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

Dzuba, S.A.; Gast, P.; Hoff, A.J.

doi:doi:10.1016/S0009-2614(97)00200-5

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(+)Q(A)(-) (D, primary electron donor; Q(A), primary electron acceptor). At temperatures 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), 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 Angstrom and that the distribution depends stepwise on the temperature. The similarity between our results and those...Show moreThe 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(+)Q(A)(-) (D, primary electron donor; Q(A), primary electron acceptor). At temperatures 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), 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 Angstrom 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.Show less

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