Exploring Delocalised Energy Transport in Bacterial Reaction Centres

光合生物，如植物，藻类和细菌，利用来自阳光的能量来驱动所有下游过程，如碳水化合物的合成，这对细胞生长，功能和修复至关重要。反应中心（RC）在接受由1000个光吸收类胡萝卜素和（细菌）叶绿素分子吸收的能量方面发挥着关键作用，并利用它来诱导电荷分离，产生电子（和空穴）。然后，电子被用来形成驱动三磷酸腺苷合成的质子的重要浓度梯度。在低光强下，捕光天线中的初始光捕获、（细菌）叶绿素分子之间的快速能量转移和RC中的电荷分离的过程是显著地100%有效的。尽管许多先前的研究RC，几个重要的因素下的化学电荷分离的高产率尚未直接实验测定。红细菌sphaeroides RC含有七个紧密包装的光吸收分子：四个细菌叶绿素-a，两个细菌脱镁叶绿素-a（细菌叶绿素色素）和15，15 '-顺式-spheroidene类胡萝卜素部分。相邻RC颜料之间的能量转移发生在约100 fs的时间尺度上（1 fs =十亿分之一秒）。支撑最终电荷分离产率的能量转移速率由RC的电子结构以及光激发分子共享电子（离域）的程度决定。在空间分离的分子中，发色团之间的相互作用是最小的，并且电子保持在它们各自的分子上。然而，在RC中，颜料之间的距离在5和10埃之间，并且通过分子间相互作用，电子可以在多个颜料上变得离域。到目前为止，还没有实验能够直接测量RC激发态的离域。超快激光光谱使用比所涉及的动力学过程更短的光脉冲，可以用来拍摄系统的快照，并推断通过系统的能量流的路径和相关的时间尺度。二维电子振动光谱是一种新兴的技术，它将被用来研究RC中激发态的空间位置随时间的变化，并改变我们对分子间相互作用和RC电子结构的认识。类胡萝卜素色素在光合作用中起着双重作用，作为辅助捕光色素和调节元件，可以保护植物免受过度阳光造成的损害。在它们的光捕获能力中，它们可以吸收部分太阳光谱，其中（细菌）叶绿素吸收较弱。类胡萝卜素通过将能量传递给（细菌）叶绿素分子来增加太阳光谱的总覆盖范围。类胡萝卜素的细菌叶绿素的能量转移机制的RC尚未得到充分的表征，可能涉及的途径，迄今被忽视。二维电子光谱将被用来跟踪类胡萝卜素和不同的细菌绿素色素之间的能量转移，揭示能量转移途径和相关的时间尺度，提高RCs的光捕获能力。拟议的实验试图改变我们目前对Rhodobacter sphaeroides RCs的电子结构和能量如何在组成光吸收分子之间转移的描述，为自然界最有效的电荷载体生成事件之一做好准备。该研究将提供RCs分子间耦合的关键设计原则，并揭示高效能量转换的蓝图。这些设计原则将是工程生物启发分子太阳能电池技术或水分解催化剂的关键。

Photosynthetic organisms such as plants, algae and bacteria, harness the energy from sunlight to drive all downstream processes such as synthesis of carbohydrates, which are essential for cell growth, function, and repair. Reaction centres (RCs) play the pivotal role of accepting energy absorbed by 1000s of light absorbing carotenoid and (bacterio)chlorophyll molecules and use it to induce charge separation, generating electrons (and holes). Electrons are then used to form a vital concentration gradient of protons that drive adenosine triphosphate synthesis. The process of initial light capture in light harvesting antenna, rapid energy transfer between (bacterio)chlorophyll molecules and charge separation in RCs is, remarkably, 100% efficient under low light intensities. Despite many prior studies of RCs, several important factors underling the high yield of chemical charge separation have yet to be directly experimentally determined.RCs of Rhodobacter sphaeroides contain seven tightly packed light absorbing molecules: four bacteriochlorophylls-a, two bacteriopheophytins-a (both bacteriochlorin pigments) and a 15,15'-cis-spheroidene carotenoid moiety. Energy transfer between adjacent RC pigments takes place on ~100 fs timescales (1 fs = one millionth billionth of a second).The rate of energy transfer, which underpins the yield of eventual charge-separation, is dictated by the electronic structure of RCs, and the extent to which photoexcited molecules share electrons (delocalisation). In the regime of spatially separated molecules, the interaction between chormophores is minimal and electrons remain localised on their respective molecules. However, in RCs the distances between pigments ranges between 5 and 10 Angstroms and through inter-molecular interactions electrons can become delocalised over multiple pigments. To date, no experiment has been able to directly measure the delocalisation of RC excited states.Ultrafast laser spectroscopies using pulses of light shorter than the dynamical processes involved can be used to take snapshots of the system and infer the route(s) and associated timescales of energy flow through the system. One such emerging technique, two-dimensional electronic-vibrational spectroscopy will be used to investigate the spatial location of excited states in RCs as a function of time, and transform our knowledge of the inter-molecular interactions and of the RC electronic structure.Carotenoid pigments play a dual role in photosynthesis, acting as both accessory light harvesting pigments and regulatory elements that can protect plants from damage caused by excessive sunlight. In their light harvesting capacity, they can absorb parts of the solar spectrum where (bacterio)chlorophyll absorption is weak. Carotenoids increase the total coverage of the solar spectrum by transferring energy to (bacterio)chlorophyll molecules. The carotenoid to bacteriochlorin energy transfer mechanisms for RCs have not been fully characterised and may involve pathways that have hitherto been ignored. Two-dimensional electronic spectroscopy will be used to follow the energy transfer between carotenoid and different bacteriochlorin pigments, revealing the energy transfer pathways and associated timescales that enhance the light harvesting capability of RCs.The proposed experiments seek to transform our current description of the electronic structure of Rhodobacter sphaeroides RCs and how energy is transferred between constituent light absorbing molecules, preparing the system for one of nature's most efficient charge-carrier generation events. The study will provide key design principles of inter-molecular couplings in RCs, and unravel the blueprint for the efficient energy transduction. These design principles will be key for engineering bio-inspired molecular solar cell technology or water splitting catalysts.