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1.
《BBA》1985,809(3):421-428
Signal II of plant photosynthesis, which is generally thought to be connected to the secondary donor complex of Photosystem II, has been investigated with EPR spectroscopy at 9 and 35 GHz. From the spectrum at 35 GHz of deuterated Chlorella vulgaris, the principle values of the g-tensor are determined to be gxx = 2.0074, gyy = 2.0044 and gzz = 2.0023. Proton hyperfine coupling tensor elements and orientations were determined from spectral simulation of random and oriented samples, assuming that Signal II is due to a plastose-miquinone cation having its π-electrons in an antisymmetric orbital as proposed by P.J. O'Malley, G.T. Babcock and R.C. Prince (Biochim. Biophys. Acta 765 (1984) 370–379). In contrast to their work, it is found that most hyperfine interaction is due to the methylene group at ring position 5 and to both hydroxyl groups. One of the hydroxyl groups shows bond bending of 35°. We presume that this is due to hydrogen bonding and that this bond stabilizes the antisymmetric orbital of the π-electrons. 相似文献
2.
The rise time of the photoinduced, reversible EPR Signal IIvf in spinach chloroplasts is found using flash excitation to be 20 ± 10 μs. The results are interpreted as evidence that the Signal IIvf radical is an electron carrier on the donor side of Photosystem II, but probably does not result from the first donor to P680+. 相似文献
3.
The EPR characteristics of oxygen evolving particles prepared from Phormidium laminosum are described. These particles are enriched in Photosystem II allowing EPR investigation of signals which were previously small or masked by those from Photosystem I in other preparations. EPR signals from a Signal II species and high potential cytochrome b-559 appear as they are photooxidised at cryogenic temperatures by Photosystem II. The Signal II species is a donor close to the Photosystem II reaction centre and may represent part of the charge accumulation system of water oxidation. An EPR signal from an iron-sulphur centre which may represent an unidentified component of photosynthetic electron transport is also described.The properties of the oxygen evolving particles show that the preparation is superior to chloroplasts or unfractionated algal membranes for the study of Photosystem II with a functional water oxidation system. 相似文献
4.
The properties of Photosystem II electron donation were investigated by EPR spectrometry at cryogenic temperatures. Using preparations from mutants which lacked Photosystem I, the main electron donor through the Photosystem II reaction centre to the quinone-iron acceptor was shown to be the component termed Signal II. A radical of 10 G line width observed as an electron donor at cryogenic temperatures under some conditions probably arises through modification of the normal pathway of electron donation. High-potential cytochrome b-559 was not observed on the main pathway of electron donation. Two types of PS II centres with identical EPR components but different electron-transport kinetics were identified, together with anomalies between preparations in the amount of Signal II compared to the quinone-iron acceptor. Results of experiments using cells from mutants of Scenedesmus obliquus confirm the involvement of the Signal II component, manganese and high-potential cytochrome b-559 in the physiological process leading to oxygen evolution. 相似文献
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6.
The proposal that EPR Signal II in spinach chloroplasts is due to a plastoquinone cation radical (O'Malley, P.J. and Babcock, G.T. (1983) Biophys. J. 41, 315a) has been investigated in further detail. The similarity in spectral shape between Signal II and the 2-methyl-5-isopropylhydroquinone cation radical is shown to arise from hyperfine coupling to one methyl group for both radicals. A well-resolved four line EPR spectrum of approximate relative intensity 1:3:3:1 for membrane orientation parallel and perpendicular to the applied magnetic field direction also indicates that the partially resolved structure of Signal II is due to hyperfine interaction with one methyl group, i.e., the 2-CH3 group of the plastoquinone cation radical. The ENDOR band observed for this coupling is similar to that observed for methyl group bands of model quinone radicals. The principal hyperfine tensor values obtained for the methyl group interactions are A⊥ = 27.2 MHz and . The large isotropic coupling value (28.6 MHz) of the plastoquinone cation radical's 2-methyl group in vivo indicates that the antisymmetric orbital is the sole contributor to the spin-density distribution of Signal II. The orientation data also suggest that the plastoquinone cation radical is oriented such that the C-CH3 bond direction, and hence the aromatic ring plane, lies perpendicular to the membrane plane. 相似文献
7.
《BBA》2013,1827(10):1165-1173
Proton matrix ENDOR was performed to investigate the protons close to the manganese cluster in oriented samples of photosystem II (PS II). Eight pairs of ENDOR signals were detected in oriented PS II membranes. At an angle of θ = 0° between the membrane normal vector n and the external field H0, five pairs of ENDOR signals were exchangeable in D2O medium and three pairs were not exchangeable in D2O medium. The hyperfine splitting of 3.60 MHz at θ = 0° increased to 3.80 MHz at θ = 90°. The non-exchangeable signals with 1.73 MHz hyperfine splitting at θ = 0°, which were assigned to a proton in an amino acid residue, were not detected at θ = 90° in oriented PS II or in non-oriented PS II. Highly resolved spectra show that only limited numbers of protons were detected by CW-ENDOR spectra, although many protons were located near the CaMn4O5 cluster. The detected exchangeable protons were proposed to arise from the protons belonging to the water molecules, labeled W1-W4 in the 1.9 Å crystal structure, directly ligated to the CaMn4O5 cluster, and nearby amino-acid residue. 相似文献
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9.
The room-temperature EPR characteristics of Photosystem II reaction center preparations from spinach, pokeweed and Chlamydomonas reinhardii have been investigated. In all preparations a light-induced increase in EPR Signal II, which arises from the oxidized form of a donor to P-680+, is observed. Spin quantitation, with potassium nitrosodisulfonate as a spin standard, demonstrates that the Signal II species, Z?, is present in approx. 60% of the reaction centers. In response to a flash, the increase in Signal II spin concentration is complete within the 98 μs response time of our instrument. The decay of Z? is dependent on the composition of the particle suspension medium and is accelerated by addition of either reducing agents or lipophilic anions in a process which is first order in these reagents. Comparison of these results with optical data reported previously (Diner, B.A. and Bowes, J.M. (1981) in Proceedings of the 5th International Congress on Photosynthesis (Akoyunoglou, G., ed.), Vol. 3, pp. 875–883, Balaban, Philadelphia), supports the identification of Z with the P-680+ donor, D1. From the polypeptide composition of the particles used in this study, we conclude that Z is an integral component of the reaction center and use this conclusion to construct a model for the organization of Photosystem II. 相似文献
10.
《BBA》1986,850(1):156-161
The orientation of the various absorbing and fluorescing dipoles in Photosystem II have been investigated by linearly polarized light spectroscopy at 5 K, performed on macroscopically oriented PS II complexes derived from Chlamydomonas reinhardtii. Linear dichroism and absorption spectra show that the QY transitions of the chlorophyll molecules are mostly tilted at less than 35° from the plane of largest cross-section of the particle (which in vivo coincides with the plane of the thylakoid membrane). The chlorophyll forms absorbing at 676 and 683 nm are oriented closer to the membrane than the forms absorbing at 665 and 670 nm which are tilted at approximately 35° from the plane. A dip observed around 680 nm in the LD/absorption spectra indicates a component tilted at a larger angle away from the membrane plane than the 676 nm- and 683 nm-absorbing species. A component weakly absorbing around 693 nm and exhibiting a negative LD (tilt larger than 35°) is clearly resolved. The amplitude of the LD at 693 nm relative to that observed at the maximum (676 nm) varies from sample to sample. In the blue spectral region, two populations of carotenoids are observed; one absorbs around 460 and 490 nm, while the other absorbs around 510 nm. They are oriented out of and near to the thylakoid plane, respectively. Comparison of polarized absorption and fluorescence spectra from the same oriented samples allows the assignment of the 695 nm fluorescence emission to the dipoles responsible for the LD signal at 693 nm. 相似文献
11.
Orientations of the active site chromophores of the mitochondrial redox carriers have been investigated in hydrated, oriented multilayers of mitochondrial membranes using optical and EPR spectroscopy. The hemes of cytochrome c oxidase, cytochrome c1, and cytochromes b were found to be oriented in a similar manner, with the normal to their heme planes lying approximately in the plane of the mitochondrial membrane. The heme of cytochrome c was either less oriented in general or was oriented at an angle closer to the plane of the mitochondrial membrane than were the hemes of the “tightly bound” mitochondrial cytochromes. EPR spectra of the azide, sulfide and formate complexes of cytochrome c oxidase in mitochondria in situ obtained as a function of the orientation of the applied magnetic field relative to the planes of the membrane multilayers showed that both hemes of the oxidase were oriented in such a way that the angle between the heme normal and the membrane normal was approx. 90°. 相似文献
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13.
《FEBS letters》1986,205(2):275-281
EPR signals in the high-spin region were studied at 10 K in photosystem II (PS II) particles and in a purified oxygen-evolving PS II reaction center complex under oxidizing conditions. PS II particles showed EPR peaks at g = 8.0 and 5.6, confirming the recent report by Petrouleas and Diner [(1986) Biochim. Biophys. Acta 849, 264-275]. Addition of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) or o-phenanthroline shifted the peaks to be closer to g = 6.0 depending on the medium pH. On the other hand, the PS II reaction center complex showed peaks at g = 6.1 and 7.8, and at g = 6.1 and 6.4, in the absence and presence of o-phenanthroline, respectively. All these peaks were found to be decreased by the illumination at 10 K. These results suggest that the high-spin signals are due to Q400, Fe(III) atom interacting with the PS II primary electron acceptor quinone QA as reported and that the Fe atom also interacts with the secondary acceptor quinone QB. This interaction seems to induce the highly asymmetric ligand coordination of the Fe atom and to be affected by DCMU and o-phenanthroline in a somewhat different manner. 相似文献
14.
《BBA》1985,808(1):171-179
The effects of selective removal of extrinsic proteins on donor side electron transport in oxygen-evolving PS II particles were examined by monitoring the decay time of the EPR signal from the oxidized secondary donor, Z+, and the amplitude of the multiline manganese EPR signal. Removal of the 16 and 24 kDa proteins by washing with 1 M NaCl inhibits oxygen evolution, but rapid electron transfer to Z+ still occurs as evidenced by the near absence of Signal IIf. The absence of a multiline EPR signal shows that NaCl washing induces a modification of the oxygen-evolving complex which prevents the formation of the S2 state. This modification is different from the one induced by chloride depletion of PS II particles, since in these a large multiline EPR signal is found. After removal of the 33 kDa protein with 1 M MgCl2, Signal IIf is generated after a light flash. Readdition of the 33 kDa component to the depleted membranes accelerates the reduction of Z+. Added calcium ions show a similar effect. These findings suggest that partial advancement through the oxygen-evolving cycle can occur in the absence of the 16 and 24 kDa proteins. The 33 kDa protein, on the other hand, may be necessary for such reactions to take place. 相似文献
15.
Howard Shields Charles McGlumphy Phillip J. Hamrick 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1982,697(1):113-120
EPR data are used to describe the conformation and identity of the atoms coordinated to Cu(II) in Cu(II)-bleomycin bound to oriented DNA fibers. The fibers were slowly drawn from viscous solutions of Cu(II)-bleomycin-DNA containing one Cu(II)-bleomycin to 200 basepairs. EPR measurements were made at room temperature and 90 K for different orientations of the external magnetic field with respect to the helical axes of the fibers. The g-values (, g⊥ =2.04) and the hyperfine constant () are consistent with values expected for Cu(II) chelated to a square planar array of ligands. In the oriented fibers, the square planar arrays do not all have the same orientations with respect to the fiber axes. At room temperature the chelated ions have rotational freedom in which the normal to the planar array has almost complete freedom of rotation about axes perpendicular to the DNA fiber axes. The normal maintains an angle of 75° with respect to the axis, in the plane of the basepair, about which it rotates. Nine superhyperfine peaks on the high field side of the EPR spectrum were partially resolved. The number and splitting (12 G) of these superhyperfine peaks indicate that four nitrogen atoms are chelated to Cu(II) in a square planar array. These data on Cu(II)-bleomycin bound to DNA give information on the orientation of the metal-containing portion of bleomycin which lies outside the double helix. 相似文献
16.
Incubation of PS II membranes with herbicides results in changes in EPR signals arising from reaction centre components. Dinoseb, a phenolic herbicide which binds to the reaction centre polypeptide, changes the width and form of the EPR signal arising from photoreduced Q?AFe. o-Phenanthroline slightly broadens the Q?AFe signal. These effects are attributed to changes in the interaction between the semi-quinone and the iron. DCMU, which binds to the 32 kDa protein, has virtually no effect on the width of the Q?AFe signal but does give rise to an increase in its amplitude. This could result from a change in redox state of an interacting component. Herbicide effects can also be seen when Q?AFe is chemically reduced and these seen to be reflected by changes in splitting and amplitude of the split pheophytin? signal. Dinoseb also results in the loss of ‘Signal II dark’, the conversion of reduced high-potential cytochrome b559 to its oxidized low-potential form and the presence of transiently photooxidized carotenoid after a flash at 25°C; these effects indicate that dinoseb may also act as an ADRY reagent. 相似文献
17.
The EPR characteristics of oxygen evolving particles prepared from Phormidium laminosum are described. These particles are enriched in Photosystem II allowing EPR investigation of signals which were previously small or masked by those from Photosystem I in other preparations. EPR signals from a Signal II species and high potential cytochrome beta-559 appear as they are photooxidised at cryogenic temperatures by Photosystem II. The Signal II species is a donor close to the Photosystem II reaction centre and may represent part of the charge accumulation system of water oxidation. An EPR signal from an iron-sulphur centre which may represent an unidentified component of photosynthetic electron transport is also described. The properties of the oxygen evolving particles show that the preparation is superior to chloroplasts or unfractionated alga membranes for the study of Photosystem II with a functional water oxidation system. 相似文献
18.
《BBA》1984,765(3):370-379
In Photosystem II, Z reduces P-680+ and gives rise to a characteristic EPR signal, termed IIvf in oxygen-evolving chloroplasts and IIf in non-oxygen-evolving chloroplasts. Previous model compound studies of Signal II have centered on the immobilized anionic and neutral forms of semiquinone radicals. These radicals, however, exhibit an essentially structureless band shape in constrast to the partially resolved hyperfine pattern observed for Signal II. In the experiments reported here, we show that some cationic semiquinone radicals (e.g., 2-methyl-5-isopropylhydroquinone cation radical) exhibit band shape and micro-wave power saturation characteristics upon immobilization which are similar to Signal II. Examination of a series of quinone cation radicals shows that a Signal-II-like spectrum is observed when significant unpaired spin density occurs at a ring carbon to which a methyl group is bound. Whether this will occur for a specific quinone depends on the extent to which the peripheral substituent pattern favors a contribution from the antisymmetric benzenoid molecular orbital to the ground state of the radical. For the 2-methyl-5-isopropylhydroquinone cation radical, for example, a 26% contribution of this orbital is estimated. A plastoquinone cation radical in which the electron-donating ability of the quinol-OH groups has been decreased is compatible with antisymmetric orbital stabilization and, therefore, is identified as the Z+ species. Hydrogen bonding of the quinol oxygen to hydrogen-donating amino acid residues in vivo plus an out-of-plane geometry for the quinol-OH groups is proposed to stabilize the antisymmetric orbital. The partially resolved structure of Signal II indicates that the antisymmetric orbital is the major contributor to the ground state; the principal hyperfine splitting in the spectrum arises from the 2-CH3 group of the plastoquinone cation radical. The estimated electrode potential of the Z+ radical is in close agreement with the in vitro electrode potential of quinone cation radicals. 相似文献
19.
From previous studies of biological semiquinones in different solvents, the origin of Signal II in chloroplasts is hypothesized to be a plastosemiquinone anion radical perturbed by a metal cation. Assuming this model, theoretical principal g factors and hyperfine splitting constants were calculated and used to simulate the random spectrum of spinach Signal II. Oriented chloroplasts were used to determine the principal angles of this model. Oriented chloroplasts from collard greens showed a different angular dependency of Signal II from those of spinach as well as the presence of added fine structure. 相似文献
20.
Redox titrations of the flash-induced formation of C550 (a linear indicator of Q?) were performed between pH 5.9 and 8.3 in Chlamydomonas Photosystem II particles lacking the secondary electron acceptor, B. One-third of the reaction centers show a pH-dependent midpoint potential (Em,7.5) = ? 30 mV) for redox couple , which varies by ?60 mV/pH unit. Two-thirds of the centers show a pH-independent midpoint potential (Emm = + 10 mV) for this couple. The elevated pH-independent Em suggests that in the latter centers the environment of Q has been modified such as to stabilize the semiquinone anion, Q?. The midpoint potentials of the centers having a pH-dependent Em are within 20 mV of those observed in chloroplasts having a secondary electron acceptor. It appears therefore that the secondary electron acceptor exerts little influence on the Em of . An EPR signal at g 1.82 has recently been attributed to a semiquinone-iron complex which comprises Q?. The similar redox behavior reported here for C550 and reported by others (Evans, M.C.W., Nugent, J.H.A., Tilling, L.A. and Atkinson, Y.E. (1982) FEBS Lett. 145, 176–178) for the g 1.82 signal in similar Photosystem II particles confirm the assignment of this EPR signal to Q?. At below ?200 mV, illumination of the Photosystem II particles produces an accumulation of reduced pheophytin (Ph?). At ?420 mV Ph? appears with a quantum yield of 0.006–0.01 which in this material implies a lifetime of 30–100 ns for the radical pair P-680+Ph?. 相似文献