A reassessment of the origin of photochemically induced dynamic nuclear polarization effects in solids

Photochemically induced dynamic nuclear polarization (photo-CIDNP) has been observed in photosynthetic reaction centers by solid-state NMR under illumination. The origin of this light-induced nuclear polarization is not yet fully understood. Based on computer simulations with parameters typical for 13 C nuclei of the special pair donor of a reaction center of purple bacteria, the previously suggested three-spin mixing mechanism and differential decay mechanism are examined in detail and all hitherto suggested mechanisms are combined into a unified approach. The contributions due to the three-spin mixing and differential decay mechanisms are found to be similar in magnitude, but to depend in different ways on the parameters of the spin system. For the first time, sign rules for these two solid-state photo-CIDNP mechanisms are derived. It is shown that the presence of multiple nuclear spins in labeled samples does not significantly affect the primary step in the generation of nuclear polarization. Experiments are suggested that could help to clarify the influence of secondary processes on the polarization patterns and to derive parameters of the spin system. It is pointed out that both the three-spin mixing and differential decay mechanism rely on overlap of the wavefunctions of the two radicals combined with a sufficient lifetime of the pair. The dependence on this subtle balance may explain why solid-state photo-CIDNP effects have to date remained a unique signature of functional photosynthetic reaction centers. (C) 2003 Elsevier B.V. All rights reserved.