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

Berry, M., C.; Bratt, P., J.; Evans, M., C.W.

Biochimica et Biophysica Acta 1319(2-3): 163-176


ISSN/ISBN: 0005-2728
DOI: 10.1016/s0005-2728(96)00157-0
Accession: 009333450

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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-Sx, 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-X 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 the procedure described by Hirsh et al. (Biochemistry 31 (1992) 532-541), provides estimates of the P700 - Fe-S-X distance of 25-35 ANG , A-1 - Fe-S-X and A-0 - Fe-S-X of 20-30 ANG . The relevance of these distances to those determined by other spectroscopic techniques and X-ray crystallography is discussed.