Abstract

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.

Highlights

  • Photosynthesis is perhaps one of the most important processes in nature

  • Evolutionary history is marked by the advent of cyanobacteria [∼ 2.4 Ga (Hofmann, 1976; Bekker et al, 2004; Rasmussen et al, 2008)] that utilizes the free energy provided by the sun to generate a highly oxidizing species that splits water, Shedding Light on Primary Donors producing reducing equivalents that are stored as NADPH or ‘biohydrogen’ for use in atmospheric CO2 fixation

  • We will focus on six reaction center (RC): (i) The heterodimers, P700 of Photosystem I (PS I), P680 of Photosystem II (PS II), P865 and P960 of the bacterial RC (bRC) from Rba. sphaeroides and Rps. viridis, respectively, and (ii) homodimers, P800 of H. modesticaldum and P840 of the green-sulfur bacterium, C. tepidum

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Summary

Shedding Light on Primary Donors in Photosynthetic Reaction Centers

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. 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. 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.

INTRODUCTION
Geometric Structure of the Primary Donor of Heterodimeric Reaction Centers
Nearest Nitrogens
Geometric Structure of the Primary Donor of Homodimeric Reaction Centers
NI NII NIII NIV NV NVI NVII NVIII
Computational Studies of the Spin Density Distribution
COMPARISON OF THE PRIMARY DONORS IN HETERO AND HOMODIMERIC REACTION CENTERS
Findings
AUTHOR CONTRIBUTIONS
Full Text
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