Photochemically induced dynamic nuclear polarization NMR on photosystem II: donor cofactor observed in entire plant.

The solid-state photo-CIDNP (photochemically induced dynamic nuclear polarization) effect allows for increase of signal and sensitivity in magic-angle spinning (MAS) NMR experiments. The effect occurs in photosynthetic reaction centers (RC) proteins upon illumination and induction of cyclic electron transfer. Here we show that the strength of the effect allows for observation of the cofactors forming the spin-correlated radical pair (SCRP) in isolated proteins, in natural photosynthetic membranes as well as in entire plants. To this end, we measured entire selectively 13C isotope enriched duckweed plants (Spirodela oligorrhiza) directly in the MAS rotor. Comparison of 13C photo-CIDNP MAS NMR spectra of photosystem II (PS2) obtained from different levels of RC isolation, from entire plant to isolated RC complex, demonstrates the intactness of the photochemical machinery upon isolation. The SCRP in PS2 is structurally and functionally very similar in duckweed and spinach (Spinacia oleracea). The analysis of the photo-CIDNP MAS NMR spectra reveals a monomeric Chl a donor. There is an experimental evidence for matrix involvement, most likely due to the axial donor histidine, in the formation of the SCRP. Data do not suggest a chemical modification of C-131 carbonyl position of the donor cofactor.

N Photo-CIDNP MAS NMR To Reveal Functional Heterogeneity in Electron Donor of Different Plant Organisms.

In plants and cyanobacteria, two light-driven electron pumps, photosystems I and II (PSI, PSII), facilitate electron transfer from water to carbon dioxide with quantum efficiency close to unity. While similar in structure and function, the reaction centers of PSI and PSII operate at widely different potentials with PSI being the strongest reducing agent known in living nature. Photochemically induced dynamic nuclear polarization (photo-CIDNP) in magic-angle spinning (MAS) nuclear magnetic resonance (NMR) measurements provides direct excess to the heart of large photosynthetic complexes (A. Diller, Alia, E. Roy, P. Gast, H.J. van Gorkom, J. Zaanen, H.J.M. de Groot, C. Glaubitz, J. Matysik, Photosynth. Res. 84, 303-308, 2005; Alia, E. Roy, P. Gast, H.J. van Gorkom, H.J.M. de Groot, G. Jeschke, J. Matysik, J. Am. Chem. Soc. 126, 12819-12826, 2004). By combining the dramatic signal increase obtained from the solid-state photo-CIDNP effect with (15)N isotope labeling of PSI, we were able to map the electron spin density in the active cofactors of PSI and study primary charge separation at atomic level. We compare data obtained from two different PSI proteins, one from spinach (Spinacia oleracea) and other from the aquatic plant duckweed (Spirodella oligorrhiza). Results demonstrate a large flexibility of the PSI in terms of its electronic architecture while their electronic ground states are strictly conserved.

Observation of the solid-state photo-CIDNP effect in entire cells of cyanobacteria Synechocystis.

Cyanobacteria are widely used as model organism of oxygenic photosynthesis due to being the simplest photosynthetic organisms containing both photosystem I and II (PSI and PSII). Photochemically induced dynamic nuclear polarization (photo-CIDNP) (13)C magic-angle spinning (MAS) NMR is a powerful tool in understanding the photosynthesis machinery down to atomic level. Combined with selective isotope enrichment this technique has now opened the door to study primary charge separation in whole living cells. Here, we present the first photo-CIDNP observed in whole cells of the cyanobacterium Synechocystis.

Differential charge polarization of axial histidines in bacterial reaction centers balances the asymmetry of the special pair.

In photosynthesis, light energy is transformed into chemical energy that sustains most forms of life on earth. Solid-state NMR spectroscopy in conjunction with density functional theory modeling can resolve electronic structure down to the atomic level in large membrane proteins. In this work, we have used this technique to address the mechanisms underlying the photochemical reactivity of the special pair in the bacterial reaction center. For charge separation, the electrostatics is important, as the Coulomb barrier must be overcome. On the basis of (15)N NMR data, we resolve a subtle charge-balancing mechanism in the ground state by an axial histidine that is connected to the central Mg(2+) on one side and hydrogen-bonded on the other side. Formation of the hydrogen bond between BChl-a-His and H(2)O leads to a difference in electron density relative to the separate BChl-a-His and H(2)O fragments, with excess positive charge on the imidazole ring. This can lower the kinetic barrier for accommodating the different length scales of electron and proton transfer for separation of spin and charge in a bidirectional proton-coupled electron-transfer mechanism.