Exploring Delocalised Energy Transport in Bacterial Reaction Centres

光合作用的生物体，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，这是细胞生长、功能和修复所必需的。反应中心(RCs)起着关键的作用，它接受数以千计的光吸收类胡萝卜素和(细菌)叶绿素分子吸收的能量，并用它来诱导电荷分离，产生电子(和空穴)。然后，电子被用来形成重要的质子浓度梯度，从而驱动三磷酸腺苷的合成。在低光强下，捕光天线的初始光捕获、(细菌)叶绿素分子间的快速能量传递和RCS中的电荷分离过程是100%有效的。尽管以前对RCS进行了许多研究，但一些重要的因素仍然没有被直接实验确定。球形红杆菌的RCs含有七个紧密堆积的光吸收分子：四个细菌叶绿素-a，两个细菌叶绿素-a(都是细菌氯色素)和一个15，15‘-顺式球状类胡萝卜素部分。相邻的RC颜料之间的能量转移发生在~100 fs的时间尺度上(1 fs=百万分之一秒)。能量转移的速度是最终电荷分离的基础，由RCS的电子结构和光激发分子共享电子的程度(离域)决定。在空间分离的分子区域中，发色团之间的相互作用是最小的，电子仍然局限在各自的分子上。然而，在RCS中，颜料之间的距离在5到10埃之间，通过分子间相互作用，电子可以在多种颜料上离域。到目前为止，还没有实验能够直接测量RC激发态的离域。使用比动力学过程更短的光脉冲的超快激光光谱仪可以用来拍摄系统的快照，并推断通过系统的能量流动的路径(S)和相关的时间尺度。一种新兴的技术，二维电子振动光谱将被用来研究RCS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电子结构的知识。类胡萝卜素在光合作用中起双重作用，既是辅助捕光色素，也是保护植物免受过度阳光伤害的调节元件。在它们的捕光能力中，它们可以吸收(细菌)叶绿素吸收较弱的部分太阳光谱。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率。类胡萝卜素到细菌素的能量传递机制尚未完全被描述，可能涉及到迄今为止被忽视的途径。二维电子光谱将被用来跟踪类胡萝卜素和不同的细菌素色素之间的能量转移，揭示增强RCS捕光能力的能量转移路径和相关的时间尺度。拟议的实验试图改变我们目前对球形红杆菌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流子产生事件之一做好准备。这项研究将提供RCS中分子间耦合的关键设计原则，并揭开高效能量转换的蓝图。这些设计原则将是设计生物灵感分子太阳能电池技术或水分离催化剂的关键。

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.