Aspects of Simulation of the EPR Spectra of the Charge-Separated States in Photosynthetic Reaction Centers

Electron paramagnetic resonance (EPR) spectra of the spin-correlated charge-separated states in photosynthetic reaction centers were numerically simulated with the unresolved hyperfine structure of the EPR lines treated, for the first time exactly, by the Monte Carlo method. The parallel computers with distributed-memory-oriented program tools were elaborated to implement these calculations. The results obtained were compared with those for the unresolved hyperfine structure of the EPR lines taken into account in the framework of a conventional approximate description (the convolution method). It is shown that both approaches lead to practically the same transient EPR spectral shapes in W- and Q-bands, while in the X-band they lead to a noticeably different spectrum shape. Our results show that the shape of the EPR spectra detected at the magnetic fields around 300 mT (X-band EPR) can be simulated well only by the Monte Carlo method, while at higher magnetic fields (Q- and W-bands) the experimental EPR spectra can be simulated reasonably well by the convolution approximation.