Nanosecond electron transfer kinetics in photosystem I following substitution of quinones for vitamin K1as studied by time resolved EPR

Abstract
Room temperature transient EPR spectra of photosystem I (PS I) particles fromSynechocystis6803 are presented. Native PS I samples and preparations depleted in the A1‐acceptor site by solvent extraction and then reconstituted with the quinones (Q) vitamin K1(VK1), duroquinone (DQ and DQd12) and naphthoquinone (NQ) have been studied. Sequential electron transfer to P+700A1(FeS) and P+700A1(FeS)is recovered only with VK1. With DQ and NQ electron transfer is restored to form the radical pair P+700Qas specified by a characteristic electron spin polarization (ESP)‐pattern, but further electron transfer is either slowed down or blocked. A qualitative analysis of the K‐band spectrum suggests that the orientation of reconstituted NQ in PS I is different from the native acceptor A1= VK1.

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