Papagiannakis Emmanouil, Kennis John T M, van Stokkum Ivo H M, Cogdell Richard J, van Grondelle Rienk
Department of Biophysics and Physics of Complex Systems, Division of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, 1081 HV, Amsterdam, The Netherlands.
Proc Natl Acad Sci U S A. 2002 Apr 30;99(9):6017-22. doi: 10.1073/pnas.092626599. Epub 2002 Apr 23.
Blue and green sunlight become available for photosynthetic energy conversion through the light-harvesting (LH) function of carotenoids, which involves transfer of carotenoid singlet excited states to nearby (bacterio)chlorophylls (BChls). The excited-state manifold of carotenoids usually is described in terms of two singlet states, S(1) and S(2), of which only the latter can be populated from the ground state by the absorption of one photon. Both states are capable of energy transfer to (B)Chl. We recently showed that in the LH1 complex of the purple bacterium Rhodospirillum rubrum, which is rather inefficient in carotenoid-to-BChl energy transfer, a third additional carotenoid excited singlet state is formed. This state, which we termed S*, was found to be a precursor on an ultrafast fission reaction pathway to carotenoid triplet state formation. Here we present evidence that S* is formed with significant yield in the LH2 complex of Rhodobacter sphaeroides, which has a highly efficient carotenoid LH function. We demonstrate that S* is actively involved in the energy transfer process to BChl and thus have uncovered an alternative pathway of carotenoid-to-BChl energy transfer. In competition with energy transfer to BChl, fission occurs from S*, leading to ultrafast formation of carotenoid triplets. Analysis in terms of a kinetic model indicates that energy transfer through S* accounts for 10-15% of the total energy transfer to BChl, and that inclusion of this pathway is necessary to obtain a highly efficient LH function of carotenoids.
蓝光和绿光可通过类胡萝卜素的光捕获（LH）功能用于光合能量转换，这涉及类胡萝卜素单重激发态向附近的（细菌）叶绿素（BChls）的转移。类胡萝卜素的激发态通常用两个单重态S(1)和S(2)来描述，其中只有后者可以通过吸收一个光子从基态填充。这两个态都能够向（B）Chl进行能量转移。我们最近表明，在红螺菌紫色细菌的LH1复合物中，类胡萝卜素到BChl的能量转移效率相当低，会形成第三个额外的类胡萝卜素激发单重态。我们将这个态称为S*，发现它是类胡萝卜素三重态形成的超快裂变反应途径上的一个前体。在这里，我们提供证据表明，在具有高效类胡萝卜素LH功能的球形红杆菌LH2复合物中，S以显著的产率形成。我们证明S积极参与向BChl的能量转移过程，因此发现了类胡萝卜素到BChl能量转移的另一条途径。在与向BChl能量转移的竞争中，裂变从S发生，导致类胡萝卜素三重态的超快形成。根据动力学模型分析表明，通过S进行的能量转移占向BChl总能量转移的10 - 15%，并且包含这条途径对于获得高效的类胡萝卜素LH功能是必要的。
