PULSED EPR STRUCTURE-ANALYSIS OF PHOTOSYSTEM-I SINGLE-CRYSTALS - LOCALIZATION OF THE PHYLLOQUINONE ACCEPTOR

A novel application of electron paramagnetic resonance (EPR) is report ed to gain three dimensional structural information on cofactors in pr oteins. The method is applied here to determine the unknown position o f the electron acceptor Q(K), a phylloquinone (vitamin K-1), in the el ectron transfer chain in photosystem I of oxygenic photosynthesis. The unusual electron spin echo (out-of-phase echo) observed for the light induced radical pair P(700)(.+)Q(K)(.-) in PS I allows the measuremen t of the dipolar coupling between the two radical pair spins which yie lds directly the distance between these two radicals. Full advantage o f the information in the out-of-phase echo modulation can be taken if measurements using single crystals are performed. With such samples, t he orientation of the principal axis of the dipolar interaction, i.e., the axis connecting P-700(.+) and Q(K)(.-), can be determined with re spect to the crystal axes system. An angle of theta = (27 +/- 5)degree s between the dipolar coupling axis and the crystallographic c-axis ha s been derived from the modulation of the out-of-phase echo. Furthermo re, the projection of the dipolar axis onto the crystallographic a,b-p lane, is found to be parallel to the a-axis. The results allow for the determination of two possible locations of Q(K) within the electron t ransfer chain of photosystem I. These two positions are related to eac h other by the pseudo C-2 symmetry Of the chlorophyll cofactors.