Excitonic interactions in the reaction centre of photosystem II studied by using circular dichroism

Short-Range Effects in the Special Pair of Photosystem II Reaction Centers: The Nonconservative Nature of Circular Dichroism

Photosystem II reaction centers extract electrons from water, providing the basis of oxygenic life on earth. Among the light-sensitive pigments of the reaction center, a central chlorophyll a dimer, known as the special pair, so far has escaped a complete theoretical characterization of its excited state properties. The close proximity of the special pair pigments gives rise to short-range effects that comprise a coupling between local and charge transfer (CT) excited states as well as other intermolecular quantum effects. Using a multiscale simulation and a diabatization technique, we show that the coupling to CT states is responsible for 45% of the excitonic coupling in the special pair. The other short-range effects cause a nonconservative nature of the circular dichroism spectrum of the reaction center by effectively rotating the electric transition dipole moments of the special pair pigments inverting and strongly enhancing their intrinsic rotational strength.

Assembly of D1/D2 complexes of photosystem II: Binding of pigments and a network of auxiliary proteins

Photosystem II (PSII) is the multi-subunit light-driven oxidoreductase that drives photosynthetic electron transport using electrons extracted from water. To investigate the initial steps of PSII assembly, we used strains of the cyanobacterium Synechocystis sp. PCC 6803 arrested at early stages of PSII biogenesis and expressing affinity-tagged PSII subunits to isolate PSII reaction center assembly (RCII) complexes and their precursor D1 and D2 modules (D1mod and D2mod). RCII preparations isolated using either a His-tagged D2 or a FLAG-tagged PsbI subunit contained the previously described RCIIa and RCII* complexes that differ with respect to the presence of the Ycf39 assembly factor and high light-inducible proteins (Hlips) and a larger complex consisting of RCIIa bound to monomeric PSI. All RCII complexes contained the PSII subunits D1, D2, PsbI, PsbE, and PsbF and the assembly factors rubredoxin A and Ycf48, but we also detected PsbN, Slr1470, and the Slr0575 proteins, which all have plant homologs. The RCII preparations also contained prohibitins/stomatins (Phbs) of unknown function and FtsH protease subunits. RCII complexes were active in light-induced primary charge separation and bound chlorophylls (Chls), pheophytins, beta-carotenes, and heme. The isolated D1mod consisted of D1/PsbI/Ycf48 with some Ycf39 and Phb3, while D2mod contained D2/cytochrome b559 with co-purifying PsbY, Phb1, Phb3, FtsH2/FtsH3, CyanoP, and Slr1470. As stably bound, Chl was detected in D1mod but not D2mod, formation of RCII appears to be important for stable binding of most of the Chls and both pheophytins. We suggest that Chl can be delivered to RCII from either monomeric Photosystem I or Ycf39/Hlips complexes.

Room temperature photooxidation of beta-carotene and peripheral chlorophyll in photosystem II reaction centre

Differential kinetic absorption spectra were measured during actinic illumination of photosystem II reaction centres and core complexes in the presence of electron acceptors silicomolybdate and ferricyanide. The spectra of samples with ferricyanide differ from those with both ferricyanide and silicomolybdate. Near-infrared spectra show temporary beta-carotene and peripheral chlorophyll oxidation during room temperature actinic illumination. Peripheral chlorophyll is photooxidized even after decay of beta-carotene oxidation activity and significant reduction of beta-carotene content in both reaction centres and photosystem II core complexes. Besides, new carotenoid cation is observed after about 1 s of actinic illumination in the reaction centres when silicomolybdate is present. Similar result was observed in PSII core complexes. HPLC analyses of illuminated reaction centres reveal several novel carotenoids, whereas no new carotenoid species were observed in HPLC of illuminated core complexes. Our data support the proposal that pigments of inner antenna are a sink of cations originating in the photosystem II reaction centre.

Spectral characteristics of PS II reaction centres: as isolated preparations and when integral to PS II core complexes

The discovery that the native PS II enzyme undergoes charge separation via an absorption extending to 730 nm has led us to re-examine the low-temperature absorption spectra of Nanba-Satoh PS II reaction centre preparations with particular focus on the long wavelength region. It is shown that these preparations do not exhibit absorption in the 700-730 nm region at 1.7 K. Absorption in the Nanba-Satoh type preparations analogous to the 'red tail' as observed in functional PS II core complexes is likely shifted to higher energy by >20 nm. Spectral changes associated with the stable reduction of pheo(a) in chemically treated reaction centre preparations are also revisited. Dithionite treatment of PS II preparations in the dark leads to changes of pigment-pigment and/or pigment-protein interactions, as evidenced by changes in absorption and CD spectra. Absorption and CD changes associated with stable Pheo(D1) photo-reduction in PS II core complexes and Nanba-Satoh preparations are compared. For Nanba-Satoh preparations, Q(y) bleaches are approximately 3x broader than in PS II core complexes and are blue-shifted by approximately 4 nm. These data are discussed in terms of current models of PS II, and suggest a need to consider protein-induced changes of some electronic properties of reaction centre pigments.

New multichannel kinetic spectrophotometer-fluorimeter with pulsed measuring beam for photosynthesis research

A multichannel kinetic spectrophotometer-fluorimeter with pulsed measuring beam and differential optics has been constructed for measurements of light-induced absorbance and fluorescence yield changes in isolated chlorophyll-proteins, thylakoids and intact cells including algae and photosynthetic bacteria. The measuring beam, provided by a short (2 micros) pulse from a xenon flash lamp, is divided into a sample and reference channel by a broad band beam splitter. The spectrum in each channel is analyzed separately by a photodiode array. The use of flash measuring beam and differential detection yields high signal-to-noise ratio (noise level of 2 x 10(-4) in absorbance units per single flash) with negligible actinic effect. The instrument covers a spectral range between 300 and 1050 nm with a spectral resolution of 2.1, 6.4 or 12.8 nm dependent on the type of grating used. The optical design of the instrument enables measuring of the difference spectra during an actinic irradiation of samples with continuous light and/or saturation flashes. The time resolution of the spectrophotometer is limited by the length of Xe flash lamp pulses to 2 micros.

Circular dichroism of the peripheral chlorophylls in photosystem II reaction centers revealed by electrochemical oxidation

Visible absorption spectra and circular dichroism (CD) of the red absorption band of isolated photosystem II reaction centers were measured at room temperature during progressive bleaching by electrochemical oxidation, in comparison with aerobic photochemical destruction, and with anaerobic photooxidation in the presence of the artificial electron acceptor silicomolybdate. Initially, selective bleaching of peripheral chlorophylls absorbing at 672 nm was obtained by electrochemical oxidation at +0.9 V, whereas little selectivity was observed at higher potentials. Illumination in the presence of silicomolybdate did not cause a bleaching but a spectral broadening of the 672-nm band was observed, apparently in response to the oxidation of carotene. The 672-nm absorption band is shown to exhibit a positive CD, which accounts for the 674-nm shoulder in CD spectra at low temperature. The origin of this CD is discussed in view of the observation that all CD disappears with the 680-nm absorption band during aerobic photodestruction.

Conformational changes and their role in non-radiative energy dissipation in photosystem II reaction centres

Accumulation of reduced pheophytin in photosystem II under illumination at low redox potential is known to be accompanied by a pronounced decrease of a chlorophyll fluorescence yield. Simultaneous measurement of this fluorescence quenching and absorbance changes in photosystem II reaction centres, in the presence of dithionite, showed each event to have a different temperature dependence. While fluorescence quenching was suppressed more than 20 times when measured at 77 K, pheophytin accumulation decreased only 5 times. At 77 K, the fluorescence was quenched considerably, but only in those reaction centres where reduced pheophytin had been accumulated at room temperature before sample freezing. This showed that the accumulation of reduced pheophytin above 240 K was accompanied by an additional, most probably conformational, change in the reaction centre that substantially enhanced non-radiative dissipation of excitation energy.

Molecular reorganization induced by Ca2+ of plant photosystem I reconstituted into phosphatidylglycerol liposomes

The interaction of divalent cations with biomembranes is important for a number of biological processes. In this study, the regulatory effect of Ca2+ on the interaction between plant spinach photosystem I (PSI) particles and negatively charged lipid phosphatidylglycerol (PG) was investigated by circular dichroism (CD) spectroscopy. It was found that in the absence of CaCl2, PG causes an increase in alpha-helix and a decrease in disordered conformations of protein secondary structures of PSI, the beta-sheet and turns being almost unaffected. Meanwhile, the same effect also enhances the excitonic interactions relating to Chl a and Chl b from the PSI core complex and external antenna light-harvesting complex (LHCI). By contrast, in the presence of CaCl2, PG hardly interferes with the structure of the proteins' skeleton of PSI, but it can depress the excitonic interactions for Chl b of LHCI and for PSI core complex Chl a at (-) 433.5 nm of the CD signal which is accompanied by a blue shift of its peak. It is most likely that the neutralization of the phosphate groups in the PSI-PG complex and the negative surface charges of PSI, and partial dehydration in the vicinity of the ester CO region of the PG polar head group by the Ca-ions modify the interaction between PSI and PG, thereby inducing molecular reorganization of protein and pigments within both the external antenna LHCI and PSI core complex in proteoliposomes.

Oxygen-evolving Photosystem II core complexes: a new paradigm based on the spectral identification of the charge-separating state, the primary acceptor and assignment of low-temperature fluorescence

We review our recent low-temperature absorption, circular dichroism (CD), magnetic CD (MCD), fluorescence and laser-selective measurements of oxygen-evolving Photosystem II (PSII) core complexes and their constituent CP 4 3, CP 47 and D1/D2/cytb(559) sub-assemblies. Quantitative comparisons reveal that neither absorption nor fluorescence spectra of core complexes are simple additive combinations of the spectra of the sub-assemblies. The absorption spectrum of the D1/D2/cytb(559) component embedded within the core complex appears significantly better structured and red-shifted compared to that of the isolated sub-assembly. A characteristic MCD reduction or 'deficit' is a useful signature for the central chlorins in the reaction centre. We note a congruence of the MCD deficit spectra of the isolated D1/D2/cytb(559) sub-assemblies to their laser-induced transient bleaches associated with P 680. A comparison of spectra of core complexes prepared from different organisms helps distinguish features due to inner light-harvesting assemblies and the central reaction-centre chlorins. Electrochromic spectral shifts in core complexes that occur following low-temperature illumination of active core complexes arise from efficient charge separation and subsequent plastoquinone anion (Q(A)(-)) formation. Such measurements allow determinations of both charge-separation efficiencies and spectral characteristics of the primary acceptor, Pheo(D1). Efficient charge separation occurs with excitation wavelengths as long as 700 nm despite the illuminations being performed at 1.7 K and with an extremely low level of incident power density. A weak, homogeneously broadened, charge-separating state of PSII lies obscured beneath the CP 47 state centered at 690 nm. We present new data in the 690-760 nm region, clearly identifying a band extending to 730 nm. Active core complexes show remarkably strong persistent spectral hole-burning activity in spectral regions attributable to CP 43 and CP 47. Measurements of homogeneous hole-widths have established that, at low temperatures, excitation transfer from these inner light-harvesting assemblies to the reaction centre occurs with approximately 70-270 ps(-1) rates, when the quinone acceptor is reduced. The rate is slower for lower-energy sub-populations of an inhomogeneously broadened antenna (trap) pigment. The complex low-temperature fluorescence behaviour seen in PSII is explicable in terms of slow excitation transfer from traps to the weak low-energy charge-separating state and transfer to the more intense reaction-centre excitations near 685 nm. The nature and origin of the charge-separating state in oxygen-evolving PSII preparations is briefly discussed.

Pathways and timescales of primary charge separation in the photosystem II reaction center as revealed by a simultaneous fit of time-resolved fluorescence and transient absorption

We model the dynamics of energy transfer and primary charge separation in isolated photosystem II (PSII) reaction centers. Different exciton models with specific site energies of the six core pigments and two peripheral chlorophylls (Chls) in combination with different charge transfer schemes have been compared using a simultaneous fit of the absorption, linear dichroism, circular dichroism, steady-state fluorescence, transient absorption upon different excitation wavelengths, and time-resolved fluorescence. To obtain a quantitative fit of the data we use the modified Redfield theory, with the experimental spectral density including coupling to low-frequency phonons and 48 high-frequency vibrations. The best fit has been obtained with a model implying that the final charge separation occurs via an intermediate state with charge separation within the special pair (RP(1)). This state is weakly dipole-allowed, due to mixing with the exciton states, and can be populated directly or via 100-fs energy transfer from the core-pigments. The RP(1) and next two radical pairs with the electron transfer to the accessory Chl (RP(2)) and to the pheophytin (RP(3)) are characterized by increased electron-phonon coupling and energetic disorder. In the RP(3) state, the hole is delocalized within the special pair, with a predominant localization at the inactive-branch Chl. The intrinsic time constants of electron transfer between the three radical pairs vary from subpicoseconds to several picoseconds (depending on the realization of the disorder). The equilibration between RP(1) and RP(2) is reached within 5 ps at room temperature. During the 5-100-ps period the equilibrated core pigments and radical pairs RP(1) and RP(2) are slowly populated from peripheral chlorophylls and depopulated due to the formation of the third radical pair, RP(3). The effective time constant of the RP(3) formation is 7.5 ps. The calculated dynamics of the pheophytin absorption at 545 nm displays an instantaneous bleach (30% of the total amplitude) followed by a slow increase of the bleaching amplitude with time constants of 15 and 12 ps for blue (662 nm) and red (695 nm) excitation, respectively.

Evidence for localisation of accumulated chlorophyll cation on the D1-accessory chlorophyll in the reaction centre of photosystem II

Absorption and circular dichroism spectra of Photosystem II (PS II) reaction centres (RC) were studied and compared with spectra calculated on the basis of point-dipole approximation. Chlorophyll cation was accumulated during a light treatment of PS II RC in the presence of artificial electron acceptor silicomolybdate. Light-induced difference spectra and their calculated counterparts revealed the location of accumulated cation at the accessory chlorophyll of the D1 protein subunit.

Theory of optical spectra of photosystem II reaction centers: location of the triplet state and the identity of the primary electron donor

Based on the structural analysis of photosystem II of Thermosynechococcus elongatus, a detailed calculation of optical properties of reaction-center (D1-D2) complexes is presented applying a theory developed previously. The calculations of absorption, linear dichroism, circular dichroism, fluorescence spectra, all at 6 K, and the temperature-dependence of the absorption spectrum are used to extract the local optical transition energies of the reaction-center pigments, the so-called site energies, from experimental data. The site energies are verified by calculations and comparison with seven additional independent experiments. Exciton relaxation and primary electron transfer in the reaction center are studied using the site energies. The calculations are used to interpret transient optical data. Evidence is provided for the accessory chlorophyll of the D1-branch as being the primary electron donor and the location of the triplet state at low temperatures.