PROBING THE ENERGY LANDSCAPE OF BACTERIAL PHOTOSYNTHETIC REACTION CENTERS AT CRYOGENIC TEMPERATURES BY ESEEM OF SPIN-POLARIZED D(-) RADICALPAIRS()Q(A)()

The energy landscape of the reaction center protein of the photosynthe tic bacterium Rhodobacter sphaeroides R-26 has been investigated with electron spin echo envelope modulation (ESEEM) spectroscopy of the mod ulations induced by spin-spin interactions (dipolar and exchange) betw een radicals in the light-induced spin-polarized radical pair D(+)Q(A) (-) (D, primary electron donor; Q(A), primary electron acceptor). At t emperatures above similar to 100 K the values of the dipolar and spin- exchange couplings between D+ and Q(A)(-) were found to be -(115 +/- 5 ) mu T and 0.7 mu T (with uncertainty of + 1.5 and -0.3 mu T), respect ively. Abrupt changes of the linewidth of the Fourier-transformed ESEE M spectrum were observed near 25, 40 and 80 K. The lineshapes could be simulated assuming that the distance between the two radicals is dist ributed within a range of about 4 Angstrom and that the distribution d epends stepwise on the temperature. The similarity between our results and those obtained with optical spectroscopies on Zn-protoporphyrin s ubstituted myoglobin suggests that the stepwise changes in distance di stribution are related to relaxation along a hierarchical self-similar pattern of minima in the multidimensional potential surface of the pr otein, and that this energy landscape is a global property of the prot ein.