The effect of magnetic isotopes on radical-pair evolution within electron-transfer proteins

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The theory of the solid-state photo-CIDNP effect at high magnetic fields has been originally based on several spin-dynamical mechanisms running in parallel: The classical radical pair mechanism (RPM) based on spin-sorting via electron-spin selective recombination of radical pairs, the three-spin mixing (TSM) transferring nuclear coherencies into nuclear spin-hyperpolarization, the differential decay (DD) mechanism based on the difference in kinetics of the two spin states of the radical pair, and the differential relaxation (DR) mechanism destroying spin-order on the triplet pathway if the molecular donor triplet lifetime is sufficiently long. Recently, the mechanisms have been re-visited by the Novosibirsk group and re-formulated in terms of level crossings (LC) and level anti-crossings (LAC) of electron-electron-nuclear states. In any case, presently the theory is limited to spin-correlated radical pairs (SCRP) evolving as three-spin electron-electron-nuclear systems. Recent experimental data on selectively 13C-labelled molecules, however, suggest that multiple-spin labelling enrichment is not "innocent" and affects the spin-evolution. Here we aim for a combined experimental and theoretical approach, carried out by the research groups in Leipzig and Novosibirsk. Well-designed sets of selectively 13C isotope labelled samples of photosynthetic reaction centers (RC) will be analyzed by field-dependent and time-resolved photo-CIDNP NMR experiments in both liquid and solid-state. The low-field region will be investigated by field-dependent optical experiments, and relaxometric NMR will allow to disentangle contributions of the different mechanisms. Theory building based on analytical methods and numerical simulation will allow to describe the efficiency of spin-mixing occurring in samples with multiple 13C labelling. Prospectively, we will aim for the understanding of proton pools in proteins as nuclear spin reservoir interacting with the two free electrons of the spin-correlated radical pair (SCRP).