Exploring Charge Migration in Light-Harvesting Complexes Using Electron Paramagnetic Resonance Line Narrowing

Abstract

The light-harvesting protein complex 1 (LH1) of the purple bacterium Rhodobacter sphaeroides exhibits EPR signals upon treatment with oxidizing reagents such as potassium ferricyanide. These signals are assigned to radical cations of the LH1's bacteriochlorophyll pigments, B880•+. An intriguing feature of the B880•+ EPR spectrum is the narrow line width exhibited relative to in vitro monomeric BChla•+ and the primary donor radical cation of the photosynthetic reaction center's “special pair”, P•+. In this paper, we investigate the temperature and oxidant concentration dependence of the B880•+ EPR spectrum with the aim of elucidating the mechanism for spectral line narrowing. The experimental data are interpreted in terms of EPR line narrowing that accompanies charge migration and spin exchange. For charge migration, the line-narrowing models are driven by standard, nonadiabatic electron transfer assisted by vibronic coupling. The results are consistent with a hypothesis that, in LH1, the EPR spectral shape is dominated by electron transfer instead of spin exchange. In addition, the electronic and energetic factors governing the inter-BChla cryogenic charge transport are explored. Using standard treatments, large reorganization energy and weak electronic coupling are obtained for the charge migration process. The EPR results support the view that highly delocalized radical cation states similar to that observed for the primary donor BChlas of the special pair of the photosynthetic reaction center do not occur in oxidized LH1 complexes in the 6−300 K temperature range. However, the EPR results are compatible with a highly asymmetrical version of the special pair. The unrealistically high value of reorganization energy for electron transfer is attributed to treating the charge migration process as if electron transfer were homogeneous. A more realistic value of reorganization energy is predicted to result if free-energy heterogeneity were to be included in modeling electron transfer in LH1.