Ultrafast transient absorption studies on Photosystem I reaction centers from Chlamydomonas reinhardtii.: 1.: A new interpretation of the energy trapping and early electron transfer steps in Photosystem I
Ultrafast transient absorption studies on Photosystem I reaction centers from Chlamydomonas reinhardtii.: 1.: A new interpretation of the energy trapping and early electron transfer steps in Photosystem I
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DOI：
10.1016/s0006-3495(03)74804-8
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发表时间：
2003-12-01
影响因子：
3.4
通讯作者：
Holzwarth, AR
中科院分区：
文献类型：
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作者：
Müller, MG;Niklas, J;Holzwarth, AR
用飞秒到纳秒时间范围内的超快瞬时吸收研究了莱茵衣藻核心光系统I(PSI)粒子的能量转移和电荷分离动力学。虽然天线中的能量转移过程被发现与以前的解释基本一致，但我们提出的证据表明，从动力学和机制方面对能量捕获和电子转移过程的解释必须与目前文献中的解释相比进行实质性的修正。我们首次解决了i)激发反应中心态的瞬时差谱，以及ii)直接从开放的PSI反应中心测量初级自由基对及其中间光谱的形成和衰减。结果表明，由于电荷分离引起的主要能量俘获寿命只有6-9ps，比目前的假设短了3倍。第一个自由基对的光谱显示出预期的680 nm处的强烈漂白带，该带在下一个电子转移步骤中再次衰减。我们进一步表明，早期的电子转移过程直到；100ps比到目前为止的假设要复杂得多。讨论了几种可能的中间氧化还原态及其顺序，其中包括P700在第一电子转移步骤中的氧化，或仅在第二电子转移步骤中的氧化，这将代表与目前假设的机制的根本变化。为了解释这些数据，我们赞成在电子转移方案中加入一个额外的氧化还原态。因此，我们在100ps的时间尺度上区分了三个不同的氧化还原中间体。然而，在这一层面上，无法就中间体的确切机制和性质得出最终结论。通过将我们的数据与文献中的荧光动力学进行比较，我们还提出了一个可逆的第一电荷分离步骤，该步骤到目前为止还不适用于开放的PSI反应中心。首次解决了在700 nm处直接激发反应中心时，发生在反应中心本身激子态间的150飞秒超快平衡过程。综上所述，这些数据要求从根本上修订目前对PSI反应中心的能量捕获和早期电子转移动力学的理解。由于它显示了到目前为止观察到的任何完整PSI粒子的最快捕获时间，所以莱茵哈蒂尼的PSI核心似乎最适合于进一步表征PSI反应中心的电子转移步骤和机制。
The energy transfer and charge separation kinetics in core Photosystem I ( PSI) particles of Chlamydomonas reinhardtii has been studied using ultrafast transient absorption in the femtosecond-to-nanosecond time range. Although the energy transfer processes in the antenna are found to be generally in good agreement with previous interpretations, we present evidence that the interpretation of the energy trapping and electron transfer processes in terms of both kinetics and mechanisms has to be revised substantially as compared to current interpretations in the literature. We resolved for the first time i), the transient difference spectrum for the excited reaction center state, and ii), the formation and decay of the primary radical pair and its intermediate spectrum directly from measurements on open PSI reaction centers. It is shown that the dominant energy trapping lifetime due to charge separation is only 6 - 9 ps, i.e., by a factor of 3 shorter than assumed so far. The spectrum of the first radical pair shows the expected strong bleaching band at 680 nm which decays again in the next electron transfer step. We show furthermore that the early electron transfer processes up to; 100 ps are more complex than assumed so far. Several possibilities are discussed for the intermediate redox states and their sequence which involve oxidation of P700 in the first electron transfer step, as assumed so far, or only in the second electron transfer step, which would represent a fundamental change from the presently assumed mechanism. To explain the data we favor the inclusion of an additional redox state in the electron transfer scheme. Thus we distinguish three different redox intermediates on the timescale up to 100 ps. At this level no final conclusion as to the exact mechanism and the nature of the intermediates can be drawn, however. From comparison of our data with fluorescence kinetics in the literature we also propose a reversible first charge separation step which has been excluded so far for open PSI reaction centers. For the first time an ultrafast 150-fs equilibration process, occurring among exciton states in the reaction center proper, upon direct excitation of the reaction center at 700 nm, has been resolved. Taken together the data call for a fundamental revision of the present understanding of the energy trapping and early electron transfer kinetics in the PSI reaction center. Due to the fact that it shows the fastest trapping time observed so far of any intact PSI particle, the PSI core of C. reinhardtii seems to be best suited to further characterize the electron transfer steps and mechanisms in the reaction center of PSI.