Pulsed EPR structure analysis of photosystem I single crystals: Localization of the phylloquinone acceptor

A novel application of electron paramagnetic resonance (EPR) is reported to gain three dimensional structural information on cofactors in proteins. The method is applied here to determine the unknown position of the electron acceptor Q(K), a phylloquinone (vitamin K₁), in the electron transfer chain in photosystem I of oxygenic photosynthesis. The unusual electron spin echo (out-of-phase echo) observed for the light induced radical pair P₇₀₀/(·+)Q(K)/(·-) in PS I allows the measurement of the dipolar coupling between the two radical pair spins which yields directly the distance between these two radicals. Full advantage of the information in the out-of-phase echo modulation can be taken if measurements using single crystals are performed. With such samples, the orientation of the principal axis of the dipolar interaction, i.e., the axis connecting P₇₀₀/(·+) and Q(K)/(·-), can be determined with respect to the crystal axes system. An angle of Θ = (27 ±5)°between the dipolar coupling axis and the crystallographic c-axis has been derived from the modulation of the out-of- phase echo. Furthermore, the projection of the dipolar axis onto the crystallographic a,b-plane, 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 transfer chain of photosystem I. These two positions are related to each other by the pseudo C₂ symmetry of the chlorophyll cofactors.