Investigation of the Stationary and Transient A1·− Radical in Trp → Phe Mutants of Photosystem I

Photosystem I (PS I) contains two symmetric branches of electron transfer cofactors. In both the A- and B-branches, the phylloquinone in the A1 site is π-stacked with a tryptophan residue and is H-bonded to the backbone nitrogen of a leucine residue. In this work, we use optical and electron paramagnetic resonance (EPR) spectroscopies to investigate cyanobacterial PS I complexes, where these tryptophan residues are changed to phenylalanine. The time-resolved optical data show that backward electron transfer from the terminal electron acceptors to P700·+ is affected in the A- and B-branch mutants, both at ambient and cryogenic temperatures. These results suggest that the quinones in both branches take part in electron transport at all temperatures. The electron-nuclear double resonance (ENDOR) spectra of the spin-correlated radical pair P700·+A1·− and the photoaccumulated radical anion A1·−, recorded at cryogenic temperature, allowed the identification of characteristic resonances belonging to protons of the methyl group, some of the ring protons and the proton hydrogen-bonded to phylloquinone in the wild type and both mutants. Significant changes in PS I isolated from the A-branch mutant are detected, while PS I isolated from the B-branch mutant shows the spectral characteristics of wild-type PS I. A possible short-lived B-branch radical pair cannot be detected by EPR due to the available time resolution; therefore, only the A-branch quinone is observed under conditions typically employed for EPR and ENDOR spectroscopies

Distance determination in spin-correlated radical pairs in photosynthetic reaction centres by electron spin echo envelope modulation

Distances between radicals in photosynthetic reaction centres can be determined from the strong 'out-of-phase' electron spin echo envelope modulation (ESEEM) caused by the electron spin-spin dipolar interaction. The precision with which the dipolar coupling can be measured in such experiments and the optimum strategy for sampling the time-domain spin echo signal are explored using the method of Cramér-Rao lower bounds. Consideration of the contributions to the ESEEM from anisotropic hyperfine interactions suggests that the precision is much higher than hitherto estimated and that the accuracy may not be affected if nuclear modulations are ignored

95 GHz ESEEM of radical pairs: a source of radical separations and relative orientations

W-band (95 GHz) electron spin echo envelope modulation (ESEEM) is proposed as a method for simultaneously determining the relative orientation and separation of the constituents of spin-correlated radical pairs in the solid state. The discussion focuses on the transient secondary radical pair in bacterial photosynthetic energy conversion. © 2001 Elsevier Science B.V

Spin-correlated radical pairs: microwave pulse effects on lifetimes, electron spin echo envelope modulations, and optimum conditions for detection by electron spin echo spectroscopy

A theoretical description is presented of the use of electron spin echo spectroscopy to study the spin-correlated radical pairs encountered in photosynthetic systems. The analysis includes an extra microwave pulse, prior to the spin echo, whose purpose is to alter the radical pair lifetime by populating spin energy levels from which the pair is unable to recombine directly. The optimal flip-angles of the three non-selective pulses are derived, the time dependence produced by spin-lattice relaxation and recombination is predicted, and the echo modulations arising from electron-electron and electron-nuclear interactions are calculated. The implications of the latter for the determination of the exchange and dipolar interactions, and hence the separation, of the two electron spins are discussed

The nuts and bolts of distance determination and zero- and double-quantum coherence in photoinduced radical pairs

We describe in some detail the new method of distance determination for a photoinduced radical pair. Emphasis is on giving the nuts and bolts of the calculations that result in analytical expressions for in- and out-of-phase electron spin echo (ESE) envelope modulations, pulse flip-angle dependencies, zero- and double-quantum coherences, and the distance between the two radicals. The theoretical results are illustrated by a set of recent experiments on photosynthetic reaction centers. © 1998 Elsevier Science B.V. All rights reserved