Exploring hyperfine interactions in spin-correlated radical pairs from photosynthetic proteins: High-frequency ENDOR and quantum beat oscillations

Electron paramagnetic resonance (EPR) and related spectroscopic tools remain among the most important probes of structure and function of natural photosynthetic systems. Indeed, the challenging questions in the study of photosynthesis have to a great extent dictated the directions taken in the development of EPR and associated spectroscopies. In this overview we demonstrate, with recent examples from our laboratories, the potential of high-frequency and time-resolved EPR spectroscopy to reveal unique information about electron transfer processes and the structure of photosynthetic systems. A common feature of these experiments is that they probe hyperfine interactions of the spincorrelated radical pair. Thus, the analysis of the results requires consideration of three interacting spins: two correlated electron spins with one nuclear spin. The results illustrate the importance of resolving nuclear hyperfine structure for obtaining details of structure-function relationships in photosynthetic electron transfer.