Institut für Physik und Astronomie, Universität Potsdam, D-14476 Potsdam-Golm, Germany.
Institut für Physik und Astronomie, Universität Potsdam, D-14476 Potsdam-Golm, Germany and Institut für Biochemie und Biologie, Universität Potsdam, D-14476 Potsdam-Golm, Germany.
Faraday Discuss. 2019 Jul 11;216(0):494-506. doi: 10.1039/c8fd00198g.
In addition to (bacterio)chlorophylls, (B)Chls, light-harvesting complexes (LHCs) bind carotenoids, and/or their oxygen derivatives, xanthophylls. Xanthophylls/carotenoids have pivotal functions in LHCs: in stabilization of the structure, as accessory light-harvesting pigments and, probably most importantly, in photoprotection. Xanthophylls are assumed to be involved in the not yet fully understood mechanism of energy-dependent (qE) non-photochemical quenching of Chl fluorescence (NPQ) in higher plants and algae. The so called "xanthophyll cycle" appears to be crucial in this regard. The molecular mechanism(s) of xanthophyll involvement in qE/NPQ have not been established, yet. Moreover, excitation energy transfer (EET) processes involving carotenoids are also difficult to study, due to the fact that transitions between the ground state (S0, 11Ag-) and the lowest excited singlet state (S1, 21Ag-) of carotenoids are optically one-photon forbidden ("dark"). Two-photon excitation spectroscopic techniques have been used for more than two decades to study one-photon forbidden states of carotenoids. In the current study, two-photon excitation profiles of LHCII samples containing different xanthophyll complements were measured in the presumed 11Ag- → 21Ag- (S0 → S1) transition spectral region of the xanthophylls, as well as for isolated chlorophylls a and b in solution. The results indicate that direct two-photon excitation of Chls in this spectral region is dominant over that by xanthophylls. Implications of the results for proposed mechanism(s) of qE/NPQ will be discussed.
除了（细菌）叶绿素（BChls）外，（B）Chls 还结合类胡萝卜素，即叶黄素，和/或它们的氧衍生物。叶黄素/类胡萝卜素在 LHCs 中具有关键作用：稳定结构、作为辅助光捕获色素，可能最重要的是在光保护中。叶黄素被认为参与了高等植物和藻类中尚未完全理解的依赖于能量的叶绿素荧光（NPQ）非光化学猝灭（qE）的机制。所谓的“叶黄素循环”在这方面似乎至关重要。叶黄素参与 qE/NPQ 的分子机制尚未建立。此外，由于类胡萝卜素的基态（S0，11Ag-）和最低激发单线态（S1，21Ag-）之间的跃迁在光学上是单光子禁阻的（“暗”），因此涉及类胡萝卜素的激发能量转移（EET）过程也很难研究。双光子激发光谱技术已经使用了二十多年来研究类胡萝卜素的单光子禁阻态。在当前的研究中，在叶黄素的假定 11Ag-→21Ag-（S0→S1）跃迁光谱区域中，测量了含有不同叶黄素补充剂的 LHCII 样品的双光子激发轮廓，以及溶液中分离的叶绿素 a 和 b。结果表明，在该光谱区域中，Chls 的直接双光子激发比叶黄素更为占主导地位。将讨论结果对拟议的 qE/NPQ 机制的影响。
