ENDOR and Special Triple Resonance Studies of Chlorophyll Cation Radicals in Photosystem 2

Abstract

Electron nuclear double resonance (ENDOR) and special triple (ST) resonance spectroscopies have been used to study the cation radicals of the primary donor, P680, and two secondary donor chlorophylls (Chl) in photosystem 2 (PS2). Two different preparations were employed, Tris-washed PS2 membranes and PS2 reaction centers (D1-D2-I-Cytb559 complex). One secondary donor Chl a cation radical, Chl1.+, was generated in the Tris-washed preparation, while the P680.+ radical cation and a further Chl a cation radical, Chl2.+, were produced in the reaction center preparation. The ENDOR spectrum of the primary donor radical cation of photosystem 1 (P700.+) is also presented for comparison. Hyperfine coupling constants for methyl groups have been measured for all three PS2 radical species and assigned by comparison with previously published spectra of Chl a radicals in vitro. Electron spin densities were calculated from these hyperfine couplings. Comparison of ENDOR spectral features with those of Chla.+ in vitro indicates similar values for Chl1.+ and Chl2.+ radicals but an apparent reduction in unpaired electron spin density for P680.+. It has been proposed from the more detailed studies of purple bacterial reaction centers that such a reduction in spin density can be interpreted as a delocalization over two Chl a molecules. Our calculations therefore suggest that P680.+ is a weakly coupled chlorophyll pair with 82% of the unpaired electron spin located on one chlorophyll of the pair at 15 K. Environmental or geometrical changes to the chlorin ring structure to give a novel monomeric primary donor are also possible.(ABSTRACT TRUNCATED AT 250 WORDS)

Radicals and Radical Pairs in Photosynthesis

“…Recently, the analysis of I4N ESEEM data indicated an asymmetric dimer for the electronic structure of P;h with the electron residing mainly on one of the dimer halves with a ratio between 3:l and 4.1 (80). This is in agreement with proton ENDOR studies of frozen PS I from which an asymmetry of 3:l was deduced (139). Single crystal IH ENDOR and I4N ESEEM was performed on PS I crystals and indicated again an asymmetric dimeric electron distribution of about 3:l (162).…”

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

“…4A). Hence, the assignment of the emissive signals to the donor implies a dimeric donor having an asymmetric electron spin distribution between two undisturbed Chl cofactors, as it also has been found with ENDOR (25)(26)(27) and by quantum-chemical calculations (28).…”

“…Thus, we propose that the electron spin density is distributed over both the donor Chl and its axial histidine. Hence, the reduction of spin density, observed by EPR and originally interpreted in terms of a weakly coupled dimer having Ϸ82% of the spin density on one Chl cofactor (25), could be explained by the model presented here implying that the rest of the spin density are localized on the axial histidine. Because the two accessory Chls, Chl D1 and Chl D2 , are not coordinated to histidines (32), the donor must be an inner Chl, either P D1 or P D2 .…”

The electronic structure of P840+• The primary donor of the Chlorobium limicola f. sp. thiosulphatophilum photosynthetic reaction centre

“…The primary donor of PSI, P700, is thought to be a chlorophyll a (Chl a) pair (for a review see [12]), while the primary donor in C. thiosulphatophilum, P840, is presumed to be a Bchl a pair [7,13,14]. An asymmetric distribution (3 : 1) of the unpaired electron spin density between the constituent Chl a monomers has been determined for the radical cation P700" [3,15]. EPR spectroscopy of the triplet states of P840 [7], P870 [16] and P700 [17,18] suggests that the electronic structure of P840 (in the triplet state) resembles that of P870 more than P700 [7].…”