Spin-correlated radical pairs: the differential effect in high-field ENDOR spectra

Abstract

A complete theoretical treatment of the new effect observed in high-field (HF) time-resolved electron-nuclear double resonance (ENDOR) spectra of the spin-correlated radical pair (SCRP) state is presented here. This effect was recently reported for the transient charge-separated state P865+QA− in purple photosynthetic reaction center proteins. One characteristic feature of this new phenomenon is the presence of specific, derivative-type lines in the ENDOR spectra. These lines can be explained by taking into account both a correlation between the electron spins of the photogenerated radical pairs and weak spin-spin interactions within these pairs. This correlation separates the emissive and absorptive lines and results in a differential effect. This effect is enhanced at HF, thus simplifying the analytical description of spectral structure, symmetry features, and interaction parameter dependence. The theoretical analysis presented here demonstrates that only particular nuclei which are essentially coupled to both correlated electron spins contribute to the derivative spectra. The differential effect in SCRP ENDOR manifests remarkable sensitivity to very small “secondary” hyperfine interactions that are usually unresolved in conventional thermalized ENDOR spectra. Application of this theory to experimental SCRP ENDOR spectra will offer a unique look at the internal structure of supramolecular complexes and provide a novel approach for probing electron transfer pathways in natural and artificial photosynthetic assemblies.