Abstract
Abstract. The electron donor in photosystem I (PSI), the chlorophyll dimer P700, is studied by photochemically induced dynamic nuclear polarization (photo-CIDNP) magic angle spinning (MAS) nuclear magnetic resonance (NMR) on selectively 13C and uniformly 15N labeled PSI core preparations (PSI-100) obtained from the aquatic plant duckweed (Spirodela oligorrhiza). Light-induced signals originate from the isotope-labeled nuclei of the cofactors involved in the spin-correlated radical pair forming upon light excitation. Signals are assigned to the two donor cofactors (Chl a and Chl a') and the two acceptor cofactors (both Chl a). Light-induced signals originating from both donor and acceptor cofactors demonstrate that electron transfer occurs through both branches of cofactors in the pseudo-C2 symmetric reaction center (RC). The experimental results supported by quantum chemical calculations indicate that this functional symmetry occurs in PSI despite similarly sized chemical shift differences between the cofactors of PSI and the functionally asymmetric special pair donor of the bacterial RC of Rhodobacter sphaeroides. This contributes to converging evidence that local differences in time-averaged electronic ground-state properties, over the donor are of little importance for the functional symmetry breaking across photosynthetic RC species.
Highlights
In the process of oxygenic photosynthesis, electrons flow from photosystem II (PSII) to photosystem I (PSI); the nomenclature, follows the order of their discovery over time (Emerson and Chalmers, 1958; Govindjee and Rabinowitch, 1960)
To further investigate the functional symmetry breaking in PSI, we have studied isotope-labeled PSI-110 samples from duckweed with photo-CIDNP magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy
Backed by 15N labeling and quantum chemical calculations, we have explored the photosynthetic machinery of PSI on 13C and 15N isotope-labeled preparations from duckweed by photo-CIDNP MAS NMR, aiming for the details of the electronic structure of the dimeric donor and the question of one- or two-sided electron transfer (ET)
Summary
In the process of oxygenic photosynthesis, electrons flow from photosystem II (PSII) to photosystem I (PSI); the nomenclature, follows the order of their discovery over time (Emerson and Chalmers, 1958; Govindjee and Rabinowitch, 1960). While the structural organization of the redox centers is virtually identical in the structures obtained from Pisum sativum and Synechococcus elongatus (Jordan et al, 2001; Amunts et al, 2007), the LHCI complex shows a high degree of variability in size, subunit composition, and number or type of bound pigments. This variation allows each organism to adjust to its specific natural habitat (Croce et al, 2007; Wientjes et al, 2009). The PSI core complex prepared from plants is sometimes denoted as the PS1-110 particle, referring roughly to the total number (∼ 110) of bound Chls (Mullet et al, 1980) and has a molecular weight of ∼ 300 kDa
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