Probing the energy landscape of bacterial photosynthetic reaction centers at cryogenic temperatures by ESEEM of spin-polarised D+QA− radical pairs

The energy landscape of the reaction center protein of the photosynthetic bacterium Rhodobacter sphaeroides R-26 has been investigated with electron spin echo envelope modulation (ESEEM) spectroscopy of the modulations induced by spin-spin interactions (dipolar and exchange) between radicals in the light-induced spin-polarized radical pair D+QA− (D, primary electron donor; QA, primary electron acceptor). At temperatures above ∼ 100 K the values of the dipolar and spin-exchange coupling between D+ and QA− were found to be −(115 ± 5) μT and 0.7 μT (with uncertainty of +1.5 and −0.3 μT), respectively. Abrupt changes of the linewidth of the Fourier-transformed ESEEM spectrum were observed near 25, 40 and 80 K. The lineshapes could be simulated assuming that the distance between the two radicals is distributed within a range of about 4 Å and that the distribution depends stepwise on the temperature. The similarity between our results and those obtained with optical spectroscopies on Zn-protoporphyrin substituted myoglobin suggests that the stepwise changes in distance distribution are related to relaxation along a hierarchical self-similar pattern of minima in the multidimensional potential surface of the protein, and that this energy landscape is a global property of the protein.