Parallel polarization electron paramagnetic resonance studies of the S1-state manganese cluster in the photosynthetic oxygen-evolving system

Magnetic properties of the S1-state manganese cluster in the oxygen-evolving photosystem II were studied by parallel polarization electron paramagnetic resonance spectroscopy. Dark minus light spectra gave rise to a broad S1-state signal with a g value of about 4.9 [Dexheimer, S. L., Klein, M. P. (1992) J. Am. Chem. Soc. 114, 2821-2826]. Temperature variation of the signal intensity between 1.9 and 10 K observed in PS II with a sucrose buffer indicates that the signal originates from an excited state with a spin S of 1 with separation from the ground state (S = 0) of about 2.5 K. The S1-state signal was also observed in the sucrose buffer supplemented by 50% glycerol. However, no S1-state signal was detected by addition of 3% methanol or 30% ethylene glycol in the sucrose buffer, although illumination at 200 K in the presence of these alcohols induced the normal multiline S2 signal. Furthermore, modification of the Mn cluster by Cl- or Ca2+ depletion from PS II membranes failed to produce a detectable S1-state signal. A possible magnetic structure of the Mn cluster responsible for the generation of the S1-state signal is discussed on the basis of these observations.

Dual-mode EPR study of new signals from the S3-state of oxygen-evolving complex in photosystem II

The light-induced new EPR signals at g = 12 and 8 were observed in photosystem II (PS II) membranes by parallel polarization EPR. The signals were generated after two flashes of illumination at room temperature, and the signal intensity had four flashes period oscillation, indicating that the signal origin could be ascribed to the S3-state. Successful simulations were obtained assuming S = 1 spin for the values of the zero-field parameters, D = +/-0.435 +/- 0. 005 cm-1 and E/D = -0.317 +/- 0.002. Orientation dependence of the g =12 and 8 signal intensities shows that the axial direction of the zero-field interaction of the manganese cluster is nearly parallel to the membrane normal.

Modification of the properties of S2 state in photosynthetic O2-evolving center by replacement of chloride with other anions

Properties of the S2 state formed in photosystem II membranes in which Cl- had been replaced by various anions were investigated by means of thermoluminescence measurements and low temperature EPR spectroscopy. The Br--substituted membranes showed the normal thermoluminescence B-band arising from S2Q-B charge recombination, whereas the SO2-4-, F--, CH3COO--, and NO-3-substituted membranes showed modified B-bands with variously upshifted peak temperatures. The extent of the peak temperature upshift varied in parallel with the extent of inhibition of O2 evolution depending on the anion species. A normal EPR S2 multiline signal was induced in Br--substituted membranes, but its amplitude was reduced to less than 10% in F--, NO-3-, CH3COO--, and SO2-4-substituted membranes, In contrast, the g = 4.1 signal from S2 was markedly enhanced in F-- and NO-3-substituted membranes, not much affected in CH3COO-- and SO2-4-substituted membranes, and decreased to 70% in Br--substituted membranes. Based on these data, the effect of various types of S2 modification on the O2-evolving activity was discussed. It was suggested that anions have an important role in regulating the interaction between the Mn atoms, and thereby adjust the redox properties of the S2 state to enable further transitions beyond S2.

Generation of triplet and cation-radical bacteriochlorophyll a in carotenoidless LH1 and LH2 antenna complexes from Rhodobacter sphaeroides

The LH1 antenna complex and a native form of the LH2 complex were isolated from the carotenoidless R26 and R26.1 mutants of Rhodobacter sphaeroides by the use of a new detergent, sucrose monocholate. One-color, pump-and-probe transient Raman spectroscopy of these complexes using 351 nm, approximately 50 ps pulses showed the generation of the triplet state of bacteriochlorophyll a (BChl a), whereas measurements using 355 nm, approximately 12 ns pulses showed the generation of BChl a cation radical. Subpicosecond to nanosecond time-resolved absorption spectroscopy using 388 nm, 200 fs pulses for excitation showed rapid (<1 ps) generation of the triplet state and fast decay (<10 ps) of the singlet state of BChl a. Microsecond absorption spectroscopy confirmed the generation of BChl a cation radical. EPR spectroscopy using 532 nm, approximately 5 ns pulses for excitation established the generation of BChl a cation radical. The EPR line width suggested that the unpaired electron is shared by two BChl a molecules. In LH1, the yield of BChl a cation radical per complex was estimated to be about 80% of that in the reaction center, and in LH2 about 50%. Thus, rapid generation of the triplet state, and its subsequent transformation into the cation-radical state of BChl a have been shown to be intrinsic properties of B870 and B850 BChl a assembly in the carotenoidless LH1 and LH2 antenna complexes. In the case of the carotenoid-containing LH2 complex, the triplet states of BChl a and carotenoid (spheroidene) were generated immediately after excitation, but the triplet-state BChl a was quenched efficiently by the carotenoid so that no BChl a cation radical was generated. Thus, the photoprotective function of the carotenoid in this antenna complex is shown.

Formate-induced inhibition of the water-oxidizing complex of photosystem II studied by EPR

The effects of various formate concentrations on both the donor and the acceptor sides in oxygen-evolving PS II membranes (BBY particles) were examined. EPR, oxygen evolution and variable chlorophyll fluorescence have been observed. It was found that formate inhibits the formation of the S(2) state multiline signal concomitant with stimulation of the Q(A)(-)Fe(2+) signal at g = 1.82. The decrease and the increase in intensities of the multiline and Q(A)(-)Fe(2+) signals, respectively, had a linear relation for formate concentrations between 5 and 500 mM. The g = 4.1 signal formation measured in the absence of methanol was not inhibited by formate up to 250 mM in the buffer. In the presence of 3% methanol the g = 4.1 signal evolved as formate concentration increased. The evolved signal could be ascribed to the inhibited centers. Oxygen evolution measured in the presence of an electron acceptor, phenyl-p-benzoquinone, was also inhibited by formate proportionally to the decrease in the multiline signal intensity. The inhibition seemed to be due to a retarded electron transfer from the water-oxidizing complex to Y(Z)(+), which was observed in the decay kinetics of the Y(Z)(+) signal induced by illumination above 250 K. These results show that formate induces inhibition of water oxidation reactions as well as electron transfer on the PS II acceptor side. The inhibition effects of formate in PS II were found to be reversible, indicating no destructive effect on the reaction center induced by formate.

EPR studies of the water oxidizing complex in the S1 and the higher S states: the manganese cluster and Y(Z) radical

The parallel polarization electron paramagnetic resonance (EPR) method has been applied to investigate manganese EPR signals of native S1 and S3 states of the water oxidizing complex (WOC) in photosystem (PS) II. The EPR signals in both states were assigned to thermally excited states with S=1, from which zero-field interaction parameters D and E were derived. Three kinds of signals, the doublet signal, the singlet-like signal and g=11-15 signal, were detected in Ca2+-depleted PS II. The g=11-15 signal was observed by parallel and perpendicular modes and assigned to a higher oxidation state beyond S2 in Ca2+-depleted PS II. The singlet-like signal was associated with the g=11-15 signal but not with the Y(Z) (the tyrosine residue 161 of the D1 polypeptide in PS II) radical. The doublet signal was associated with the Y(Z) radical as proved by pulsed electron nuclear double resonance (ENDOR) and ENDOR-induced EPR. The electron transfer mechanism relevant to the role of Y(Z) radical was discussed.

Positions of Q(A)and Chl(Z)Relative to Tyrosine Y(Z)and Y(D)in Photosystem II Studied by Pulsed EPR

PELDOR (Pulsed Electron eLectron DOuble Resonance) was applied to determinethe distance of between Y(Z)and Q(A) (-)inY(D)-less mutant of Chlamydomonas reinhardtiiin Tris-treatedand Zn-substituted preparation of photosystem II. The value of distance wasfound to be 34.5 ± 1 Â. A '2+1' electron spin echo method has beenapplied to measure the orientation of the radius-vector RfomY(D)to Chl(Z)in a membrane-oriented photosystem II. The anglebetween Rand the membrane normal nwas determined to be 50 ±5(°), using the distance 29.4 ± 0.5 Â determined in non-orientedPS II.

Pulsed EPR studies of doublet signal and singlet-like signal in oriented Ca2+-depleted PS II membranes: location of the doublet signal center in PS II

Doublet signal and singlet-like signal induced in Ca(2+)-depleted PS II were investigated by pulsed EPR in one-dimensionally oriented photosystem (PS) II membranes. The doublet signal showed marked angular dependent change in its spectrum in term of the applied magnetic field, indicating that the magnetic dipole-dipole interaction is mainly responsible for the doublet signal. The singlet-like signal also showed angular dependence, which was less pronounced than that of the doublet signal. The parameters of dipole and exchange interactions used to simulate the doublet signal indicate that the signal arises from a magnetically coupled organic radical pair. Angular dependence of the doublet signal indicates that the radius vector of the radical pair (r) and the normal of the thylakoid membrane is at an angle of 65 degrees. Pulsed ELDOR studies in the oriented membranes indicate that the vector (R) connecting the doublet-signal center with the Y(D)(*) radical and the plane of the thylakoid membrane are at an angle of 8 degrees. Furthermore, the angle between the projections of the R and r vectors on the plane of the thylakoid membrane was determined to be 64 degrees. The location of the doublet-signal species in PS II is discussed.

The position of cytochrome b(559) relative to Q(A) in photosystem II studied by electron-electron double resonance (ELDOR)

The electron-electron double resonance (ELDOR) method was applied to measure the dipole interaction between cytochrome (Cyt) b(+)(559) and the primary acceptor quinone (Q(-)(A)), observed at g=2.0045 with the peak to peak width of about 9 G, in Photosystem II (PS II) in which the non-heme Fe(2+) was substituted by Zn(2+). The paramagnetic centers of Cyt b(+)(559)Y(D)Q(-)(A) were trapped by illumination at 273 K for 8 min, followed by dark adaptation for 3 min and freezing into 77 K. The distance between the pair Cyt b(+)(559)-Q(-)(A) was estimated from the dipole interaction constant fitted to the observed ELDOR time profile to be 40+/-1 A. In the membrane oriented PS II particles the angle between the vector from Q(A) to Cyt b(559) and the membrane normal was determined to be 80+/-5 degrees. The position of Cyt b(559) relative to Q(A) suggests that the heme plane is located on the stromal side of the thylakoid membrane. ELDOR was not observed for Cyt b(+)(559) Y(D) spin pair, suggesting the distance between them is more than 50 A.

Matrix line in pulsed electron-nuclear double resonance spectra

The intense line in Mims and Davies electron-nuclear double resonance (ENDOR) spectra due to the hyperfine interactions of an unpaired electron with distant matrix nuclei is shown to originate from a simultaneous inversion of a large number of nuclear spins by a radiofrequency pulse. Theoretical expressions describing the matrix ENDOR effect are derived and verified experimentally.

Light-induced high-spin signals from the oxygen evolving center in Ca2+-depleted photosystem II studied by dual mode electron paramagnetic resonance spectroscopy

Illuminating Ca2+-depleted photosystem (PS) II membranes generated two new EPR signals at g = 11 and 15 by perpendicular and parallel polarization modes, respectively. Two turnovers of the oxygen evolving center (OEC) beyond the modified S2' state are required for the appearance of these signals. The formation of the signals correlated with that of an asymmetric (singlet-like) EPR signal observed at g approximately 2. Spectral simulation indicated that both signals arose from a transition between |2(+/-)> levels with intradoublet splitting of Delta = 0.276 cm-1 in an S = 2 spin system. Furthermore, the two signals in parallel and perpendicular modes were formed at the same time, indicating that the same metal center was responsible. The molecular z-axis of the S = 2 spin system for the signals was almost parallel to the plane of thylakoid membranes. These results indicate that the Mn cluster in the photosynthetic oxygen evolving center is the source of the new EPR species which may be a Mn(IV)-Mn(IV) or Mn(III)-Mn(III) dimer or a Mn(III) monomer. Redox events of the Mn cluster in the Ca2+-depleted PS II are discussed based on these observations.

pH-dependent characteristics of Y(Z) radical in Ca(2+)-depleted photosystem II studied by CW-EPR and pulsed ENDOR

The Y(Z)-tyrosine radical was trapped by freezing immediately after illumination in Ca(2+)-depleted Photosystem II (PS II) membranes and the pH-dependent characteristics of the radical were investigated using CW-EPR and pulsed ENDOR. The spectrum of the Y*(Z) radical trapped in the Y*(Z)S(1) state at pH 5.5 was cation-like as reported in Mn-depleted PS II (H. Mino et al., Spectrochim. Acta A 53 (1997) 1465-1483). By illuminating the PS II-retaining S(2) state, the Y*(Z) radical and a broad doublet signal formed in the g approximately 2 region were trapped concomitantly. The spectrum of the trapped Y*(Z) radical in the Y*(Z)S(2) state was cation-like at pH 5.5 but the pulsed ENDOR measurements reveals the involvement of the neutral Y*(Z) radical in the doublet signal. At pH 7.0, the resulting Y*(Z) signal was the mixture of the cation-like and neutral radical spectra, and considerably different from the neutral radical found in Mn-depleted PS II. pH-Dependent changes in the properties of the Y*(Z) radical are discussed in relation to the redox events occurring in Ca(2+)-depleted PS II.

The functional sites of chlorophylls in D1 and D2 subunits of photosystem II identified by pulsed EPR

The functional site of ChlZ, an auxiliary electron donor to P680+, was determined by pulsed ELDOR applied to a radical pair of YD * and Chlz+ in oriented PS II membranes from spinach. The radical-radical distance was determined to be 29.5 A and its direction was 50 degrees from the membrane normal, indicating that a chlorophyll on the D2 protein is responsible for the EPR Chlz+ signal. Spin polarized ESEEM (Electronin Spin Echo Envelop Modulation) of a 3Chl and QA - radical pair induced by a laser flash was observed in reaction center D1D2Cytb559 complex, in which QA was functionally reconstituted with DBMIB and reduced chemically. QA -ESEEM showed a characteristic oscillating time profile due to dipolar coupling with 3Chl. By fitting with the dipolar interaction parameters, the distance between 3Chl and QA - was determined to be 25.9 A, indicating that the accessory chlorophyll on the D1 protein is responsible for the 3Chl signal.