PARENT SESSION
Posters P5A Type II reaction centres : Structure. Abstracts (289-312)

Pulsed high-frequency EPR study of the secondary electron-transfer pathways of Photosystem II. K. Lakshmi^*,1, Oleg Poluektov², Arlene Wagner², Michael Reifler¹, Marion Thurnauer², Gary Brudvig¹, ¹ Department of Chemistry, New Haven, CT, USA² Chemistry Department, Argonne, IL, USA

ABSTRACT- 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 b₅₅₉ are alternate electron donors in a secondary photoprotection mechanism.¹ Several possible sequential and branched pathways for secondary electron transfer have been proposed in PS II. However, it is essential to determine the location of the redox active Car cofactors and the kinetics of electron transfer from the Chl and Car cofactors to clarify the precise sequence of events in the secondary photoprotection mechanism of PS II. Previous studies from our laboratories have demonstrated that high-frequency EPR spectroscopy (at 130 GHz), together with deuteration of PS II, yields resolved Car⁺ and Chl⁺ EPR signals.² We have used pulsed high-frequency EPR spectroscopy to measure the location of the carotenoid and chlorophyll radicals relative to other paramagnetic cofactors in PS II.³ In the present work, pulsed high-frequency EPR kinetic measurements are used to study the sequence of electron transfers in the secondary electron-donation pathways of PS II. This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under contract W-31-109-Eng-38 (ANL) and DE-F-G02-01ER15281 (Yale). References: 1. Frank and Brudvig (2004) Biochemistry ASAP article; 2. Lakshmi et al. (2000) J. Phys. Chem. B 104, 10445; 3. Lakshmi et al. (2003) J. Amer. Chem. Soc. 125, 5005.

KEY WORDS: Photoprotection, Photosystem II, Carotenoid, EPR spectroscopy