Welc Renata, Luchowski Rafal, Grudzinski Wojciech, Puzio Michal, Sowinski Karol, Gruszecki Wieslaw I
Department of Biophysics, Institute of Physics, Maria Curie-Sklodowska University , 20-031 Lublin, Poland.
Faculty of Pharmacy, Medical University , Lublin, Poland.
J Phys Chem B. 2016 Dec 29;120(51):13056-13064. doi: 10.1021/acs.jpcb.6b10393. Epub 2016 Dec 15.
The main physiological function of LHCII (light-harvesting pigment-protein complex of photosystem II), the largest photosynthetic antenna complex of plants, is absorption of light quanta and transfer of excitation energy toward the reaction centers, to drive photosynthesis. However, under strong illumination, the photosynthetic apparatus faces the danger of photodegradation and therefore excitations in LHCII have to be down-regulated, e.g., via thermal energy dissipation. One of the elements of the regulatory system, operating in the photosynthetic apparatus under light stress conditions, is a conversion of violaxanthin, the xanthophyll present under low light, to zeaxanthin, accumulated under strong light. In the present study, an effect of violaxanthin and zeaxanthin on the molecular organization and the photophysical properties of LHCII was studied in a monomolecular layer system with application of molecular imaging (atomic force microscopy, fluorescence lifetime imaging microscopy) and spectroscopy (UV-Vis absorption, FTIR, fluorescence spectroscopy) techniques. The results of the experiments show that violaxanthin promotes the formation of supramolecular LHCII structures preventing dissipative excitation quenching while zeaxanthin is involved in the formation of excitonic energy states able to quench chlorophyll excitations in both the higher (B states) and lower (Q states) energy levels. The results point to a strategic role of xanthophylls that are not embedded in a protein environment, in regulation of the photosynthetic light harvesting activity in plants.
LHCII（光系统II的捕光色素-蛋白复合体）是植物最大的光合天线复合体，其主要生理功能是吸收光量子并将激发能传递至反应中心，以驱动光合作用。然而，在强光照射下，光合装置面临光降解的危险，因此LHCII中的激发必须被下调，例如通过热能耗散。在光胁迫条件下，光合装置中运行的调节系统的一个要素是将低光条件下存在的叶黄素紫黄质转化为强光下积累的玉米黄质。在本研究中，利用分子成像（原子力显微镜、荧光寿命成像显微镜）和光谱学（紫外-可见吸收光谱、傅里叶变换红外光谱、荧光光谱）技术，在单分子层系统中研究了紫黄质和玉米黄质对LHCII分子组织和光物理性质的影响。实验结果表明，紫黄质促进超分子LHCII结构的形成，防止耗散性激发猝灭，而玉米黄质参与激子能态的形成，能够猝灭高能级（B态）和低能级（Q态）的叶绿素激发。结果表明，未嵌入蛋白质环境的叶黄素在调节植物光合捕光活性中具有重要作用。
