Light-Harvesting, Energy and Charge Transfer and Primary Processes of Photosynthesis
The properties of chlorophyll molecules in photosynthetic complexes are tuned by binding to specific protein environments. This principle developed evolutionarily in the biosphere and has resulted in pigmentprotein structures with the highest efficiency with regards to lightharvesting, transfer of electronically excited states, and the transformation of light into electrochemical free energy. Up until today it remains impossible to assemble a nanomachine of a complexity, stability and regulatory control that can compare to the ones present in a single working thylakoid membrane. A full understanding of the functional relevance of molecular interactions in photosynthetic pigment-protein- complexes (PPCs) might therefore be useful in many areas of application. Artificially designed PPCs can work as nanoscaled tunable antenna systems for photovoltaics, e.g. in the form of light-harvesting complexes with high exciton diffusion coefficients that thus rise and/or regulate the light absorption cross-section. Other applications might include switchable PPCs that undergo transitions from an inactive to an active state when illuminated or when in contact with certain environments. Such PPCs could be applicable in the photodynamic diagnostics and therapy of diseases such as skin cancer.
Photosynthesis appears to be the most important process in solar energy exploitation of the biosphere (Renger, 2008b). It can be described as the light-induced chemical reaction of water with carbon dioxide and glucose:
The photosynthetical reaction summarized by chemical eq. 1 is highly endergonic. It is driven by Gibbs free energy from solar radiation on earth as it is being absorbed by green plants. The light energy is absorbed by chromophores that are bound to the photosynthetic complexes of the photosystems PS I and PS II. These chromophores are mainly chlorophyll and carotenoid molecules. In the following, we will focus on describing PS II as a large fraction of ROS, including singlet oxygen, is produced there. The redox chemistry of PS I lies beyond the scope of this book; therefore a detailed description of PS I will not be presented here. Instead, the reader is referred to the existing literature on the light-harvesting complex and RC of PS I (Byrdin, 1999, and references therein).
The electronically excited singlet states formed by light absorption are not completely transformed into Gibbs free energy of the excited Chl molecules. A fraction is emitted as red fluorescence; the dynamics of the fluorescence emission of all samples containing PS I and PS II is mainly determined by the properties (organization and coupling) of the photosynthetic pigment-protein complexes of PS II (see Schatz et al., 1988). Therefore the structure, electrochemistry and function of PS II will be described briefly.
The PS II core complexes from Thermosynechococcus elongatus, a thermophilic cyanobacterium, was resolved at 1.9 A° resolution (see Umena et al., 2011), which showed that the dimer PS II unit is composed of two times 19 protein subunits and two times 25 integral lipids. At room temperature each monomer contains 35 Chl a molecules, 2 pheophytin (Phe) molecules, 11 carotenoid molecules, 2 plastoquinones (PQ), the Mn4CaO5 cluster, bicarbonate, oxygen atoms, calcium atoms, chloride ions, haem and non haem iron, and at least 1,350 water molecules forming a strong hydrogen-bonding network. The PS II core complex contains the reaction center (RC) (D1/D2/cytb559), the inner antenna (CP43 and CP47), and three peripheral (PsbO, PsbP, PsbQ) proteins; this is the minimal unit capable of oxidizing water and reducing plastoquinone molecules (see Figure 17, first published by Mamedov et al., 2015).
Figure 17. Schematic representation of the core proteins of photosystem II (PS II) in higher plants and green algae illustrating the pathway of the electron flow through PS II by black arrows. Mn4CaO5 denotes the inorganic core of the water oxidizing Mn complex; YZ the redox active tyrosine residue on the D1 protein and YD, a tyrosine, on the D2 protein. PQ the mobile plastoquinone electron carrier CP43 and CP47, the chlorophyll binding core proteins (see text), the light-harvesting complex are denoted LHC; in addition PsbO, PsbP, and PsbQ, the extrinsic proteins of PS II and the redox active cytochrome b559 (Mamedov et al., 2015). Image reproduced with permission.
In Figure 17, the electron transfer is denoted in the form of black arrows that indicate the ET steps that are typically described by rate equations. ET mostly involves the D1 protein, except for Qa, which is on D2. Figure 17 also shows a bicarbonate (HCO3) ion, which is known to be bound on a non-heme-iron that sits between Qa and Qb. HCO3- has been shown to play an essential role in electron and proton transport on the electron acceptor side of PS II. Although the light-harvesting systems of the PS II exhibit strong variations between different photosynthetic organisms (e.g. cyanobacteria and higher plants) the architecture of the core complex shown in Figure 17 is very similar among all oxygenic photosynthetic organisms. After light absorption inside the PS II, an excited single state of the chlorophyll localizes inside the RC. The molecular identities of the excited state 1P680* of the RC, from where electron transfer starts, and the state P680+L where the hole stabilizes, are different. Together with the strongly coupled Chl dimer Pd1/Pd2, which is called a “special pair”, the Chl molecules ChW, ChW, ChlD1 and ChlD2 and different site energies form an energetic “trap” that is represented by the reaction center in comparison to the excited states in the Chl antenna (for further details see Renger and Renger, 2008, and references therein).
PS II and PS I are functionally connected by the Cytb6f complex in which plastoquinol PQH2 formed at PS II is oxidized and the electrons are transferred to PS I via plastocyanin (PC) as mobile carrier (see Figure 4). This process is coupled with proton transport from the stroma to the lumen, thus increasing the proton concentration in the lumen. At PS I, the light driven reaction leads to the reaction of NADP+ to NADPH. The proton gradient provides the driving force for the ATP synthase where ATP is formed from ADP + P. NADPH is used in the dark reactions for CO2 reduction to produce glucose inside the chloroplast stroma. The spatial separation between lumen and stroma and the oriented arrangement of PS I, PS II and Cytb6f is the essential symmetry break inside the chloroplast volume, which in its turn enables a directional electron flow that is coupled with the formation of a transmembrane electrochemical potential difference. In this way the absorbed Gibbs free energy of the photons is partially and transiently “stored”.
The antenna complexes permit a very efficient adaptation of anoxy- genic photosynthetic bacteria (Law and Cogdell, 2007), cyanobacteria (Mimuro et al., 2008) and plants (van Amerongen and Croce, 2008) to different and widely varying illumination conditions. At low light intensities, the few electronically excited singlet states are funnelled with high efficiency to the photochemically active pigment of the RCs, where the photochemical charge separation takes place (Parson, 2008; Setif and Leibl, 2008). An opposite effect is induced under strong illumination; i.e., the radiationless decay of the superfluous excited singlet states is stimulated by opening dissipative channels, and, in addition, (bacterio) chlorophyll triplets formed via intersystem crossing are effectively quenched by carotenoids (Cars) (Belyaeva et al., 2008, 2011, 2014, 2015).
In contrast to the reactions in the antenna, only the excited states in RCs, PS I and PS II are transformed into electrochemical Gibbs free energy via electron transfer. This gives rise to the formation of primary cation-anion radical pairs, which is followed by stabilization steps under the participation of secondary acceptor components (see Parson, 2008; Setif and Leibl, 2008, and references therein). In plants, both of these types of pigment-protein complexes (antennas and photosystems) are highly hydrophobic. Therefore they solve as integral proteins inside the lipophilic, intrinsic part of thylakoid membrane.
Figure 18 represents the protein and pigment protein complexes that form the PS II in a different view than Figure 17. The electron flow/pathways between the cofactors shown in Figure 18 are indicated as black arrows. Next to the Chl-containing core antenna, the LHC-II (Lhcb) complexes are also in contact with protein structures, shown in black in Figure 18, that surround the core complex and contain nearly no pigments. These proteins most likely stabilize the structure of the core complex and they work, like the protein complex S, as linker proteins and seem to be very important for the energy transfer processes. The compounds of the core complex of the PS II (see Figure 17) consists of the reaction center (D1, donor side, yellow and D2, acceptor side, orange in Figure 18) and the Chl-containing core proteins CP 43 (dark green in Figure 18, most probably containing 13 Chl molecules) and CP 47 (red in Figure 18, most probably containing 16 Chl molecules); these form the core antenna. As mentioned above, the core complex exhibits a common architecture among all oxygen photosynthetic organisms (and is also very similar in anoxygenic photosynthetic bacteria) and it has remained nearly unchanged throughout evolution. The main differences are found in the oligomeric macrostructure of the core complex that leads to dimeric or tetrameric supercomplexes as found in several species, e.g. the cyanobacteria Prochloron didemni and A.marina (Bibby et al., 2003; Chen et al., 2005). In marked contrast, the antenna complexes of cyanobacteria, green sulphur bacteria, purple bacteria and higher plants (just to mention a few examples) are very different in composition and/or architecture.
Figure 18. Subunits of the PS II of higher plants, including the LHC-II complexes, as light-harvesting systems and the core complexes (for details see text). Image reproduced with permission.
The substrate water (H2O) interacts with the water oxidising complex (WOC) that contains four manganese ions (see Figure 18). The interaction between H2O and the WOC elevates the energetic state of the H2O molecule. The interaction process activates the release of two electrons per molecule H2O to an intermediate acceptor YZ which fulfills the role of an electron donator for P680+. P680 is the lower excitonic state of the two Chl a molecules forming the “special pair” PDi/PD2 inside the RC. The oxidation of P680 is possible via interaction with pheophytin (Pheo). The formation of the first radical ion pair P680+Pheo- is the primary charge separation. The energy of this radical pair strongly depends on the state of the environment. After formation of the primary radical pair, the environment relaxes and shifts the energy level of P680+Pheo- to a lower value (Renger and Holzwarth, 2005).
Due to the strong interaction between P680+ and Pheo- the recombination rate of P680+Pheo- is comparatively high. The charge separation is more stable after the relaxation of the environment and the subsequent “charge stabilization” process, when Pheo- releases an electron to the plastoquinone QA. In Chemical equation 3 charge separation and stabilisation are indicated. Strongly coupled molecules with partial excitonical coupling and EET fast in comparison to the resolution limit of TWCSPC are indicated with a double arrow (« ) while localized electron transitions are indicated with simple arrows for reactions that occur in both directions (—^-»). The index N in Chemical equation 3 indicates that a high number of Chl molecules (N = 100-400) are coupled inside and between the PS II subunits. The electronically excited complexes are marked with an asterisk (*).
chemical eq. 3: most important steps of the charge separation in the reaction center after absorption of light energy in the antenna which interacts strongly with the P680 inside the reaction center. The electron is released towards pheophytin and (Pheo) and plastoquinone (QA).
The primary donor P680 and the primary acceptor Pheo are bound to the D1 protein as shown in Figure 17 and Figure 18. D1 is the donor side of the reaction center. Qa is located inside the D2 protein on the acceptor side. From there the electron is transferred towards the plastoquinone molecule at the Qb site, and then further on to mobile PQ carriers (see Figure 17 and Figure 18).