Department of Biophysical Chemistry, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Groningen, The Netherlands.
Biophys J. 2011 Mar 2;100(5):1372-80. doi: 10.1016/j.bpj.2011.01.030.
Photosystem I (PSI) plays a major role in the light reactions of photosynthesis. In higher plants, PSI is composed of a core complex and four outer antennas that are assembled as two dimers, Lhca1/4 and Lhca2/3. Time-resolved fluorescence measurements on the isolated dimers show very similar kinetics. The intermonomer transfer processes are resolved using target analysis. They occur at rates similar to those observed in transfer to the PSI core, suggesting competition between the two transfer pathways. It appears that each dimer is adopting various conformations that correspond to different lifetimes and emission spectra. A special feature of the Lhca complexes is the presence of an absorption band at low energy, originating from an excitonic state of a chlorophyll dimer, mixed with a charge-transfer state. These low-energy bands have high oscillator strengths and they are superradiant in both Lhca1/4 and Lhca2/3. This challenges the view that the low-energy charge-transfer state always functions as a quencher in plant Lhc's and it also challenges previous interpretations of PSI kinetics. The very similar properties of the low-energy states of both dimers indicate that the organization of the involved chlorophylls should also be similar, in disagreement with the available structural data.
光系统 I（PSI）在光合作用的光反应中起着重要作用。在高等植物中，PSI 由一个核心复合物和四个外部天线组成，这些天线组装成两个二聚体，即 Lhca1/4 和 Lhca2/3。对分离的二聚体的时间分辨荧光测量显示出非常相似的动力学。使用目标分析解析了单体间的转移过程。它们的速率与观察到的向 PSI 核心的转移速率相似，表明两种转移途径之间存在竞争。似乎每个二聚体都采用了不同的构象，这些构象对应于不同的寿命和发射光谱。Lhca 复合物的一个特殊特征是存在一个低能量的吸收带，该吸收带源于叶绿素二聚体的激子态，与电荷转移态混合。这些低能带具有高振子强度，并且在 Lhca1/4 和 Lhca2/3 中都是超辐射的。这挑战了低能量电荷转移态在植物 LHC 中总是作为猝灭剂的观点，也挑战了以前对 PSI 动力学的解释。两个二聚体的低能态非常相似的性质表明，所涉及的叶绿素的组织也应该相似，这与现有的结构数据不一致。
