¹⁵ N photochemically induced dynamic nuclear polarization magic-angle spinning NMR analysis of the electron donor of photosystem II

Abstract

In natural photosynthesis, the two photosystems that operate in series to drive electron transport from water to carbon dioxide are quite similar in structure and function, but operate at widely different potentials. In both systems photochemistry begins by photo-oxidation of a chlorophyll a, but that in photosystem II (PS2) has a 0.7 eV higher midpoint potential than that in photosystem I (PS1), so their electronic structures must be very different. Using reaction centers from 15 N-labeled spinach, these electronic structures are compared by their photochemically induced dynamic nuclear polarization (photo-CIDNP) in magic-angle spinning (MAS) NMR measurements. The results show that the electron spin distribution in PS1, apart from its known delocalization over 2 chlorophyll molecules, reveals no marked disturbance, whereas the pattern of electron spin density distribution in PS2 is inverted in the oxidized radical state. A model for the donor of PS2 is presented explaining the inversion of electron spin density based on a tilt of the axial histidine toward pyrrole ring IV causing -overlap of both aromatic systems.photosynthesis ͉ photosystem I ͉ solid-state NMR ͉ electron transfer ͉ redox potential I n photosynthesis, the two photosystems that operate in series to drive electron transport from water to carbon dioxide are similar in structure and function, but operate at widely different potentials. In both photosystems, photochemistry begins by photo-oxidation of a chlorophyll a (Chl). The oxidized electron donor of photosystem II (PS2) is the strongest oxidizing agent known in living nature, having a redox potential of ϩ 1.2V (1), required for water oxidation. The electronically excited donor of photosystem I (PS1), probably the most reducing compound in living nature (2), initiates the dark reaction. The question arises what factors tune those electronic properties. The spatial structure of PS2 (Fig. 1) shows two inner Chls (P D1 and P D2 ), two accessory Chls (Chl D1 and Chl D2 ), two pheophytin a (Phe) cofactors and two quinones in an arrangement similar to that in bacterial reaction centers (RCs) of purple bacteria.Photochemically induced dynamic nuclear polarization (photo-CIDNP) magic-angle spinning (MAS) NMR is an optical solidstate NMR method using the high electron polarization in the correlated electron pair and allows for strong increase of sensitivity and selectivity (4, 5) allowing to study the electronic structure of photosynthetic cofactors in great detail. The solid-state photo-CIDNP effect, discovered in 1994 (6), relies on different mechanisms called three-spin mixing (7), differential decay (8), and differential relaxation (9, 10). Recently, the contribution of these three mechanisms has been analyzed by field-dependent measurements on unlabeled RCs of the purple bacterium Rhodobacter sphaeroides (11,12). The chemical shift refers to the electronic ground-state after the photocycle, the photo-CIDNP signal intensity is linked to the intermediate radical state. Analytical expressions imply tha...

15N Photo-CIDNP MAS NMR To Reveal Functional Heterogeneity in Electron Donor of Different Plant Organisms

“…3) which is assigned to the N-I of P B (Table 1) is Comparing the chemical shifts observed in duckweed with those observed in spinach (Fig. 3) [24], similar transitions are observed (Table 1). Such finding is expected because in all cases, both donor and primary acceptor cofactors consist of plant Chl a (and Chl a 0 ).…”

“…None of the signals in the spectrum obtained from spinach PSI (Table 1; Fig. 3b) was assigned to the N-III of the primary donor P B which is expected as an emissive signal around 190 ppm (Table 1) [24]. In contrast to the high similarity in chemical shifts, the intensity patterns between duckweed and spinach appear to be substantially different.…”

“…The 15 N chemical shifts assignment of the photo-CIDNP signals observed in duckweed PSI are given in Table 1, and are compared with 15 N chemical shifts assignment of spinach PSI [24] and liquid MAS NMR data of free Chl a in solution [35]. If both branches participate in the ET, photo-CIDNP signals from up to three active Chl a and one active Chl a 0 cofactors can be expected from PSI with a total maximum of sixteen nitrogen.…”

“…15 N-labelled spinach (Spinacia oleracea) plants were grown as described by Diller et al [24]. 15 N-labelled duckweed (S. oligorrhiza) plants were grown under aseptic conditions on half-strength Hunter's medium [29] under continuous light (20 lEm -2 s -1 ) at 25°C.…”

“…In solid-state NMR upon illumination with continuous white light, photo-CIDNP has been observed by applying 15 N magic-angle spinning (MAS) NMR to four photosynthetic RCs: (i) Rhodobacter (R.) sphaeroides R26 [17][18][19][20], wild type (WT) [21][22][23], PSI [2,24], and PSII [24]. Photo-CIDNP is caused by the strong electron polarization in the initial radical pair and subsequently transferred to nuclei, where it is detected by NMR as a signal enhancement up to a factor of about 10,000 [25,26].…”