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[sub Z] and CHI[sub D]), β-carotene (Car) and cytochrome b[sub 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[sup +] and Chi[sup +] 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[sup +] and Chal[sup +] 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[sup +] and Chl[sup +] 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[sup +] and Chl[sup +] by the non-heme Fe(II) are smaller than the relaxation enhancement of Tyr[sub D]• and P[sub 865, sup +] by the non-heme Fe(II) in PS II and in the reaction center from Rhodobacter sphaeroides, respectively, indicating that the Car[sup +]-Fe(II) and Chl[sup +]-Fe(II) distances are greater than the known Tyr[sub D]•-Fe(II) and P[sub 865, sup +]-Fe(II) distances. The Car[sup +] radical exhibits a greater relaxation enhancement by Fe(II) than the Chl[sup +] radical, consistent with Car being an earlier electron donor to P[sub 680, sup +] than Chh 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... [ABSTRACT FROM AUTHOR]