When the primary electron-donation pathway from the water-oxidation complex in photosystem II (PS II) is inhibited, chlorophyll (ChlZ and ChlD), β-carotene (Car) and cytochrome b559 are alternate electron donors that are believed to function in a photoprotection mechanism. Previous studies have demonstrated that high-frequency EPR spectroscopy (at 130 GHz), together with deuteration of PS II, yields resolved Car+ and Chl+ EPR signals (Lakshmi et al. J. Phys. Chem. B 2000, 104, 10 445-10 448). The present study describes the use of pulsed high-frequency EPR spectroscopy to measure the location of the carotenoid and chlorophyll radicals relative to other paramagnetic cofactors in Synechococcus lividus PS II. The spin-lattice relaxation rates of the Car+ and Chl+ radicals are measured in manganese-depleted and manganese-depleted, cyanide-treated PS II; in these samples, the non-heme Fe(II) is high-spin (S = 2) and low-spin (S = 0), respectively. The Car+ and Chl+ radicals exhibit dipolar-enhanced relaxation rates in the presence of high-spin (S = 2) Fe(II) that are eliminated when the Fe(II) is low-spin (S = 0). The relaxation enhancements of the Car+ and Chl+ by the non-heme Fe(II) are smaller than the relaxation enhancement of TyrD· and P865+ by the non-heme Fe(II) in PS II and in the reaction center from Rhodobacter sphaeroides, respectively, indicating that the Car+-Fe(II) and Chl+-Fe(II) distances are greater than the known TyrD·-Fe(II) and P865+-Fe(II) distances. The Car+ radical exhibits a greater relaxation enhancement by Fe(II) than the Chl+ radical, consistent with Car being an earlier electron donor to P680+ than Chl. On the basis of the distance estimates obtained in the present study and by analogy to carotenoid-binding sites in other pigment-protein complexes, possible binding sites are discussed for the Car cofactors in PS II. The relative location of Car+ and Chl+ radicals determined in this study provides valuable insight into the sequence of electron transfers in the alternate electron-donation pathways of PS II.
ASJC Scopus subject areas
- Colloid and Surface Chemistry