Relaxation properties of the photosystem 1 electron transfer components: Indications of the relative positions of the electron transfer cofactors in photosystem 1

Pulsed EPR spectroscopy was used to investigate the relaxation properties of the electron transfer components of photosystem 1. The magnetic dipolar interactions between the quinone, A(1), and the redox active chlorophylls, P700 and A(0), and the iron-sulphur component Fe-S-X, were investigated. The spin lattice relaxation transients for A(0)(-.), A(1)(-.) and P700(+.) were non-single-exponential in the presence of reduced iron-sulphur centres, suggesting a dipolar magnetic interaction. However, when the iron-sulphur centres were removed the relaxation transients for these species became mono-exponential demonstrating that the iron sulphur centres were the relaxation enhancing species. When Fe-S, was reduced it became the dominant relaxation enhancing species. Qualitatively the relaxation enhancement is similar for A(0)(-.) and A(1)(-.) with the least effect on P700(+.). Quantitative analysis based on thr procedure described by Hirsh et al. (Biochemistry 31 [1992] 532-541), provides estimates of the P700 - Fe-S-X distance of 25-35 Angstrom, A(1) - Fe-S-X and A(0) Fe-S-X of 20-30 Angstrom. The relevance of these distances to those determined by other spectroscopic techniques and X-ray crystallography is discussed.