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

在高磁场下的固态光CIDNP效应的理论最初是基于几个并行运行的自旋动力学机制：经典的自由基对机制（RPM）是通过自由基对的电子自旋选择性复合实现的自旋分选，三自旋混合机制（TSM）将核相干性转变为核自旋超极化，基于自由基对两种自旋态动力学差异的微分衰变（DD）机制，以及如果分子供体三重态寿命足够长则破坏三重态路径上自旋序的微分弛豫（DR）机制。最近，新西伯利亚小组重新审视了这些机制，并从电子-电子-核态的能级交叉（LC）和能级反交叉（LAC）的角度重新阐述了这些机制。在任何情况下，目前的理论仅限于自旋相关自由基对（SCRP）演变为三自旋电子-电子-核系统。然而，最近的实验数据选择性13 C标记的分子，表明，多自旋标记富集是不是“无辜的”，并影响自旋演化。在这里，我们的目标是结合实验和理论的方法，在莱比锡和新西伯利亚的研究小组进行。精心设计的选择性13 C同位素标记的光合反应中心（RC）的样品集将进行分析，在液体和固体状态下的场依赖性和时间分辨的光CIDNP NMR实验。低场区域将通过依赖于场的光学实验进行研究，弛豫NMR将允许解开不同机制的贡献。基于分析方法和数值模拟的理论构建将允许描述在具有多个13 C标记的样品中发生的自旋混合的效率。因此，我们的目标是理解蛋白质中的质子池作为核自旋库与自旋相关自由基对（SCRP）的两个自由电子相互作用。

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).