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

Abstract

The concept of introducing an additional, stable paramagnetic species into photosynthetic reaction centres to increase the information content of their spin polarized transient EPR spectra is investigated theoretically. The light-induced electron transfer in such systems generates a series of coupled three-spin states consisting of sequential photoinduced radical pairs coupled to the stable spin which acts as an "observer". The spin polarized transient EPR spectra are investigated using the coupled three-spin system P* I -Qp in pre-reduced bacterial reaction centres as a specific example which has been studied experimentally. The evolution of the spin system and the spin polarized EPR spectra of PI -Qp and Qq following recombination of the radical pair (P = primary donor, I = primary acceptor, QA = quinone acceptor) are calculated numerically by solving the equations of motion for the density matrix. The net polarization of the observer spin is also calculated analytically by perturbation theory for the case of a single, short-lived, charge-separated state The result bears a close resemblance to the chemically induced nuclear polarization (CIDNP) generated in photolysis reactions in which a nuclear spin plays the role of the observer interacting with the radical pair intermediates. However, because the Zeeman frequencies of the three electron spins involved are usually quite similar, the polarization of the electron observer spin in strong magnetic fields can reflect features of the CIDNP effect in both, high and low magnetic fields. The dependence of the quinone spin polarization on the exchange couplings in the three-spin system is investigated by numerical simulations, and it is shown that the observed emissive polarization pattern is compatible with either sign, positive or negative, for a range of exchange couplings, Jp" in the primary pair. The microwave frequency and orientation dependence of the spectra are discussed as two of several possible criteria for determining the sign of JPY .

Interpretation of multifrequency transient EPR spectra of the P 700 + A0Q K − state in photosystem I complexes with a sequential correlated radical pair model: Wild type versus A0 mutants

“…In this case, electron transfer past the primary acceptor is blocked. Here, ah opportunity is provided for direct information on the spin exchange interaction in a now longer-lived primary radical pair [2,3]. In this case, suitable energetics will allow electron-hole recombination of the primary pair state to form the excited triPlet state 3P70 o of the primary donor, which transient electron paramagnetic resonance (TR-EPR) spectroscopy can readily identify as a recombination product [4].…”

Electron spin polarization in an excited triplet-radical pair system: Generation and decay of the state

“…2)) of the [-T-l/2) spins states of s 3 spin, respectively. This is why the quickly created excess of the (T)D_~/2 population is transferred to the [+ 1/2) spin state of 3-NOPy [22,24]. At later time, the contribution from the quartet subspace dominates because of the faster depopulation of the doublet subspace.…”

“…These parameters agree well with the experimentally determined lifetime of 70 ns [30] of the I(nD~~/z ) spin state. When kr~ I ranges from 1.5.10 6 to 2.5" 107 S -1 , k D and /ce are variable within the intervals (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25). 106 and (7-10).…”

“…The magnetic dipolar, isotropic Heisenberg exchange, and anisotropic Zeeman electron spin interactions are taken into account in a full measure. In this aspect, we follow the theory for the transient EPR spectra of coupled three-spin systems developed by Salikhov, van der Est, and Stehlik [24]. The model is based on the stochastic Liouville equation (SLE) as applied to the diffusively rotating system where the intersystem crossing processes, "spin-selective" and "spin-nonselective", between the singlet and triplet states of ZnTPP are considered in terms of a kinetic approach.…”

Influence of nitroxides on CIDNP of micellised radical pairs and short-lived biradicals

“…Nitroxides also have wide application as spin labels in biology [4,5] andas pH probes in biological systems [6]. A number of papers have been published to date on the influence of nitroxides on magnetic and spin effects [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] in chemical reactions. There are two theoretical models to account for this influence.…”