Heathcote P, Williams-Smith D L, Evans M C
Biochem J. 1978 Feb 15;170(2):373-8. doi: 10.1042/bj1700373.
An e.p.r. spectrum of the reduced form of the electron-transport component (X), thought to be the primary electron acceptor of Photosystem I, was obtained. By using line-shape simulations of this component and the free-radical e.p.r. signal I of the oxidized reaction-centre chlorophyll (P700), it was possible to determine the ratio of the number of electron spins to which these signals correspond in Photosystem-I particles under a variety of conditions. On illumination at cryogenic temperatures of Photosystem-I preparations, in which both bound iron-sulphur centres A and B were reduced, the measured ratio of free radical to component X varied between 1.04 and 2.23, with an average value of 1.54 +/- 0.18 where a Gaussian line-shape is assumed for the component-X signal in the simulation. The error in this measurement is estimated to be up to 50%. In a similar way component X and centre A of the bound iron-sulphur protein were quantified, the ratio between these two components varying between 1.26 and 0.61 with an average value of 0.75 +/- 0.06. These results indicate that the quantitative relationship, in terms of net electron spins, between centre A, component X and P700 is of the order to be expected if component X is indeed the primary electron acceptor in Photosystem I and a component of the photosynthetic electron-transport chain.
获得了电子传递组分（X）还原形式的电子顺磁共振谱，该组分被认为是光系统I的主要电子受体。通过对该组分以及氧化态反应中心叶绿素（P700）的自由基电子顺磁共振信号I进行线形模拟，得以确定在各种条件下这些信号在光系统I颗粒中所对应的电子自旋数之比。在低温下对光系统I制剂进行光照时，其中结合的铁硫中心A和B均被还原，所测得的自由基与组分X的比例在1.04至2.23之间变化，若在模拟中假设组分X信号为高斯线形，则其平均值为1.54±0.18。该测量的误差估计高达50%。以类似方式对结合的铁硫蛋白的组分X和中心A进行了定量，这两个组分之间的比例在1.26至0.61之间变化，平均值为0.75±0.06。这些结果表明，就净电子自旋而言，如果组分X确实是光系统I中的主要电子受体以及光合电子传递链的一个组分，那么中心A、组分X和P700之间的定量关系符合预期的量级。
