Sharp R R, Yocum C F
Biochim Biophys Acta. 1980 Aug 5;592(1):185-95. doi: 10.1016/0005-2728(80)90124-3.
Proton spin-lattice relaxation rates (R1) have been measured in a variety of dark-adapted chloroplast suspensions over a range of field stengths between 1 and 15 kG (4-65 MHz). When the effects of EDTA or Tris washing on chloroplast relaxivities are compared, the pool of Mn associated with oxygen evolution is seen not to contribute significantly to relaxivity. Instead, nearly all of the observed relaxivity, which is characterized by a paramagnetic maximum near 20.7 MHz in the field dispersion profile of R1, appears to arise from contaminating non-functional Mn(II) that can be removed by EDTA during the isolation procedure. These observations, which contradict previous reports ascribing chloroplast relaxivity to the water-oxidizing system, require a reevaluation of proposed models, derived from NMR studies, of the state of Mn in the water-splitting reaction. Chloroplasts from which loosely bound non-functional Mn has been removed by EDTA washing do show an enhancement of relaxivity when exposed to NH2OH at concentrations known to inactivate water oxidation. This NH2OH-induced relaxivity is comprised of Mn(II) in two distinct paramagnetic sites. One site is chelatable by EDTA, whereas the other site is not. This finding suggests that some Mn(II) tightly bound to thylakoid membranes can contribute to relaxivity after inactivation of the oxygen-evolving reaction.
在1至15千高斯（4 - 65兆赫兹）的磁场强度范围内，对多种暗适应叶绿体悬浮液的质子自旋 - 晶格弛豫率（R1）进行了测量。当比较乙二胺四乙酸（EDTA）或三羟甲基氨基甲烷（Tris）洗涤对叶绿体弛豫性的影响时，发现与氧气释放相关的锰池对弛豫性的贡献并不显著。相反，几乎所有观察到的弛豫性，其特征是在R1的场频散曲线中接近20.7兆赫兹处有一个顺磁最大值，似乎源于在分离过程中可被EDTA去除的污染性无功能锰（II）。这些观察结果与先前将叶绿体弛豫性归因于水氧化系统的报道相矛盾，需要重新评估从核磁共振研究得出的关于水分解反应中锰状态的模型。经EDTA洗涤去除了松散结合的无功能锰的叶绿体，在暴露于已知能使水氧化失活的浓度的羟胺（NH2OH）时，确实显示出弛豫性增强。这种由NH2OH诱导的弛豫性由两个不同顺磁位点的锰（II）组成。一个位点可被EDTA螯合，而另一个位点则不能。这一发现表明，一些紧密结合在类囊体膜上的锰（II）在氧气释放反应失活后可对弛豫性有贡献。
