF161D1 mutant contain a redox-active amino acid.

Electron transfer in photosystem II at cryogenic temperatures.

The photochemistry in photosystem II of spinach has been characterized by electron paramagnetic resonance (EPR) spectroscopy in the temperature range of 77-235 K, and the yields of the photooxidized species have been determined by integration of their EPR signals. In samples treated with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), a single stable charge separation occurred throughout the temperature range studied as reflected by the constant yield of the Fe(II)-QA-EPR signal. Three distinct electron donation pathways were observed, however. Below 100 K, one molecule of cytochrome b559 was photooxidized per reaction center. Between 100 and 200 K, cytochrome b559 and the S1 state competed for electron donation to P680+. Photooxidation of the S1 state occurred via two intermediates: the g = 4.1 EPR signal species first reported by Casey and Sauer [Casey, J. L., & Sauer, K. (1984) Biochim. Biophys. Acta 767, 21-28] was photooxidized between 100 and 160 K, and upon being warmed to 200 K in the dark, this EPR signal yielded the multiline EPR signal associated with the S2-state. Only the S1 state donated electrons to P680+ at 200 K or above, giving rise to the light-induced S2-state multiline EPR signal. These results demonstrate that the maximum S2-state multiline EPR signal accounts for 100% of the reaction center concentration. In samples where electron donation from cytochrome b559 was prevented by chemical oxidation, illumination at 77 K produced a radical, probably a chlorophyll cation, which accounted for 95% of the reaction center concentration. This electron donor competed with the S1 state for electron donation to P680+ below 100 K.(ABSTRACT TRUNCATED AT 250 WORDS)

Light-induced EPR signal I and signal II in wheat etioplasts greened under intermittent light without or with subsequent continuous light

The behaviour of light-induced electron paramagnetic resonance (EPR) Signal I and Signal II in wheat plastids lacking O 2 evolving capacity due to development under intermittent light (IL plastids) is compared with that of plastids whose water-splitting system has been photoactivated by a brief subsequent continuous light (ICL plastids). EPR spectra in the absence or presence of DCMU as well as the observations of rise kinetics over a long time course clearly reveal that in ICL plastids Signal I (identified with P 700 +) is the result of competing processes that oxidize P 700 on the one hand and reduce it on the other. In IL plastids the high Signal I level indicates a high steady-state concentration of oxidized P 700 due to the absence of normal electron flow from Photosystem II (PS II). The three kinetic components (dark, slow-decaying and fast-decaying) of Signal II are observed both in IL and ICL plastids. Signal II is not chemically induced as Signal I by ferricyanide. Protease eliminates Signal II but leaves Signal I intact. Only the fast Signal II is abolished by DCMU and electron donors to PS II. It is assumed to reflect the oxidation state of a component functioning on the PS II oxidizing side.

Primary acceptor in bacterial photosynthesis: obligatory role of ubiquinone in photoactive reaction centers of Rhodopseudomonas spheroides.

Reaction centers were found to bind two ubiquinones, both of which could be removed by o-phenanthroline and the detergent lauryldimethylamine oxide. One ubiquinone was more easily removed than the other. The low-temperature light-induced optical and electron paramagnetic resonance (EPR) changes were eliminated and restored upon removal and readdition of ubiquinone and were quantitatively correlated with the amount of tightly bound ubiquinone. We, therefore, conclude that this ubiquinone plays an obligatory role in the primary photochemistry. The easily removed ubiquinone is thought to be the secondary electron acceptor. The low-temperature charge recombination kinetics, as well as the optical and EPR spectra, were the same for untreated reaction centers and for those reconstituted with ubiquinone. This indicates that extraction and reconstitution were accomplished without altering the conformation of the active site. Reaction centers reconstituted with other quinones also showed restored photochemical activity, although they exhibited changes in their low-temperature recombination kinetics and light-induced (g = 1.8) EPR signal is interpreted in terms of a magnetically coupled ubiquinone--Fe2+ acceptor complex. A possible role of iron is to facilitate electron transfer between the primary and secondary ubiquinones.

New light-induced EPR signal in Anacystis nidulans.

IT has been known for more than 10 years that there are two light-induced electron paramagnetic resonance (EPR) signals in plants. A recent review has summarized what is known about them1. My purpose here is to characterize in a preliminary way a hitherto unreported light-induced EPR signal in Anacystis nidulans, which I name signal III. This signal can readily be distinguished from the other two signals. I do not know why nobody has detected it before, but it is real, and its source is being investigated.

AN ATTEMPT AT QUANTITATION OF THE SHARP LIGHT-INDUCED ELECTRON PARAMAGNETIC RESONANCE SIGNAL IN PHOTOSYNTHETIC MATERIALS.

Abstract Quantitative determinations were made of the number of spins responsible for the sharp, fast decaying EPR signal induced by light in photosynthetic material. The species resposible for the observed EPR signal is present at the level of approx. 1 per 100 molecules of chlorophyll. Values ranging between 1 and 4 were obtained for the ratio between free radicals and molecules of photoconverter P700. In view of the difficulties and errors inherent in the quantitation procedures involved, this finding does not exclude the possibility that the oxidized of P700 is the species responsible for the observed EPR signal, as the correlation of several properties of the radical and P700 had previously suggested. The presence of radical species other than or in addition to P700+ is, however, also compatible with the results.