Electron-spin resonance studies of the bound iron-sulfur centers in Photosystem I II. Correlation of P-700 triplet production with urea/ferricyanide inactivation of the iron-sulfur clusters

Abstract

Photosystem I charge separation in a subchloroplast particle isolated from spinach was investigated by electron spin resonance (ESR) spectroscopy following graduated inactivation of the bound iron-sulfur centers by urea-ferricyanide treatment. Previous work demonstrated a differential decrease in iron-sulfur centers A, B and X which indicated that center X serves as a branch point for parallel electron flow through centers A and B (Golbeck, J.H. and Warden, J.T. (1982) Biochim. Biophys. Acta 681, 77–84). We now show that during inactivation the disappearance of iron-sulfur centers A, B, and X correlates with the appearance of a spin-polarized triplet ESR signal with | D| = 279·10 −4 cm −1 and | E| = 39·10 −4 cm −1. The triplet resonances titrate with a midpoint potential of +380 ± 10 mV. Illumination of the inactivated particles results in the generation of an asymmetric ESR signal with g = 2.0031 and ΔH pp = 1.0 mT. Deconvolution of the P-700 + contribution to this composite resonance reveals the spectrum of the putative primary acceptor species, A 0, which is characterized by g = 2.0033 ± 0.0004 and ΔH pp = 1.0 ± 0.2 mT. The data presented in this report do not substantiate the participation of the electron acceptor A 1 in PS I electron transport, following destruction of the iron-sulfur cluster corresponding to center X. We suggest that A 1 is closely associated with center X and that this component is decoupled from the electron-transport path upon destruction of center X. The inability to photoreduce A 1 in reaction centers lacking a functional center X may result from alteration of the reaction center tertiary structure by the urea-ferricyanide treatment or from displacement of A 1 from its binding site.