Electron paramagnetic resonance of the primary radical pair [D·+φA·−] in reaction centers of photosynthetic bacteria

The transient EPR spectra and spin dynamics of coupled three-spin systems in photosynthetic reaction centres

“…sphaeroides at 70 K, and J 1 = -( 1.7-1.6) mT for Rps. viridis at 190 K. It was suggested in [15] that the discrepancy between these results and the positive sign of J 1 reported in [14] might be due to differences in the preparations, however this seems quite unlikely since the spectrum of P+1 -and the Q spin polarization pattern are not known to depend strongly on preparation details. More importantly, the discrepancy raises questions concerning the validity of the parameters obtained from the analysis of the Q spin polarization and provides additional motivation to reconsider this case in more detail here.…”

“…With the broader concept of a general observer spin in mind, we will concentrate on the well-studied case in which the pre-reduced primary quinone acceptor plays the role of the observer [11][12][13][14][15][16]. In reaction centres of purple bacteria (bRC) the electron transfer follows the reaction scheme [1,2] where P is the special pair bacteriochlorophyll dimer which acts as the primary donor, B is the accessory bacteriochlorophyll, I is pheophytin and Q A is the primary quinone acceptor.…”

“…Information about the magnetic interactions in the primary pair P+I -has been obtained in modified bRC. Pre-reduction of Q A blocks the electron transfer past pheophytin and leads to an increase in the lifetime of P+I- [15] but can be assumed not to disturb the preceding electron transfer steps [14]. Moreover, the pre-reduced quinone, Qp, can serve as an observer spin (see, e.g., [11][12][13][14][15][16], and the recent review [2]).…”

“…Pre-reduction of Q A blocks the electron transfer past pheophytin and leads to an increase in the lifetime of P+I- [15] but can be assumed not to disturb the preceding electron transfer steps [14]. Moreover, the pre-reduced quinone, Qp, can serve as an observer spin (see, e.g., [11][12][13][14][15][16], and the recent review [2]). Upon illumination of the reaction centres, the pre-reduced quinone becomes spin polarized and its EPR spectrum is emissive when detected after the recombination of the primary pair and return to the ground state [11,12,14].…”

“…This theoretical prediction has been confirmed experimentally [ 19] with negative J-values found for radical pairs in micelles. In recent direct detection EPR experiments on the primary pair P+I - [15,16], the analysis of the observed spectra also resulted in negative values for the exchange integral: J 1 = -(0.8-0.6) mT for Rb. sphaeroides at 70 K, and J 1 = -( 1.7-1.6) mT for Rps.…”