ORIENTATION OF P700, THE PRIMARY ELECTRON-DONOR OF PHOTOSYSTEM-I

Ordered multilayers of Photosystem I particles were obtained by partial drying on mylar sheets. This material was used to study the orientation of P700, the primary electron donor, by monitoring its light-induced triplet state at 4 K by EPR. The orientation dependence of the P700 triplet state was interpreted as indicating that P700 is oriented perpendicular to the mylar sheet. From a comparison of the orientation dependence of the iron-sulphur centres with data reported earlier in ordered chloroplast membranes, it is shown that the Photosystem I particles are ordered on mylar with an orientation comparable to that in the native membrane. Thus it is deduced that P700 is oriented perpendicular to the thylakoid membrane. © 1990.