| 1. |
- Cox, Nicholas, et al.
(författare)
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The S-1 split signal of photosystem II : a tyrosine-manganese coupled interaction
- 2009
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Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier BV. - 0005-2728 .- 1879-2650. ; 1787:7, s. 882-889
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Tidskriftsartikel (refereegranskat)abstract
- Detailed optical and EPR analyses of states induced in dark-adapted PS II membranes by cryogenic illumination permit characterization and quantification of all pigment derived donors and acceptors, as well as optically silent (in the visible, near infrared) species which are EPR active. Near complete turnover formation of Q(A)(-) is seen in all centers, but with variable efficiency, depending on the donor species. In minimally detergent-exposed PS II membranes, negligible (<5%) oxidation of chlorophyll or carotenoid centers occurs for illumination temperatures 5-20 K. An optically silent electron donor to P680(+) is observed with the same decay kinetics as the S-1 split signal. Cryogenic donors to P680(+) seen are: (i) transient (t(1/2)similar to 150 s) tyrosine related species, including 'split signals' (similar to 15% total centers), (ii) reduced cytochrome b(559) (similar to 30-50% centers), and (iii) an organic donor, possibly an amino acid side chain, (similar to 30% centers).
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| 2. |
- Han, Guangye, et al.
(författare)
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Direct quantification of the four individual S states in Photosystem II using EPR spectroscopy
- 2008
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Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier BV. - 0005-2728 .- 1879-2650. ; 1777:6, s. 496-503
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Tidskriftsartikel (refereegranskat)abstract
- Car, carotenoid; Chl, chlorophyll; ChlZ, secondary chlorophyll electron donor to P680+; Cytb559, cytochrome b559; EPR, electron paramagnetic resonance; DMSO, dimethylsulfoxide; MES, 2-(N-morpholino) ethanesulfonic acid; NIR, near-infrared; OEC, oxygen evolving complex; P680, primary electron donor chlorophylls in PSII; PpBQ, phenyl-p-benzoquinone; PSII, Photosystem II; QA and QB, primary and secondary plastoquinone acceptors of Photosystem II; YD, tyrosine 161 of the PSII D2 polypeptide; YZ, tyrosine 161 of the PSII D1 polypeptide
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| 3. |
- Havelius, Kajsa G.V. 1977-
(författare)
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EPR Studies of Photosystem II : Characterizing Water Oxidizing Intermediates at Cryogenic Temperatures
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The principles of natures own light-driven water splitting catalyst, Photosystem II (PSII), can in the future inspire us to use water as electron and proton source to generate light-driven H2 production. To mimic this challenging step, it is important to understand how the enzyme system can oxidize water. The mechanism of light-driven water oxidation in PSII is in this thesis addressed by EPR spectroscopy. P680+ is a strong oxidant formed by light-oxidation of the chlorophyll species P680 positioned in the center of PSII. The redox active tyrosine-Z (YZ) can reduce P680+ and the YZ• radical is formed. This transient radical is further reduced by the CaMn4-cluster, which is the binding site of the substrate water molecules. In a cyclic process called the S-cycle, this catalytic cluster accumulates four oxidizing equivalents to evolve one molecule of O2 and to oxidize two molecules of water. We can induce the YZ• radical at cryogenic temperatures in the different oxidation states of the catalytic S-cycle and observe this in metalloradical EPR signals. These metalloradical EPR signals are here characterized and used to deduce mechanistic information from the intact PSII. The "double nature" of these spin-spin interaction signals, so called split EPR signals, makes them excellent probes to both YZ oxidation and, when YZ• is present, also to the S-states of the CaMn4-cluster. The metalloradical EPR signals presented here, form a way to study the transient YZ• radical in active PSII that has not been depleted of the catalytic metal cluster. This depleting method that has often been used in the past to study YZ is not representing studies of a mechanistically relevant material. The previously suggested disorder around YZ and accessibility to the bulk can be artifactual properties induced in the mechanistically defect PSII. On the contrary, our observation that proton coupled electron transfer from YZ to the light induced P680+ can occur in a high yield at cryogenic temperatures, suggests a well ordered catalytic site in the protein positioned for optimal performance. The optimized positioning of the redox components found in PSII might be a feature also important to build in an efficient water oxidizing catalyst.
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| 4. |
- Havelius, Kajsa G. V., et al.
(författare)
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Metalloradical EPR Signals from the Y-Z center dot S-State Intermediates in Photosystem II
- 2010
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Ingår i: Applied Magnetic Resonance. - : Springer Science and Business Media LLC. - 0937-9347 .- 1613-7507. ; 37:1-4, s. 151-176
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Forskningsöversikt (refereegranskat)abstract
- The redox-active tyrosine residue (Y-Z) plays a crucial role in the mechanism of the water oxidation. Metalloradical electron paramagnetic resonance (EPR) signals reflecting the light-induced Y-Z center dot in magnetic interaction with the CaMn4-cluster in the particular S-state, Y-Z center dot S-X intermediates, have been found in intact photosystem II. These so-called split EPR signals are induced by illumination at cryogenic temperatures and provide means to both study the otherwise transient Y-Z center dot and to probe the S-states with EPR spectroscopy. The illumination used for signal induction grouped the observed split EPR signals in two categories: (i) Y-Z in the lower S-states was oxidized by P680(+) formed via charge separation, while (ii) Y-Z in the higher S-states was oxidized by an excited, highly oxidizing Mn species. Applied mechanistic studies of the Y-Z center dot S-X intermediates in the different S-states are reviewed and compared to investigations in photosystem II at physiological temperature. Addition of methanol induced S-state characteristic changes in the split signals' formation which reflect changes in the magnetic coupling within the CaMn4-cluster due to methanol binding. The pH titration of the split EPR signals, on the other hand, could probe the proton-coupled electron transfer properties of the Y-Z oxidation. The apparent pK (a)s found for decreased split signal induction were interpreted in the fate of the phenol proton.
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| 5. |
- Havelius, Kajsa G. V., et al.
(författare)
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pH Dependent Competition between YZ and YD in Photosystem II Probed by Illumination at 5 K
- 2007
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Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 46:26, s. 7865-7874
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Tidskriftsartikel (refereegranskat)abstract
- The photosystem II (PSII) reaction center contains two redox active tyrosines, YZ and YD, situated on the D1 and D2 proteins, respectively. By illumination at 5 K, oxidation of YZ in oxygen-evolving PSII can be observed as induction of the Split S1 EPR signal from YZ* in magnetic interaction with the CaMn4 cluster, whereas oxidation of YD can be observed as the formation of the free radical EPR signal from YD*. We have followed the light induced induction at 5 K of the Split S1 signal between pH 4-8.5. The formation of the signal, that is, the oxidation of YZ, is pH independent and efficient between pH 5.5 and 8.5. At low pH, the split signal formation decreases with pKa approximately 4.7-4.9. In samples with chemically pre-reduced YD, the pH dependent competition between YZ and YD was studied. Only YZ was oxidized below pH 7.2, but at pH above 7.2, the oxidation of YD became possible, and the formation of the Split S1 signal diminished. The onset of YD oxidation occurred with pKa approximately 8.0, while the Split S1 signal decreased with pKa approximately 7.9 demonstrating that the two tyrosines compete in this pH interval. The results reflect the formation and breaking of hydrogen bonds between YZ and D1-His190 (HisZ) and YD and D2-His190 (HisD), respectively. The oxidation of respective tyrosine at 5 K demands that the hydrogen bond is well-defined; otherwise, the low-temperature oxidation is not possible. The results are discussed in the framework of recent literature data and with respect to the different oxidation kinetics of YZ and YD.
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| 6. |
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| 7. |
- Havelius, Kajsa G. V., et al.
(författare)
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The formation of the split EPR signal from the S-3 state of Photosystem II does not involve primary charge separation
- 2011
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Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier BV. - 0005-2728 .- 1879-2650 .- 0006-3002 .- 1878-2434. ; 1807:1, s. 11-21
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Tidskriftsartikel (refereegranskat)abstract
- Metalloradical EPR signals have been found in intact Photosystem II at cryogenic temperatures. They reflect the light-driven formation of the tyrosine Z radical (Y-z(center dot)) in magnetic interaction with the CaMn4 cluster in a particular S state. These so-called split EPR signals, induced at cryogenic temperatures, provide means to study the otherwise transient Y-z(center dot) and to probe the S states with EPR spectroscopy. In the S-0 and S-1 states, the respective split signals are induced by illumination of the sample in the visible light range only. In the S-3 state the split EPR signal is induced irrespective of illumination wavelength within the entire 415-900 nm range (visible and near-IR region) [Su, J. H., Havelius, K. G. V., Ho, F. M., Han, G., Mamedov, F., and Styring, S. (2007) Biochemistry 46. 10703-10712]. An important question is whether a single mechanism can explain the induction of the Split S-3 signal across the entire wavelength range or whether wavelength-dependent mechanisms are required. In this paper we confirm that the Y-z(center dot) radical formation in the S-1 state, reflected in the Split S-1 signal, is driven by P680-centered charge separation. The situation in the S-3 state is different. In Photosystem II centers with pre-reduced quinone A (Q(A)), where the P680-centered charge separation is blocked, the Split S-3 EPR signal could still be induced in the majority of the Photosystem II centers using both visible and NIR (830 nm) light. This shows that P680-centered charge separation is not involved. The amount of oxidized electron donors and reduced electron acceptors (Q(A)(-)) was well correlated after visible light illumination at cryogenic temperatures in the S-1 state. This was not the case in the S-3 state, where the Split S-3 EPR signal was formed in the majority of the centers in a pathway other than P680-centered charge separation. Instead, we propose that one mechanism exists over the entire wavelength interval to drive the formation of the Split S-3 signal. The origin for this, probably involving excitation of one of the Mn ions in the CaMn4 cluster in Photosystem II, is discussed.
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| 8. |
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| 9. |
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| 10. |
- Leidel, Nils, et al.
(författare)
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Electronic Structure of an [FeFe] Hydrogenase Model Complex in Solution Revealed by X-ray Absorption Spectroscopy Using Narrow-Band Emission Detection
- 2012
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 134:34, s. 14142-14157
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution X-ray absorption spectroscopy with narrow-band X-ray emission detection, supported by density functional theory calculations (XAES-DFT), was used to study a model complex, ([Fe-2(mu-adt)(CO)(4)(PMe3)(2)] (1, adt = S-CH2-(NCH2Ph)-CH2-S), of the [FeFe] hydrogenase active site. For 1 in powder material (1(powder)), in MeCN solution (1'), and in its three protonated states (1H, 1Hy, 1HHy; H denotes protonation at the adt-N and Hy protonation of the Fe-Fe bond to form a bridging metal hydride), relations between the molecular structures and the electronic configurations were determined. EXAFS analysis and DFT geometry optimization suggested prevailing rotational isomers in MeCN, which were similar to the crystal structure or exhibited rotation of the (CO) ligands at Fe1 (1(CO), 1Hy(CO)) and in addition of the phenyl ring (1H(CO,ph), 1HHy(CO,ph)), leading to an elongated solvent-exposed Fe-Fe bond. Isomer formation, adt-N protonation, and hydride binding caused spectral changes of core-to-valence (pre-edge of the Fe K-shell absorption) and of valence-to-core (K beta(2,5) emission) electronic transitions, and of K alpha RIXS data, which were quantitatively reproduced by DFT. The study reveals (1) the composition of molecular orbitals, for example, with dominant Fe-d character, showing variations in symmetry and apparent oxidation state at the two Fe ions and a drop in MO energies by similar to 1 eV upon each protonation step, (2) the HOMO-LUMO energy gaps, of similar to 2.3 eV for 1(powder) and similar to 2.0 eV for 1', and (3) the splitting between iron d(z(2)) and d(x(2-)y(2)) levels of similar to 0.5 eV for the nonhydride and similar to 0.9 eV for the hydride states. Good correlations of reduction potentials to LUMO energies and oxidation potentials to HOMO energies were obtained. Two routes of facilitated bridging hydride binding thereby are suggested, involving ligand rotation at Fe1 for 1Hy(CO) or adt-N protonation for 1HHy(CO,ph). XAES-DFT thus enables verification of the effects of ligand substitutions in solution for guided improvement of [FeFe] catalysts.
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| 11. |
- Leidel, Nils, et al.
(författare)
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High-valent [MnFe] and [FeFe] cofactors in ribonucleotide reductases
- 2012
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Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier BV. - 0005-2728 .- 1879-2650. ; 1817:3, s. 430-444
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Tidskriftsartikel (refereegranskat)abstract
- Ribonucleotide reductases (RNRs) are essential for DNA synthesis in most organisms. In class-Ic RNR from Chlamydia trachomatis (Ct), a MnFe cofactor in subunit R2 forms the site required for enzyme activity, instead of an FeFe cofactor plus a redox-active tyrosine in class-la RNRs, for example in mouse (Mus musculus, Mm). For R2 proteins from Ct and Mm, either grown in the presence of, or reconstituted with Mn and Fe ions, structural and electronic properties of higher valence MnFe and FeFe sites were determined by X-ray absorption spectroscopy and complementary techniques, in combination with bond-valence-sum and density functional theory calculations. At least ten different cofactor species could be tentatively distinguished. In Cr R2, two different Mn(IV)Fe(III) site configurations were assigned either L4MnIV(mu O)(2)(FeL4)-L-III (metal-metal distance of similar to 2.75 angstrom, L = ligand) prevailing in metal-grown R2, or L4MnIV(mu O)(mu OH)(FeL4)-L-III (similar to 2.90 angstrom) dominating in metal-reconstituted R2. Specific spectroscopic features were attributed to an Fe(IV)Fe(III) site (similar to 2.55 angstrom) with a L4FeIV(mu O)(2)(FeL3)-L-III core structure. Several Mn,Fe(III)Fe(III) (similar to 2.9-3.1 angstrom) and Mn,Fe(III)Fe(II) species (similar to 3.3-3.4 angstrom) likely showed 5-coordinated Mn(III) or Fe(III). Rapid X-ray photoreduction of iron and shorter metal-metal distances in the high-valent states suggested radiation-induced modifications in most crystal structures of R2. The actual configuration of the MnFe and FeFe cofactors seems to depend on assembly sequences, bound metal type, valence state, and previous catalytic activity involving subunit RI. In Ct R2, the protonation of a bridging oxide in the Mn-IV(mu O)(mu OH)Fe-III core may be important for preventing premature site reduction and initiation of the radical chemistry in R1.
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| 12. |
- Leidel, Nils, et al.
(författare)
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Site-Selective X-ray Spectroscopy on an Asymmetric Model Complex of the [FeFe] Hydrogenase Active Site
- 2012
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 51:8, s. 4546-4559
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Tidskriftsartikel (refereegranskat)abstract
- The active site for hydrogen production in [FeFe] hydrogenase comprises a diiron unit. Bioinorganic chemistry has modeled important features of this center, aiming at mechanistic understanding and the development of novel catalysts. However, new assays are required for analyzing the effects of ligand variations at the metal ions. By high-resolution X-ray absorption spectroscopy with narrow-band X-ray emission detection (XAS/XES = XAES) and density functional theory (DFT), we studied an asymmetrically coordinated [FeFe] model complex, [(CO)(3)Fe(I)1-(bdtCl(2))-Fe-2(I)(CO)(Ph2P-CH2-NCH3-CH2-PPh2)] (1, bdt = benzene-1,2-dithiolate), in comparison to iron-carbonyl references. K beta emission spectra (K beta(1,3), K beta') revealed the absence of unpaired spins and the low-spin character for both Fe ions in 1. In a series of low-spin iron compounds, the K beta(1,3) energy did not reflect the formal iron oxidation state, but it decreases with increasing ligand field strength due to shorter iron-ligand bonds, following the spectrochemical series. The intensity of the valence-to-core transitions (K beta(2,5)) decreases for increasing Fe-ligand bond length, certain emission peaks allow counting of Fe-CO bonds, and even molecular orbitals (MOs) located on the metal-bridging bdt group of 1 contribute to the spectra. As deduced from 3d -> 1s emission and 1s -> 3d absorption spectra and supported by DFT, the HOMO-LUMO gap of 1 is about 2.8 eV. K beta-detected XANES spectra in agreement with DFT revealed considerable electronic asymmetry in 1; the energies and occupancies of Fe-d dominated MOs resemble a square-pyramidal F(0) for Fe1 and an octahedral Fe(II) for Fe2. EXAFS spectra for various K beta emission energies showed considerable site-selectivity; approximate structural parameters similar to the crystal structure could be determined for the two individual iron atoms of 1 in powder samples. These results suggest that metal site- and spin-selective XAES on [FeFe] hydrogenase protein and active site models may provide a powerful tool to study intermediates under reaction conditions.
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| 13. |
- Sjöholm, Johannes, et al.
(författare)
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Effects of pH on the S3 State of the Oxygen Evolving Complex in Photosystem II Probed by EPR Split Signal Induction
- 2010
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Ingår i: Biochemistry. - Easton : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 49:45, s. 9800-9808
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Tidskriftsartikel (refereegranskat)abstract
- The electrons extracted from the CaMn4 cluster during water oxidation in photosystem II are transferred to P-680(+) via the redox-active tyrosine D1-Tyr161 (Y-z). Upon Y-z oxidation a proton moves in a hydrogen bond toward D1-His190 (His(z)). The deprotonation and reprotonation mechanism of Y-z-OH/Y-z-O is of key importance for the catalytic turnover of photosystem II. By light illumination at liquid helium temperatures (similar to 5 K) Y-z can be oxidized to its neutral radical, Y-z(center dot). This can be followed by the induction of a split EPR signal from Y-z(center dot) in a magnetic interaction with the CaMn4 cluster, offering a way to probe for Y-z oxidation in active photosystem II. In the S-3 state, light in the near-infrared region induces the split S-3 EPR signal, S-2'Y-z(center dot). Here we report on the pH dependence for the induction of S-2'Y-z(center dot) between pH 4.0 and pH 8.7. At acidic pH the split S-3 EPR signal decreases with the apparent pK(a) (pK(app)) similar to 4.1. This can be correlated to a titration event that disrupts the essential H-bond in the Y-z-His(z) motif. At alkaline pH, the split S-3 EPR signal decreases with the pK(app) similar to 7.5. The analysis of this pH dependence is complicated by the presence of an alkaline-induced split EPR signal (pK(app) similar to 8.3) promoted by a change in the redox potential of Y-z. Our results allow dissection of the proton-coupled electron transfer reactions in the S-3 state and provide further evidence that the radical involved in the split EPR signals is indeed Y-z(center dot).
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| 14. |
- Sjöholm, Johannes, et al.
(författare)
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The S0 state of the water oxidizing complex in photosystem II : pH dependence of the EPR Split signal, induction and mechanistic implications
- 2009
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Ingår i: Biochemistry. - Easton : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 48:40, s. 9393-9404
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Tidskriftsartikel (refereegranskat)abstract
- Water oxidation in photosystem II is catalyzed by the CaMn4 cluster. The electrons extracted from the CaMn4 cluster are transferred to P-680(+) via the redox-active tyrosine residue D1-Tyr161 (Y-Z). The oxidation of Y-Z is coupled to a deprotonation creating the neutral radical Y-Z(center dot). Light-induced oxidation of Y-Z is possible down to extreme temperatures. This call be observed as a split EPR signal from Y-Z(center dot) in a magnetic interaction with the CaMn4 cluster, offering a way to probe for Y-Z oxidation in active PSII. Here we have used the split S-0 EPR signal to study the mechanism of Y-Z oxidation at 5 K in the S-0 state. The state of the hydrogen bond between Y-Z and its proposed hydrogen bond partner D1-His190 is investigated by varying the pH. The split S-0 EPR signal was induced by illumination at 5 K between pH 3.9 and pH 9.0. Maximum signal intensity was observed between pH 6 and pH 7. On both the acidic and alkaline sides the signal intensity decreased with the apparent pK(a)s (pK(app)) similar to 4.8 and similar to 7.9, respectively. The illumination protocol used to induce the split S-0 EPR signal also induces a mixed radical signal in the g similar to 2 region. One part of this signal decays with similar kinetics as the split S-0 EPR signal (similar to 3 min, at 5 K) and is easily distinguished from a stable radical originating from Car/Chi. We suggest that this fast-decaying radical originates from Y-Z(center dot). The pH dependence of the light-induced fast-decaying radical was measured in the same pH range. as for the split S-0 EPR signal. The pK(app) for the light-induced fast-decaying radical was identical at acidic pH (similar to 4.8). At alkaline pH the behavior was more complex. Between pH 6.6 and pH 7.7 the signal decreased with pK(app) similar to 7.2. However, above pH 7.7 the induction of the radical species was pH independent. We compare our results with the pH dependence of the split S-1 EPR signal induced at 5 K and the S-0 -> S-1 and S-1 -> S-2 transitions at room temperature. The result allows mechanistic conclusions concerning differences between the hydrogen bond pattern around Y-Z in the S-0 and S-1 states.
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| 15. |
- Sjöholm, Johannes, et al.
(författare)
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Visible light induction of an EPR split signal in photosystem II in the S2 state reveals the importance of charges in the oxygen evolving center during catalysis : a unifying model
- 2012
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Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 51:10, s. 2054-2064
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Tidskriftsartikel (refereegranskat)abstract
- Cryogenic illumination of Photosystem II (PSII) can lead to the trapping of the metastable radical Y-z(center dot), the radical form of the redox-active tyrosine residue D1-Tyr161 (known as Y-z). Magnetic interaction between this radical and the CaMn4 cluster of PSII gives rise to so-called split electron paramagnetic resonance (EPR) signals with characteristics that are dependent on the S state. We report here the observation and characterization of a split EPR signal that can be directly induced from PSII centers in the S-2 state through visible light illumination at 10 K. We further show that the induction of this split signal takes place via a Mn-centered mechanism, in the same way as when using near-infrared light illumination [Koulougliotis, D., et al. (2003) Biochemistry 42, 3045-3053]. On the basis of interpretations of these results, and in combination with literature data for other split signals induced under a variety of conditions (temperature and light quality), we propose a unified model for the mechanisms of split signal induction across the four S states (S-0, S-1, S-2, and S-3). At the heart of this model is the stability or instability of the Y-z(center dot)(D1-His190)(+) pair that would be formed during cryogenic oxidation of Y-Z. Furthermore, the model is closely related to the sequence of transfers of protons and electrons from the CaMn4, cluster during the S cycle and further demonstrates the utility of the split signals in probing the immediate environment of the oxygen-evolving center in PSII.
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| 16. |
- Su, Ji-Hu, et al.
(författare)
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Formation Spectra of the EPR Split Signals from the S(0), S(1), and S(3) States in Photosystem II Induced by Monochromatic Light at 5 K
- 2007
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Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 46:37, s. 10703-10712
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Tidskriftsartikel (refereegranskat)abstract
- The interaction EPR split signals from photosystem II (PSII) have been reported from the S0, S1, and S3 states. The signals are induced by illumination at cryogenic temperatures and are proposed to reflect the magnetic interaction between YZ* and the Mn4Ca cluster. We have investigated the formation spectra of these split EPR signals induced in PSII enriched membranes at 5 K using monochromatic laser light from 400 to 900 nm. We found that the formation spectra of the split S0, split S1, and split S3 EPR signals were quite similar, but not identical, between 400 and 690 nm, with maximum formation at 550 nm. The major deviations were found between 440 and 480 nm and between 580 and 680 nm. In the regions around 460 and 680 nm the amplitudes of the formation spectra were 25-50% of that at 550 nm. A similar formation spectrum was found for the S2-state multiline EPR signal induced at 0 degrees C. In general, the formation spectra of these signals in the visible region resemble the reciprocal of the absorption spectra of our PSII membranes. This reflects the high chlorophyll concentration necessary for the EPR measurements which mask the spectral properties of other absorbing species. No split signal formation was found by the application of infrared laser illumination between 730 and 900 nm from PSII in the S0 and S1 states. However, when such illumination was applied to PSII membranes poised in the S3 state, formation of the split S3 EPR signal was observed with maximum formation at 740 nm. The quantum yield was much less than in the visible region, but the application of intensive illumination at 830 nm resulted in accumulation of the signal to an amplitude comparable to that obtained with illumination with visible light. The split S3 EPR signal induced by NIR light was much more stable at 5 K (no observable decay within 60 min) than the split S3 signal induced by visible light (50% of the signal decayed within 30 min). The split S3 signals induced by each of these light regimes showed the same EPR spectral features and microwave power saturation properties, indicating that illumination of PSII in the S3 state by visible light or by NIR light produces a similar configuration of YZ* and the Mn4Ca cluster.
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| 17. |
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