Amesz J, De Grooth B G
Biochim Biophys Acta. 1975 Feb 17;376(2):298-307. doi: 10.1016/0005-2728(75)90022-5.
Absorbance changes in the region 500-565 nm and at 702 nm, brought about by excitation of Photosystems 1 and 2, respectively, were measured in spinach chloroplasts at minus 50 degrees C. Either dark-adapted chloroplasts were used or chloroplasts preilluminated with a number of short saturating flashes just before cooling. Both photosystems were found to cause a light-induced increase of absorbance at 518 nm (due to "P518"). The System 1-induced change was not affected by pre-illumination. It decayed within 1 s in the dark and showed similar kinetics as P700. Experiments in the presence of external electron acceptors (methylviologen or Fe(CN)6-3-) suggested that P518 was not affected by the redox state of the primary electron acceptor of System 1. The absorbance increase at 518 nm due to System 2 decayed in the dark with a half-time of several min. The kinetics were similar to those of C-550, the presumed indicator of the primary electron acceptor of System 2. After two flashes preillumination the changes due to P518 and C-550 were reduced by about 40%, and a relatively slow, System 2-induced oxidation of cytochrome b559 occurred which proceeded at a similar rate as the increase in yield of chlorophyll a fluorescence. The results indicate that at minus 50 degrees C two different photoreactions of System 2 occur. One consists of a photoreduction of the primary electron acceptor associated with C-550, accompanied by the oxidation of an unknown electron donor; the other is less efficient and results in the photooxidation of cytochrome b559.
分别通过光系统1和光系统2的激发所引起的500 - 565纳米区域以及702纳米处的吸光度变化，在零下50摄氏度的菠菜叶绿体中进行了测量。使用的要么是暗适应的叶绿体，要么是在冷却前用若干短时间饱和闪光预先照射过的叶绿体。发现两个光系统都会导致518纳米处（由于“P518”）吸光度的光诱导增加。光系统1诱导的变化不受预先照射的影响。它在黑暗中1秒内衰减，并且显示出与P700相似的动力学。在存在外部电子受体（甲基紫精或铁氰化物离子）的情况下进行的实验表明，P518不受光系统1初级电子受体氧化还原状态的影响。由于光系统2导致的518纳米处吸光度增加在黑暗中以几分钟的半衰期衰减。动力学与C - 550相似，C - 550被认为是光系统2初级电子受体的指标。经过两次闪光预先照射后，由于P518和C - 550引起的变化减少了约40%，并且发生了相对缓慢的、光系统2诱导的细胞色素b559氧化，其进行速率与叶绿素a荧光产率的增加相似。结果表明，在零下50摄氏度时，光系统2发生两种不同的光反应。一种包括与C - 550相关的初级电子受体的光还原，伴随着一个未知电子供体的氧化；另一种效率较低，导致细胞色素b559的光氧化。
