A short note on orientation selection in the DEER experiments on a native cofactor and a spin label in the reaction center ofRhodobacter sphaeroides
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The analysis of the two-frequency pulsed electron paramagnetic resonance (EPR) (double electron-electron spin resonance, DEER) investigation on the coupling between the semiquinone anion state of the primary acceptor (QA) and the spin label at the cysteine 156 in the H-subunit in the photosynthetic reaction center (RC) fromRhodobacter sphaerodes (R26) (I. V. Borovykh, S. Ceola, P. Gajula, P. Gast, H. J. Steinhoff, M. Huber: J. Magn. Reson. 180, 178–185, 2006) is reinvestigated to include orientation selection. The combination of the EPR properties of the two radicals and the pump and observer frequencies suggests that such an effect could play a role even at the X-band (9 GHz) EPR fields and frequencies employed. The magnitude of the effect is estimated from the structures obtained from the molecular-dynamics (MD) simulations from the previous study: the distance change is small (around 2%) and the distance of 3.05 nm obtained then is possibly underestimated by 0.06 nm. Thus, the difference of at least 0.2 nm between the measured distance and the average distance of 2.8 nm found by the MD simulation remains, suggesting a significant difference between the measurement and the X-ray structure of the RC, as discussed previously.
KeywordsElectron Paramagnetic Resonance Electron Paramagnetic Resonance Spectrum Nitroxide Electron Paramagnetic Resonance Signal Spin Label
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