New insights into the photochemistry of carotenoid spheroidenone in light-harvesting complex 2 from the purple bacterium Rhodobacter sphaeroides

详细信息    查看全文

• 作者：Dariusz M. Niedzwiedzki ; Preston L. Dilbeck ; Qun Tang…
• 关键词：Carotenoids ; Energy transfer ; Ultrafast spectroscopy ; Light ; harvesting complex ; Purple bacteria
• 刊名：Photosynthesis Research
• 出版年：2017
• 出版时间：March 2017
• 年：2017
• 卷：131
• 期：3
• 页码：291-304
• 全文大小：2139KB
• 刊物类别：Biomedical and Life Sciences
• 刊物主题：Plant Sciences; Plant Physiology; Biochemistry, general; Plant Genetics & Genomics;
• 出版者：Springer Netherlands
• ISSN：1573-5079
• 卷排序：131

文摘

Light-harvesting complex 2 (LH2) from the semi-aerobically grown purple phototrophic bacterium Rhodobacter sphaeroides was studied using optical (static and time-resolved) and resonance Raman spectroscopies. This antenna complex comprises bacteriochlorophyll (BChl) a and the carotenoid spheroidenone, a ketolated derivative of spheroidene. The results indicate that the spheroidenone-LH2 complex contains two spectral forms of the carotenoid: (1) a minor, “blue” form with an S₂ (11B_u+) spectral origin band at 522 nm, shifted from the position in organic media simply by the high polarizability of the binding site, and (2) the major, “red” form with the origin band at 562 nm that is associated with a pool of pigments that more strongly interact with protein residues, most likely via hydrogen bonding. Application of targeted modeling of excited-state decay pathways after carotenoid excitation suggests that the high (92%) carotenoid-to-BChl energy transfer efficiency in this LH2 system, relative to LH2 complexes binding carotenoids with comparable double-bond conjugation lengths, derives mainly from resonance energy transfer from spheroidenone S₂ (11B_u+) state to BChl a via the Q_x state of the latter, accounting for 60% of the total transfer. The elevated S₂ (11B_u+) → Q_x transfer efficiency is apparently associated with substantially decreased energy gap (increased spectral overlap) between the virtual S₂ (11B_u+) → S₀ (11A_g−) carotenoid emission and Q_x absorption of BChl a. This reduced energetic gap is the ultimate consequence of strong carotenoid–protein interactions, including the inferred hydrogen bonding.