The dependence of reaction center and antenna triplets on the redox state of Photosystem I

Abstract

The formation of chlorophyll triplet states during illumination of Photosystem I reaction center samples depends upon the redox state of P-700, X and ferredoxin Centers A and B. When the reaction centers are in the states P-700 +A 1XFd BFd − A and P-700 A 1XFd − BFd − A prior to illumination, we observe electron paramagnetic resonance (EPR) spectra from a triplet species which has zero-field splitting parameters (| D| and | E|) larger than those of either the chlorophyll a or chlorophyll b monomer triplet, and a polarization which results from population of the triplet spin sublevels by an intersystem crossing mechanism. We interpret this triplet as arising from photoexcited chlorophyll antenna species associated with reaction centers in the states P-700 +Fd − A and P-700 +X −, respectively, which undergo de-excitation via intersystem crossing. When the reaction centers are in the states P-700A 1XFd − BFd − A and P-700A 1X −Fd − BFd − A prior to illumination, we observe a triplet EPR signal with a polarization which results from population of the triplet spin sublevels by radical pair recombination, and which has a | D| value similar to that of chlorophyll a monomer. We interpret this triplet (the radical pair-polarized triplet) as arising from 3 P-700 which has been populated by the process P-700 + A − 1 → 3P-700 A 1 . We observe both the radical pair-polarized triplet and the chlorophyll antenna triplet when the reaction centers are in the state P-700 A 1XFd − BFd − A, presumably because the processes P-700 +A − 1X → P-700 +A 1X − and P-700 + A − 1 X → 3P-700 A 1 X have similar rate constants when Centers A and B are reduced, i.e., the forward electron transfer time from A − 1 to X is apparently much slower in the redox state P-700 A 1XFd − BFd − A than it is in state P-700 A 1XFd BFd A. The amplitude of the radical pair-polarized triplet EPR signal does not decrease in the presence of a 13.5-G-wide EPR signal centered at g 2.0 which was recorded in the dark prior to triplet measurements in samples previously frozen under intense illumination. This g 2.0 signal, which has been attributed to phototrapped A − 1 (Heathcote, P., Timofeev, K.N. and Evans, M.C.W. (1979) FEBS Lett. 101, 105–109), corresponds to as many as 12 spins per P-700 and can be photogenerated during freezing without causing any apparent attenuation of the radical pair-polarized triplet amplitude. We conclude that species other than A − 1 contribute to the g 2.0 signal.