Niedzwiedzki Dariusz M, Dilbeck Preston L, Tang Qun, Martin Elizabeth C, Bocian David F, Hunter C Neil, Holten Dewey
Department of Chemistry, Washington University in St. Louis, St. Louis, MO, 63130, USA.
Photosynthetic Antenna Research Center, Washington University in St. Louis, Campus Box 1138, St. Louis, MO, 63130, USA.
Photosynth Res. 2017 Mar;131(3):291-304. doi: 10.1007/s11120-016-0322-2. Epub 2016 Nov 16.
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 S (1B ) 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 S (1B ) state to BChl a via the Q state of the latter, accounting for 60% of the total transfer. The elevated S (1B ) → Q transfer efficiency is apparently associated with substantially decreased energy gap (increased spectral overlap) between the virtual S (1B ) → S (1A ) carotenoid emission and Q absorption of BChl a. This reduced energetic gap is the ultimate consequence of strong carotenoid-protein interactions, including the inferred hydrogen bonding.
利用光学（静态和时间分辨）光谱以及共振拉曼光谱，对在半好氧条件下生长的球形红细菌中的捕光复合物2（LH2）展开了研究。该天线复合物由细菌叶绿素（BChl）a和类胡萝卜素球形烯酮组成，球形烯酮是球形烯的一种酮化衍生物。结果表明，球形烯酮-LH2复合物包含两种类胡萝卜素光谱形式：（1）一种次要的“蓝色”形式，其S（1B ）光谱起源带位于522 nm处，相较于在有机介质中的位置，仅因结合位点的高极化率而发生了位移；（2）主要的“红色”形式，其起源带位于562 nm处，与一组与蛋白质残基相互作用更强的色素有关，很可能是通过氢键作用。对类胡萝卜素激发后的激发态衰变途径进行靶向建模的结果表明，相较于结合具有可比双键共轭长度类胡萝卜素的LH2复合物，该LH2系统中类胡萝卜素到BChl的高能量转移效率（92%）主要源于球形烯酮S（1B ）态通过BChl a的Q态向BChl a的共振能量转移，占总转移的60%。S（1B ）→Q转移效率的提高显然与类胡萝卜素虚拟S（1B ）→S（1A ）发射与BChl a的Q吸收之间的能隙大幅减小（光谱重叠增加）有关。这种减小的能隙是类胡萝卜素与蛋白质之间强相互作用的最终结果，包括推测的氢键作用。
