Intensity Dependence of the Excited State Lifetimes and Triplet Conversion Yield in the Fenna鈥揗atthews鈥揙lson Antenna Protein

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• 作者：Gregory S. Orf ; Dariusz M. Niedzwiedzki ; Robert E. Blankenship
• 刊名：Journal of Physical Chemistry B
• 出版年：2014
• 出版时间：February 27, 2014
• 年：2014
• 卷：118
• 期：8
• 页码：2058-2069
• 全文大小：921K
• 年卷期：v.118,no.8(February 27, 2014)
• ISSN：1520-5207

文摘

The Fenna鈥揗atthews鈥揙lson (FMO) protein is a soluble light-harvesting, bacteriochlorophyll a (BChl a) containing antenna complex found in green sulfur bacteria. We have measured time-resolved fluorescence and transient absorption at variable laser intensities at 298 and 77 K using FMO protein from Chlorobaculum tepidum prepared in both oxidizing and reducing environments. Fitting of the spectroscopic data shows that high laser intensities (i.e., above 10¹³ photons 脳 cm^鈥? delivered per laser pulse) distort the intrinsic decay processes in this complex. At high laser intensities, both oxidized and reduced FMO samples behave similarly, exhibiting high levels of singlet鈥搒inglet annihilation. At lower laser intensities, the reduced protein mainly displays a singlet excited state lifetime of 2 ns, although upon oxidation, a 60 ps lifetime dominates. We also demonstrate that the apparent quantum yield of singlet鈥搕riplet intersystem crossing in the reduced FMO complex is 11% in the most favorable low laser intensities, with this yield decreasing and the probability of singlet鈥搒inglet annihilation yield increasing as laser intensity increases. After correcting for stimulated emission effects in the experiments, the actual maximum triplet yield is calculated to be 27%. Experiments at 77 K demonstrate that BChl a triplet states in FMO are localized on pigments no. 4 or 3, the lowest energy pigments in the complex. This study allows for a discussion of how BChl triplets form and evolve on the picosecond-to-nanosecond time scale, as well as whether triplet conversion is a physiologically relevant process.