Chlorophylls (Chl)s exist in a variety of flavors and are ubiquitous in both the energy and electron transfer processes of photosynthesis. The functions they perform often occur on the ultrafast (fs–ns) time scale and until recently, these have been difficult to measure in real time. Further, the complexity of the binding pockets and the resulting protein-matrix effects that alter the respective electronic properties have rendered theoretical modeling of these states difficult. Recent advances in experimental methodology, computational modeling, and emergence of new reaction center (RC) structures have renewed interest in these processes and allowed researchers to elucidate previously ambiguous functions of Chls and related pheophytins. This is complemented by a wealth of experimental data obtained from decades of prior research. Studying the electronic properties of Chl molecules has advanced our understanding of both the nature of the primary charge separation and subsequent electron transfer processes of RCs. In this review, we examine the structures of primary electron donors in Type I and Type II RCs in relation to the vast body of spectroscopic research that has been performed on them to date. Further, we present density functional theory calculations on each oxidized primary donor to study both their electronic properties and our ability to model experimental spectroscopic data. This allows us to directly compare the electronic properties of hetero- and homodimeric RCs.

Photosystem I (PSI) is a naturally occurring light-harvesting complex that drives oxygenic photosynthesis through a series of photoinitiated transmembrane electron transfer reactions that occur with a high quantum efficiency. Understanding the mechanism by which this process occurs is fundamental to understanding the near-unity quantum efficiency of PSI and in turn could lead to further insight into PSI-based technologies for solar energy conversion. In this article, we have applied two-dimensional electronic spectroscopy to PSI complexes isolated from two different cyanobacterial strains to gain further insight into the ultrafast energy transfer in PSI. The PSI complexes studied differ in the number and absorption of the red chlorophylls, chlorophylls that lie to lower energies than the reaction center. By applying a global analysis to the 2D electronic spectra of the PSI complexes we extract 2D decay associated spectra (2D-DAS). Through analysis of the 2D-DAS we observe a 50 fs relaxation among the bulk antenna chlorophylls in addition to two pathways of energy equilibration involving the red chlorophylls: a fast 200 fs equilibration followed by a 2-4 ps equilibration. As demonstrated with a model system, the λ, λ coordinates of the cross-peaks in the 2D-DAS spectra indicate that the two equilibration pathways involve different chlorophyll molecules.

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The molecular wire-appended naphthoquinone 1-[15-(3-methyl-1,4-naphthoquinone-2-yl)]pentadecyl disulfide [(NQ(CH2)15S)2] has been incorporated into the A1A and A1B sites of Photosystem I (PS I) in the menB variant of Synechocystis sp. PCC 6803. Transient electron paramagnetic resonance studies show that the naphthoquinone headgroup displaces plastoquinone-9 from the A1A (and likely A1B) sites to a large extent. When a Pt nanoparticle is attached to the molecular wire by reductive cleavage of the disulfide and reaction with the resulting thiol, the PS I-NQ(CH2)15S-Pt nanoconstruct evolves dihydrogen at a rate of 67.3 μmol of H2 (mg of Chl)(-1) h(-1) [3.4 e(-) (PS I)(-1) s(-1)] after illumination for 1 h at pH 6.4. No dihydrogen is detected if wild-type PS I, which does not incorporate the quinone, is used or if either (NQ(CH2)15S)2 or the Pt nanoparticle is absent. Time-resolved optical studies of the PS I-NQ(CH2)15S-Pt nanoconstruct show that the lifetimes of the forward electron transfer to and reverse electron transfer from the iron-sulfur clusters are the same as in native PS I. Thus, electrons are not shuttled directly from the quinone to the Pt nanoparticle during either forward or reverse electron transfer. It is found that the rate of dihydrogen evolution in the PS I-NQ(CH2)15S-Pt nanoconstruct depends strongly on the concentration the sacrificial electron donor cytochrome c6. These observations can be explained if the iron-sulfur clusters are involved in stabilizing the electron; the ~50 ms residence time of the electron on FA or FB is sufficiently long to allow cytochrome c6 to reduce P700(+), thereby eliminating the recombination channel. In the absence of P700(+), slow electron transfer through the molecular wire to the Pt catalyst can occur, and hence, H2 evolution is observed.

This research addresses one of the most compelling issues in the field of photosynthesis, namely, the role of the accessory chlorophyll molecules in primary charge separation. Using a combination of empirical and computational methods, we demonstrate that the primary acceptor of photosystem (PS) I is a dimer of accessory and secondary chlorophyll molecules, Chl 2A and Chl 3A , with an asymmetric electron charge density distribution. The incorporation of highly coupled donors and acceptors in PS I allows for extensive delocalization that prolongs the lifetime of the charge-separated state, providing for high quantum efficiency. The discovery of this motif has widespread implications ranging from the evolution of naturally occurring reaction centers to the development of a new generation of highly efficient artificial photosynthetic systems.While the kinetics and thermodynamics of each ET step beyond A 0 are well characterized (