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Photosystem I (PS I) in plants contains 18 different protein subunits. Only three subunits are directly involved in electron transport, while the remaining subunits have a multitude of other functions. Although the PS I complex in plants shows many specific adaptations, it is fascinating that the overall structure and function has been extremely well conserved through the billion years of evolution that separates plants and cyanobacteria. Important roles of the small accessory subunits include efficient docking of the electron donor plastocyanin and the electron acceptor ferredoxin. These functions are conserved between plants and cyanobacteria but plant PS I has a number of specific adaptations, particularly the PS I-N subunit, which is only present in eukaryotes and is involved in the efficient oxidation of plastocyanin.

This review discusses energy and electron transfer in Photosystem I (PS I) complexes by means of ultrafast timeresolved optical techniques. In particular, this article addresses directly observable initial (sub)picosecond electronic excitation equilibration among different antenna chlorophyll forms and energy trapping by the charge separation process followed by picosecond electron transfer to the secondary electron acceptor A. There is still no general agreement on the energy trapping classification in PS I; the validity of diffusion-limited, trap-limited, and mixedenergy trapping models is tested against the available experimental data. Ultrafast experiments on branch-specific complementary mutants of the reaction center open the unique possibility of differentiating between the two highly symmetrical branches of the reaction center, revealing the directionality of electron transfer in PS I. Finally, the most recent optical data questions the conventional sequence of electron transfer steps, and suggests the intriguing possibility that one additional intermediate radical pair may exist that was not previously observed.

In cyanobacteria, longwavelength chlorophylls (LWC) are located in the Photosystem I (PS I) core complex,whereas in plants and algae they are distributed between the PS I core and the light-harvesting complexes (LHC I). LWC are most probably aggregates of (excitonically) coupled chlorophylls, mainly dimers or trimers. The total number of LWC is rather small (≤10% of the total chlorophylls). Depending on their location in the PS I antenna and their distance from P700, they can play a crucial role in the kinetics of energy transfer and in the trapping of the excitation energy by charge separation. Energy absorbed by LWC is transferred uphill to P700 with high efficiency at room temperature, thereby increasing the cross-section for the absorption of red light. LWC are involved also in the dissipation of excess energy, thus protecting the reaction center. Under physiological conditions, the excitations within the PS I antenna are nearly thermally equilibrated over the different spectral forms and the excitation energy is efficiently trapped via charge separation in the reaction center. When the photochemistry in the reaction center is blocked, the excitations migrate to the LWCand are quenched either by P700 or by the P700 triplet state depending on the state of P700.

The electron transfer chain of Photosystem I (PS I) is made up of six chlorophylls (P, A, and A), two quinones (A), and three iron–sulfur centers (FX, F, and F). The chlorophylls and quinones formtwo potential pathways for electron transfer from P to F. The electron transfer chain chlorophylls are also excitonically coupled as indicated by the spectrally broad transient bleaching observed between 660 and 700 nm upon initial excitation. This article reviews how specific mutagenesis of the ligands of each cofactor is being used to address some of the outstanding questions raised by the structural model of Photosystem I.

Isoprenoid quinones (phylloquinone, plastoquinone) and their derivatives (α-, β-, and γ-tocopherol) play crucial roles in oxygenic photosynthesis. Phylloquinone (vitamin K) and plastoquinone-9 are cofactors of Photosystem I (PS I) and Photosystem II (PS II) complexes, respectively, and mediate electron transfer within and between complexes, while the roles of tocopherols are yet to be fully identified. Traditionally, the biosynthetic pathways of these quinones have been studied by direct enzymatic assays or, since the late 1960s, by using isotopic tracer compounds. Recent progress in the genome sequencing of 14 cyanobacteria has provided a newtool for the identification of genes encoding enzymes of the biosynthetic pathways of these quinones; comparative genomics, in combination with reverse genetics, has recently provided a wealth of new information. With the exception of , phylloquinone biosynthesis in cyanobacteria has been shown to be very similar to menaquinone biosynthesis in . Metabolic engineering of the pathway resulted in the incorporation of a variety of quinone species of either biotic or abiotic origin into the A1 site of Photosystem I, and the resulting strains are important tools for the investigation of electron transfer around the A1 quinone. Plastoquinone-9 biosynthesis in cyanobacteria differs from that in higher plants. Comparative genome analysis has revealed the presence of conserved open reading frames, which encode proteins that share sequence similarity with those required for ubiquinone biosynthesis in . Possible applications of metabolic engineering of the plastoquinone-9 and α-tocopherol biosynthetic pathways for studies of oxygenic photosynthesis are also discussed.

All known photosynthetic reaction centers have symmetric structures, using two similar or identical integral membrane subunits to form a dimeric core, which binds the cofactors through which electrons are shuttled across the membrane. This symmetric arrangement gives rise to two similar branches of cofactors, down which light-driven electron transfer could proceed. The first three members of each branch are chlorins, while the third is a quinone. It is known that the initial electron transfer occurs almost exclusively along one of the two branches in the wellcharacterized Type 2 reaction centers, although the origins of this strong asymmetry are still debated. Photosystem I is the best characterized representative of the Type 1 reaction centers, but many aspects of electron transfer directionality remain unresolved. Recent time-resolved absorption studies suggest that electron transfer can make use of both cofactor branches of Photosystem I at room temperature. Here, we will present the results that led to this proposal and discuss this model in the light of the recent studies aimed at testing its validity.

The primary donor P700 in Photosystem I (PS I) is a heterodimer comprised of a chlorophyll and a chlorophyll . The electronic structure of this species, which is related to its function , can be studied by EPR techniques applied to the paramagnetic states P700 (cation radical) and P700 (triplet state) of the primary donor. In the case of P700 observables are the electronic tensor and the electron-nuclear hyperfine and nuclear quadrupole coupling tensors; in the case of P700 the electron–electron dipolar coupling tensor serves as an additional probe. In this contribution, the determination of the magnetic resonance parameters by EPR techniques are described. Conclusions about the electronic structure, in particular about the spin and charge density distribution in this species, are drawn. The results are corroborated by studies of model systems and of the primary donor in genetically modified photosystem I preparations, which gives information on the effect of the protein surroundings. Emphasis is placed on a theoretical description of P700 in its various states, which is based on a comparison with molecular orbital calculations. Implications of the experimental findings for the functional properties of the primary donor in photosystem I are discussed.

Over the last two decades FTIR difference spectroscopy has emerged as a prominent technique to investigate the electronic structure and the bonding interactions of P700, the primary electron donor of photosystem I. In this chapter, the advances in the field during this period are reviewed and discussed in the light of the structural model of P700 derived from X-ray crystallography. The effect on the FTIR difference spectra of mutations of the axial ligands of the chlorophyll molecules in P700 as well as of amino acid residues in hydrogen bonding interaction with the carbonyl groups of P700 is analyzed.

The mechanism of charge separation and the stabilization of separated charges in photosystem I (PS I) is considered in comparison with reaction centers (RCs) in bacteria. The analysis of the X-ray crystal structures of the RCs together with psec and fsec studies of charge separation coupled to nuclear motion in the system provides new insight into the problem. A psec study of PS I RCs has shown that the primary charge separation takes place between P700*. and the A-A complex. The three-dimensional structure of both the primary electron donor and acceptor shows a possible pathway for electron transfer between P700 and the A-A complex that is governed by nuclear motions. A fsec study of a coherent formation of the nuclear wavepacket on the potential energy surface of the excited state of the primary electron donor P* and of the charge separated state PA (where A is the primary electron acceptor) in native, pheophytin-modified and mutant reaction centers of was compared with X-ray and psec data for PS I RCs. A mechanism of the charge separation and stabilization of separated charges in PS I RCs is proposed.

Fourier transform infrared (FTIR) difference spectroscopy is a useful tool for the study of the structural details of electron transfer cofactors (and their binding sites) in photosynthetic complexes. To date, most FTIR difference spectroscopic studies of photosynthetic complexes have been static experiments in the sense that a steady-state population of an excited state species is photo-accumulated. In intact Photosystem I (PS I) particles the P700A state is short-lived, and not easily studied using static FTIR difference techniques.