Electronic structure of the P₇₀₀ special pair from high-frequency electron paramagnetic resonance spectroscopy

Enhanced spectral resolution of high-frequency electron paramagnetic resonance (EPR) spectroscopy allows for detailed g-tensor analysis of the active paramagnetic pigments in photosynthetic reaction centers. This analysis has been made for the cation radical and triplet state of the primary donor in photosystem I (P₇₀₀) compared to the same species from chlorophyll a (Chla) in vitro. The data prove that the electronic structures of the primary donor cation and triplet state are very different from those of monomeric Chla. The g-value anisotropy of P₇₀₀ ^+̇ is smaller, and the g_z component considerably deviates from that of Chla^+̇. The triplet state of P₇₀₀ is also different from Chla^T. In the present study, this difference is resolved because of the high g-value resolution of the high-frequency EPR and characterized by the switching of the directions of the X and Y g-tensor axes with respect to the zero-field axes. These results can be explained either by a delocalized electronic character of the P₇₀₀ ^+̇ and P₇₀₀ ^T states or by a heteromeric model of the primary donor in photosystem I.