Intermediary Electron Acceptor Research Articles

Manipulating Charge Transfer and Transport via Intermediary Electron Acceptor Channels Enables 19.3% Efficiency Organic Photovoltaics

AbstractBalancing and improving the open‐circuit voltage (Voc) and short‐circuit current density (Jsc) synergistically has always been the critical point for organic photovoltaics (OPVs) to achieve high efficiencies. Here, this work adopts a ternary strategy to regulate the trade‐off between Voc and Jsc by combining the symmetric‐asymmetric non‐fullerene acceptors that differ at terminals and alkyl side chains to build the ternary OPV (TOPV). It is noticed that the reduced energy disorder and the enhanced luminescence efficiency of TOPV enable a mitigated energy loss and a higher Voc. Meanwhile, the third component, which is distributed at the host donor–acceptor interface, acts as the charge transport channel. The prolonged exciton lifetime, the boosted charge mobility, and the depressed charge recombination promote the TOPV to obtain an improved Jsc. Finally, with synergistically improved Voc and Jsc, the TOPV delivers an optimal efficiency of 19.26% (certified as 19.12%), representing one of the highest values reported so far.

Bicarbonate stimulates the electron donation from Mn2+ to [formula omitted] in isolated D1/D2/cytochrome b559 complex

Influence of bicarbonate on the efficiency of the electron donation from Mn2+ to P680+ in isolated D1/D2/cytochrome b559 complex was investigated. All the experiments were carried out in a medium depleted of HCO3-/CO2. Kinetics of photoinduced absorbance changes (ΔA) at different wavelengths and decrease of chlorophyll fluorescence yield (−ΔF) related to photoaccumulation of reduced pheophytin, the intermediary electron acceptor of photosystem II (PSII), in the presence of Mn2+ under anaerobic conditions were measured. Addition of bicarbonate (1mM) increased the amplitude of these ΔA and −ΔF at least by a factor of 3. Measurements of the photoinduced ΔA, related to photooxidation of the primary electron donor of PSII, chlorophyll P680, were done in the presence of silicomolybdate as electron acceptor. These results show that the addition of 0.05mM Mn2+ alone or jointly with 1mM bicarbonate induces a 20% and 70%-decrease of the magnitude of the ΔA at 680nm. The effect of Mn2+ (in the presence and absence of bicarbonate) was completely eliminated by the addition of 12mM EDTA. All these bicarbonate effects were not observed if MgCl2 or formate were used instead of MnCl2 and bicarbonate, respectively. In the absence of Mn2+, bicarbonate induced none of the mentioned above effects (increase of photoaccumulation of reduced pheophytin and decrease of photooxidation of P680). The presented data suggest that bicarbonate stimulates the electron donation from Mn2+ to D1/D2/cyt b559 reaction center evidently due to formation of easily oxidizable Mn-bicarbonate complexes.

Steady-State FTIR Spectra of the Photoreduction of QA and QB in Rhodobacter sphaeroides Reaction Centers Provide Evidence against the Presence of a Proposed Transient Electron Acceptor X between the Two Quinones

In the reaction center (RC) of the photosynthetic bacterium Rhodobacter sphaeroides, two ubiquinone molecules, QA and QB, play a pivotal role in the conversion of light energy into chemical free energy by coupling electron transfer to proton uptake. In native RCs, the transfer of an electron from QA to QB takes place in the time range of 5-200 micros. On the basis of time-resolved FTIR step-scan measurements in native RCs, a new and unconventional mechanism has been proposed in which QB- formation precedes QA- oxidation [Remy, A., and Gerwert, K. (2003) Nat. Struct. Biol. 10, 637-644]. The IR signature of the proposed transient intermediary electron acceptor (denoted X) operating between QA and QB has been recently measured by the rapid-scan technique in the DN(L210) mutant RCs, in which the QA to QB electron transfer is slowed 8-fold compared to that in native RCs. This IR signature has been reported as a difference spectrum involving states X+, X, QA, and QA- [Hermes, S., et al. (2006) Biochemistry 45, 13741-13749]. Here, we report the steady-state FTIR difference spectra of the photoreduction of either QA or QB measured in both native and DN(L210) mutant RCs in the presence of potassium ferrocyanide. In these spectra, the CN stretching marker modes of ferrocyanide and ferricyanide allow the extent of the redox reactions to be quantitatively compared and are used for a precise normalization of the QA-/QA and QB-/QB difference spectra. The calculated QA- QB/QA QB- double-difference spectrum in DN(L210) mutant RCs is closely equivalent to the reported QA- X+/QA X spectrum in the rapid-scan measurement. We therefore conclude that species X+ and X are spectrally indistinguishable from QB and QB-, respectively. Further comparison of the QA- QB/QA QB- double-difference spectra in native and DN(L210) RCs also allows the possibility that QB- formation precedes QA- reoxidation to be ruled out for native RCs.

Time-resolved spectroscopy at 10 K of the Photosystem II reaction center; deconvolution of the red absorption band

The red absorption band of isolated Photosystem II reaction centers at 10 K and its flash-induced changes due to formation of the primary radical pair, P +I −, and subsequent recombination to the triplet state, P T, were analysed. It is concluded that the primary electron donor P consists of two chlorophyll molecules, which show exciton interaction and are responsible both for the main long wavelength absorption band at 679.6 nm and a shoulder at 683.6 nm (these wavelengths vary somewhat between preparations). The angle between their Q y transitions is about 60° and the exciton splitting about 85 cm −1. Both bands are bleached in the states P + and P T and replaced by a single band at 678 nm, attributed to one of the two chlorophylls, while the oxidized or triplet state is localized on the other, contributing little absorbance in the Q y region. The pheophytin which acts as the intermediary electron acceptor I has a 6.4 nm wide Q y band at 676.5 nm with about 2 3 of the amplitude of a chlorophyll Q y band. For the remaining pigments gaussian curve fitting of the absorption spectrum led to the following tentative assignments. The Q y band of the other pheophytin is spectrally indistinguishable from that of I. All accessory chlorophylls absorb in the 670 nm region; a good fit was obtained with two 4.7 nm wide bands peaking at 672.6 and 669.4 nm, respectively, and at least one Q y band at slightly shorter wavelength. However, neither an artifactual origin of the latter band nor the presence of two such bands can be excluded. The pigments of the PS II reaction center show little exciton interaction.

Characterization of bonding interactions of the intermediary electron acceptor in the reaction center of Photosystem II by FTIR spectroscopy

Molecular changes associated with the photoreduction of the pheophytin a intermediary electron acceptor in films of Photosystem II reaction center (D1D2 RC) were characterized by FTIR spectroscopy. Upon accumulation at 240 K of the photoreduced acceptor, three negative carbonyl bands are observed at 1739 cm −1, 1721 cm −1 and 1677 cm −1 in the light-minus-dark FTIR spectrum of D1D2 RC. The redox-induced FTIR spectrum of the pheophytin a anion generated electrochemically in tetrahydrofuran shows only two negative bands at 1743 cm −1 and 1706 cm −1 which are assigned to changes of absorption of the 10a-ester C=O and 9-keto C=O, respectively. These assignments are based upon the comparison between FTIR data obtained on radicals of pheophytin a and its pyroderivative lacking the 10a-ester C=O. Thus, the 1677 cm −1 band observed in vivo reflects an interacting 9-keto C=O in D1D2 RC. The close similarity observed between: (i) FTIR spectra obtained on Photosystem II and Rps. viridis reaction centers and (ii) amino-acid sequences of the L and D1 polypeptides leads to the assignment of the 1721 cm −1 band in D1D2 RC to a protein-bound 10a-ester C=O of the acceptor and the 1739 cm −1 band to a contribution from the protonated carboxylic group of Glu D1-130 which is proposed to be H-bonded to the 9-keto C=O of the pheophytin acceptor, in the same way as in the Rps. viridis reaction center, Glu L104 is interacting with the 9-keto C=O of H L. The FTIR data indicate that the interactions of the 9-keto C=O and of the 10a-ester of the intermediary acceptor with the protein are stronger in D1D2 RC than in Rps. viridis. These stronger interactions could account, at least in part, for the difference in accessibility to 1H- 2H exchange of the H-bonded proton of the Glu D1-130 side-chain in D1D2 RC compared to Rps. viridis reaction center.

Burning of a narrow spectral hole at 1.7 K in the absorbance band of the primary electron donor of Rhodopseudomonas viridis reaction centers with blocked electron transfer

Burning of spectral holes within the absorption band of the primary electron donor P in reaction centers of Rhodopseudomonas viridis is studied using selective excitation at 1014 nm and 1.7 K. A narrow (≤ 1.0 cm −1) spectral hole with relatively weak electron-phonon and vibronic coupling is burned if the intermediary electron acceptor I is pre-reduced. The hole is broadened up to ∼90–150 cm −1 if I is neutral and the electron-transfer chain is open. Broadening of the hole is discussed in terms of the fast (∼70–110 fs) electron hopping between P* and P +BL − (BL, bacteriochlorophyll monomer in L protein subunit).

Isolation of a photosystem II reaction center consisting of D-1 and D-2 polypeptides and cytochrome b -559

A photosystem II reaction center complex consisting of D-1 and D-2 polypeptides and cytochrome b-559 was isolated from spinach grana thylakoids, treated with 4% (wt/vol) Triton X-100, by ion-exchange chromatography using DEAE-Toyopearl 650S. The isolated complex appears to contain five chlorophyll a, two pheophytin a, one beta-carotene, and one or two cytochrome b-559 heme(s) (molar ratio) and exhibits a reversible absorbance change attributable to the photochemical accumulation of reduced pheophytin typical for the intermediary electron acceptor of photosystem II reaction center. These results strongly suggest that the site of primary charge separation in photosystem II is located on the heterodimer composed of D-1 and D-2 subunits.

Light-induced Fourier transform infrared (FTIR) spectroscopic investigations of primary reactions in photosystem I and photosystem II

Light-induced Fourier transform infrared (FTIR) difference spectroscopy has been applied for the first time to primary reactions in green plant photosynthesis. Photooxidation of the primary electron donor (P700) in photosystem I-enriched particles as well as in thylakoids, and photoreduction of the pheophytin (Pheo) intermediary electron acceptor in photosystem II-enriched particles, have led to reproducible difference spectra. In the spectral range investigated (between 1800 and 1000 cm −1) several bands are tentatively attributed to changes in intensity and position of the keto and ester carbonyl vibrations of the chlorophyll or Pheo molecule(s) involved. For some of these groups, possible interpretations in terms of changes of their environment or type of bonding to the protein are given. The intensity of the differential features in the amide I and amide II spectral region allows the exclusion of the eventuality of large protein conformational changes occurring upon primary charge separation.

LIGHT‐INDUCED FOURIER TRANSFORM INFRARED SPECTROSCOPIC INVESTIGATIONS OF THE INTERMEDIARY ELECTRON ACCEPTOR REDUTION IN BACTERIAL PHOTOSYNTHESIS

Abstract— Molecular changes associated with the light‐induced reduction of the intermediary electron acceptor I (bacteriopheophytin, BPh) in bacterial photosynthesis were studied by means of Fourier transform infrared (FTIR) difference spectroscopy. Chromatophore membranes and reconstituted reaction centers (RCs) of Rhodopseudomonas viridis were prereduced with sodium dithionite and illuminated in order to trap photochemically the state I−. Fourier transform infrared spectra of these samples were recorded before, during and after illumination, with an accuracy better than 10−3 absorbance units. Difference spectra of I− in chromatophores and in RCs closely correspond to each other. In the carbonyl stretching frequency region between 1640 and 1750 cm−1, bands are tentatively attributed to a shift (from 1713 to 1683 cm−1) of a keto carbonyl group, a change of an acetyl carbonyl grou at 1656 cm−1 and a decrease in absorbance strength of ester carbonyl groups (at 1746 and 1732 cm−P) after reduction of I. These groups likely belong to the BPh molecule, although at least one of the ester carbonyls could be assigned to an amino acid side chain. The absence of strong bands in the amide I and amide II region excludes large protein conformational changes associated with I reduction.

Evidence for two tightly bound iron-quinones in the electron acceptor complex of photosystem II

The EPR signal of the iron-quinone electron acceptor of photosystem II in higher plant chloroplasts is normally difficult to observe. If the preparation is washed with formate to remove bound CO 2 the signal becomes larger. Using formate-washed photosystem II particles from peas the redox potential of the ironquinone complex has been determined. Two waves are observed in the titrations at E m ~0 and ~ −250 mV. The ability to reduce the pheophytin intermediary electron acceptor by illumination at 200 K is largely associated with the −250 mV step. It is suggested that there are two tightly bound quinones in the reaction centre and that the low-potential component is Qa. It is unlikely that the high-potential component is the gating quinone (Qb) as it is not displaced by DCMU and is present in Chlamydomonas reinhardtii preparations which lack the gating quinone.

Electron acceptors of photosystem 2 in the cyanobacterium Phormidium laminosum

Photosystem 2 preparations with very high rates of oxygen evolution from the thermophilic cyanobacterium Phormidium laminosum have been studied by EPR spectrometry. In the presence of DCMU the g = 1.82 signal of the iron—quinone electron acceptor (Q) can be observed. It is proposed that DCMU is necessary to disrupt a magnetic interaction, between the semiquinone forms of Q and the secondary acceptor B, which otherwise prevents detection of the Q −Fe signal. A doublet EPR signal arising from magnetic interaction between Q −Fe and the reduced intermediary electron acceptor pheophytin (I −), and a spin-polarized triplet signal assumed to arise from the back reaction between I − and P680 + can also be seen. Preliminary redox titrations of Q reduction have been carried out, indicating E m ⋍ 0 mV.

Linear dichroism and orientation of pheophytin, the intermediary electron acceptor in photosystem II reaction centers

Specific molecular organization of reaction centers which provide the high quantum efficiency of lightinduced charge separation in photosynthesis has been intensively studied by polarized light spectroscopy [l-lo]. Orientation of electric transition dipole moments corresponding to the primary electron donors and acceptors in the reaction centers of photosynthetic bacteria and photosystem I of higher plants has been analyzed on the basis of linear dichroism (LD) and photoselection data [l-6]. The transition dipole of the 820 nm absorption band of the oxidized primary electron donor, chlorophyll P680”, in the reaction center of photosystem II (PS II) lies almost parallel to the thylakoid membrane plane [ 71. Here, we report LD measurements of lightinduced absorbance changes related to photoreduction of pheophytin, the intermediary electron acceptor of PS II, in oriented subchloroplast particles. Light-dark difference absorption spectra of pheophytin photoreduction in reaction centers of PS II reported here, consist of 2 components: (1) Corresponding to light-induced reduction of pheophytin itself with an absorption band near 680 nm and the Q, transition dipole almost perpendicular to the membrane plane. The Qx transition dipole of the pheophytin molecule is approximately parallel to thylakoid membrane plane. (2) Representing a blue shift of a chlorophyll absorption band centered at 680 nm and polarized approximately parallel to the membrane, which has been tentatively ascribed to P680 or another chlorophyll molecule within or near to the reaction center of PS II. Sub-chloroplast particles enriched in PS II reaction centers were prepared as in [8]. The procedure involved treatment of pea chloroplasts with detergents digltonin (0.4%) and Triton X-100 (O.l%), and the particles were sedimented by centrifugation at 20 000 X g [8]. This preparation is designated DT-20. Samples were oriented in polyacrylamide gel as in [4]. A rectangular sample was squeezed 1 S-fold along its x-axis and elongated in z-direction, while the y-size was kept constant. Light-dark absorbance changes, AA, were measured with the apparatus in [ 81. A non-polarized beam of saturating actinic red light passed through the sample along its x-axis; the monochromatic, linearly polarized measuring beam was propagated in y-direction. Two values of absorbance changes, U, and L\A,, were registered when the electric vector of the measuring light beam was parallel to the x-axis of the sample or to its z-axis, respectively [ 51. Light-induced accumulation of reduced pheophytin in the reaction center of PS II is observed at low redox potential, El,, when the electron acceptor Q is chemically reduced in the dark [8]. To fullfil these conditions, the polymerized samples were pre-incubated at 2°C in buffer solutions with dithionite at Eh = -450 mV as in [9].

Interaction between the intermediary electron acceptor (pheophytin) and a possible plastoquinone-iron complex in photosystem II reaction centers

Photoreduction of the intermediary electron acceptor, pheophytin (Pheo), in photosystem II reaction centers of spinach chloroplasts or subchloroplast particles (TSF-II and TSF-IIa) at 220 K and redox potential E(h) = -450 mV produces an EPR doublet centered at g = 2.00 with a splitting of 52 G at 7 K in addition to a narrow signal attributed to Pheo([unk]) (g = 2.0033, DeltaH approximately 13 G). The doublet is eliminated after extraction of lyophilized TSF-II with hexane containing 0.13-0.16% methanol but is restored by reconstitution with plastoquinone A (alone or with beta-carotene) although not with vitamin K(1). TSF-II and TSF-IIa are found to contain approximately 2 nonheme Fe atoms per reaction center. Incubation with 0.55 M LiClO(4) plus 2.5 mM o-phenanthroline (but not with 0.55 M LiClO(4) alone) decreases this value to approximately 0.6 and completely eliminates the EPR doublet, but photoreduction of Pheo is not significantly affected. Partial restoration of the doublet (about 25%) was achieved by subsequent incubation with 0.2 mM Fe(2+), but not with either Mn(2+) or Mg(2+). The Fe removal results in the development of a photoinduced EPR signal (g = 2.0044 +/- 0.0003, DeltaH = 9.2 +/- 0.5 G) at E(h) = 50 mV, which is not observed after extraction with 0.16% methanol in hexane. It is ascribed to plastosemiquinone no longer coupled to Fe in photosystem II reaction centers. The results show that a complex of plastoquinone and Fe can act as the stable "primary" electron acceptor in photosystem II reaction centers and that the interaction of its singly reduced form with the reduced intermediary acceptor, Pheo([unk]), is responsible for the EPR doublet.

Nanosecond fluorescence and absorbance changes in photosystem II at low redox potential: Pheophytin as an intermediary electron acceptor

The electron-acceptor system in photosystem II (PS II) reaction centers of green plants is similar to that in bacterial reaction centers. In both kinds of reaction centers, a quinone (Q) acts as an early electron acceptor (ubiquinone or menaquinone in bacterial [l] and plastoquinone in PS II [2,3]). If the quinone is in the reduced form, continuous illumination causes the accumulation of a bacteriopheophytin (BPh) radical anion (BPh’-) in bacterial reaction centers [4-61 and of a pheophytin (Ph) radical anion (Ph.-) in PS II [7-l 31. The radical anions of BPh [5,14,15] and of Ph [12,13] participate inan exchange interaction with the radical anion of the quinone, coupled to a non-heme Fe atom, indicating that the distance between (B)Ph and Q is very short. The reduction of the quinone increases the formation of pigment triplet states in both bacteria [4,16] and PS II [29]. This is inhibited upon photoaccumulation of BPH’or Ph.-. Reduction of Q also is accompanied by the appearance of ns delayed fluorescence, which decreases during photoaccumulation of BPh’[4] or Ph.[9]. The delayed fluorescence has a characteristic magnetic field dependence in bacterial reaction centers [ 17-191 and in PS II [ 181 and an activation energy of 0.04-0.12 eV [4,9]. When the quinone in bacterial reaction centers is reduced, flash excitation generates a state ‘PF’, which has a lifetime of -10 ns [ 16,201. The transient state

Effect of photoreduction of the photosystem-II intermediary electron acceptor (pheophytin) on triplet state of carotenoids

FEBS LettersVolume 118, Issue 1 p. 123-126 Full-length articleFree Access Effect of photoreduction of the photosystem-II intermediary electron acceptor (pheophytin) on triplet state of carotenoids V.V. Klimov, V.V. Klimov Charles F. Kettering Research Laboratory, Yellow Springs, OH 45387, USA Permanent address: Institute of Photosynthesis, USSR Academy of Sciences, Poustchino (Moscow Region), USSR Search for more papers by this authorB. Ke, Corresponding Author B. Ke Charles F. Kettering Research Laboratory, Yellow Springs, OH 45387, USAAddress correspondence and reprint requests to B. K.Search for more papers by this authorE. Dolan, E. Dolan Charles F. Kettering Research Laboratory, Yellow Springs, OH 45387, USASearch for more papers by this author V.V. Klimov, V.V. Klimov Charles F. Kettering Research Laboratory, Yellow Springs, OH 45387, USA Permanent address: Institute of Photosynthesis, USSR Academy of Sciences, Poustchino (Moscow Region), USSR Search for more papers by this authorB. Ke, Corresponding Author B. Ke Charles F. Kettering Research Laboratory, Yellow Springs, OH 45387, USAAddress correspondence and reprint requests to B. K.Search for more papers by this authorE. Dolan, E. Dolan Charles F. Kettering Research Laboratory, Yellow Springs, OH 45387, USASearch for more papers by this author First published: August 25, 1980 https://doi.org/10.1016/0014-5793(80)81232-4Citations: 22 AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL References 1 J. Amesz, L.N.M. Duysens, Primary Processes of Photosynthesis. J. Barber Top. Photosynth. 2, (1977), Elsevier/North-Holland Amsterdam, New York 149– 185. 2 D.B. Knaff, R. Malkin, Curr. Top. Bioenerget., 7, (1978), 139– 172. 3 V.V. Klimov, A.V. Klevanik, V.A. Shuvalov, A.A. Krasnovsky, FEBS Lett., 82, (1977), 183– 186. 4 A.V. Klevanik, V.V. 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Effect of detergents on the reliability of a chemical assay for P-700

A chemical assay for P-700 was developed using 0.36 mM potassium ferricyanide as oxidant and 1.6 mM sodium ascorbate as reductant. The major difference from other chemical assays for P-700 is procedural. The method is designed to take advantage of the availability of microprocessor-linked spectrophotometers to obtain greater accuracy by minimizing the spectral changes due to irreversibly oxidized antenna chlorophyll molecules. The value measured for the P-700 concentration in a sample of chloroplasts was not changed by the presence of EDTA, Mg 2+ or sucrose in the assayed solution. Similarly, half of the detergents tested (Triton X-100, Nonidet P-40, digitonin, Deriphat 160, Miranol S2M-SF and Miranol M2M) did not alter the value when added to the chloroplasts. The remainder of the detergents examined caused a significant decrease or increase in the value for P-700 content. Sodium dodecyl sulfate, of particular interest due to its widespread use, caused a doubling in the amount of apparent P-700. This effect may be due to this detergent and some others enabling an additional long wavelength form of chlorophyll, possibly an intermediary electron acceptor in Photosystem I, to be chemically oxidized and reduced under the assay conditions.

EPR properties of an intermediary electron acceptor (pheophytin) in photosystem-II reaction centers at cryogenic temperatures

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Properties of the EPR spectrum of the intermediary electron acceptor (A 1) in several different photosystem I particle preparations

Properties of the EPR spectrum of the intermediary electron acceptor (A 1) in several different photosystem I particle preparations

Magnetic field-induced increase of the yield of (bacterio)chlorophyll emission of some photosynthetic bacteria and of Chlorella vulgaris

In photosynthetic bacteria, in which the iron-ubiquinone complex X is prereduced, a magnetic field induces an increase of the emission yield, which is correlated with the decrease in reaction center triplet yield reported previously (Hoff, A.J., Rademaker, H., van Grondelle, R. and Duysens, L.N.M. (1977) Biochim. Biophys. Acta 460, 547–554). Our results support the hypothesis that under these conditions charge recombination of the oxidized primary donor and the reduced primary acceptor predominantly generates the excited singlet state of the reaction center bacteriochlorophyll. In Chlorella vulgaris and spinach chloroplasts, at 120 K, the magnetic field has an effect similar to that found in bacteria, which suggests that an intermediary electron acceptor between P-680 and Q is present in Photosystem II also.

Kinetic and spectral properties of the intermediary electron acceptor A 1 in photosystem I : Subnanosecond spectroscopy

Kinetic and spectral properties of the intermediary electron acceptor A 1 in photosystem I : Subnanosecond spectroscopy