Pulsed EPR spectroscopy on short-lived intermediates in photosystem I

The application of pulsed electron paramagnetic resonance spectroscopy on s hort-lived intermediates in Photosystem I is reviewed. The spin polarizatio n in light-induced radical pairs gives rise to a phase shifted 'out-of-phas e' electron spin echo signal. This echo signal shows a prominent modulation of its intensity as a function of the spacing between the two microwave pu lses. Its modulation frequency is determined by the electron-electron spin couplings within the radical pair. Thereby, the measurement of the dipolar coupling gives direct information about the spin-spin distance and can ther efore be used to determine cofactor distances with high precision. Applicat ion of this technique to the radical pair F-700(.+) A(1)(.-) in Photosystem I is discussed. Moreover, if oriented samples (e.g. single crystals) are u sed, the angular dependence of the dipolar coupling can be used to derive t he orientation of the axis connecting donor and acceptor with respect to an external (crystal) axes system. Using out-of-phase electron spin echo enve lope modulation spectroscopy, the localization of the secondary acceptor qu inone A(1) has become possible. (C) 2001 Elsevier Science B.V. All rights r eserved.