Site, Rate, and Mechanism of Photoprotective Quenching in Cyanobacteria

详细信息    查看全文

• 作者：Lijin Tian ; Ivo H. M. van Stokkum ; Rob B. M. Koehorst ; Aniek Jongerius ; Diana Kirilovsky ; Herbert van Amerongen
• 刊名：Journal of the American Chemical Society
• 出版年：2011
• 出版时间：November 16, 2011
• 年：2011
• 卷：133
• 期：45
• 页码：18304-18311
• 全文大小：973K
• 年卷期：v.133,no.45(November 16, 2011)
• ISSN：1520-5126

文摘

In cyanobacteria, activation of the Orange Carotenoid Protein (OCP) by intense blue-green light triggers photoprotective thermal dissipation of excess absorbed energy leading to a decrease (quenching) of fluorescence of the light harvesting phycobilisomes and, concomitantly, of the energy arriving to the reaction centers. Using spectrally resolved picosecond fluorescence, we have studied cells of wild-type Synechocystis sp. PCC 6803 and of mutants without and with extra OCP (螖OCP and OverOCP) both in the unquenched and quenched state. With the use of target analysis, we managed to spectrally resolve seven different pigment pools in the phycobilisomes and photosystems I and II, and to determine the rates of excitation energy transfer between them. In addition, the fraction of quenched phycobilisomes and the rates of charge separation and quenching were resolved. Under our illumination conditions, 72% of the phycobilisomes in OverOCP appeared to be substantially quenched. For wild-type cells, this number was only 29%. It is revealed that upon OCP activation, a bilin chromophore in the core of the phycobilisome, here called APC^Q₆₆₀, with fluorescence maximum at 660 nm becomes an effective quencher that prevents more than 80% of the excitations in the phycobilisome to reach Photosystems I and II. The quenching rate of its excited state is extremely fast, that is, at least (240 卤 60 fs)^鈭?. It is concluded that the quenching is most likely caused by charge transfer between APC^Q₆₆₀ and the OCP carotenoid hECN in its activated form.