Leiden Institute of Chemistry, Leiden University, 2300 RA Leiden, The Netherlands.
J Am Chem Soc. 2012 Apr 4;134(13):5921-30. doi: 10.1021/ja2117377. Epub 2012 Mar 21.
Photo-CIDNP (photochemically induced dynamic nuclear polarization) can be observed in frozen and quinone-blocked photosynthetic reaction centers (RCs) as modification of magic-angle spinning (MAS) NMR signal intensity under illumination. Studying the carotenoidless mutant strain R26 of Rhodobacter sphaeroides, we demonstrate by experiment and theory that contributions to the nuclear spin polarization from the three-spin mixing and differential decay mechanism can be separated from polarization generated by the radical pair mechanism, which is partially maintained due to differential relaxation (DR) in the singlet and triplet branch. At a magnetic field of 1.4 T, the latter contribution leads to dramatic signal enhancement of about 80,000 and dominates over the two other mechanisms. The DR mechanism encodes information on the spin density distribution in the donor triplet state. Relative peak intensities in the photo-CIDNP spectra provide a critical test for triplet spin densities computed for different model chemistries and conformations. The unpaired electrons are distributed almost evenly over the two moieties of the special pair of bacteriochlorophylls, with only slight excess in the L branch.
光诱导动态核极化（Photo-CIDNP）可在冷冻和醌阻断的光合反应中心（RC）中观察到，即在光照下魔角旋转（MAS）NMR 信号强度的修饰。通过实验和理论研究，我们证明了缺失类胡萝卜素的红细菌突变株 R26，三自旋混合和差分衰减机制对核自旋极化的贡献可以与自由基对机制产生的极化分离，由于单重态和三重态分支中的差分弛豫（DR），该机制部分得以维持。在 1.4 T 的磁场下，后一种贡献导致约 80000 的信号显著增强，并且超过其他两种机制。DR 机制编码了供体三重态中自旋密度分布的信息。光 CIDNP 光谱中的相对峰强度为不同模型化学和构象计算的三重态自旋密度提供了关键测试。不成对的电子几乎均匀地分布在细菌叶绿素特殊对的两个部分上，在 L 支上只有轻微的过剩。
