Pulsed high-frequency EPR study on the location of carotenoid and chlorophyll cation radicals in photosystem II.

When the primary electron-donation pathway from the water-oxidation complex in photosystem II (PS II) is inhibited, chlorophyll (Chl(Z) and Chl(D)), beta-carotene (Car) and cytochrome b(559) 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 Tyr(D)(*) and P(865)(+) by the non-heme Fe(II) in PS II and in the reaction center from Rhodobactersphaeroides, respectively, indicating that the Car(+)-Fe(II) and Chl(+)-Fe(II) distances are greater than the known Tyr(D)(*)-Fe(II) and P(865)(+)-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 P(680)(+) 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.