Spin in Photosynthetic Electron Transport

Photosynthetic charge separation also leads to electron spin separation. The two separated electron spins form a spin-correlated radical pair initially in a pure singlet state. This high electron spin order can be detected by EPR spectroscopy as CIDEP (chemically induced dynamic electron polarization). The radical pair undergoes spin evolution leading to periodic intersystem crossing. Interactions with nuclear spins lead to CIDNP (chemically induced dynamic nuclear polarization), which is often referred to as “photo-CIDNP” if it is of photochemical origin. NMR spectroscopy is able to observe such nuclear spin hyperpolarization. Both CIDEP EPR and CIDNP NMR allow for the early steps of photosynthesis to be studied in great detail. History and examples of these studies are presented. Finally, the question whether the occurrence of the spin-polarization is simply a by-product of the charge separation is discussed. Concepts for a possible functional relevance are proposed.