Pigments Controlling the Quantum Efficiency of Photosynthetic Light Harvesting

光合作用的生物过程是地球上所有生命赖以生存的基础。自从氧合光合作用出现以来，放氧生物为所有异养生物提供了食物和利用食物所需的氧气。最近，光合作用生物为人类提供了大量有用的化合物，包括燃料、药物、纤维、色素和动物饲料，显然，植物衍生商品在未来绿色经济中的重要性只会增加。地球上氧合光合作用的激增可能归因于光合作用分子机制的有效进化设计，以及它们对不断变化的环境条件的适应性。叶绿体的光合膜，俗称类囊体膜，是所有生物膜中最复杂的，极大地丰富了各种蛋白质复合体的集合。这些蛋白质复合体代表了复杂的、多步骤的光合作用过程所必需的分子机制，执行各种任务，如捕光、电子传递和合成重要的生化化合物。为了发挥这些作用，各种膜蛋白结合了许多辅助因子，如叶绿素、类胡萝卜素、脂类、水和各种金属离子。类囊体膜中发现的主要色素-脂蛋白复合体之一是光系统II的捕光复合体LHCII。这种复合体是由三个相同的蛋白质组成的三聚体，每个蛋白质结合18个光合色素分子，收集类囊体膜接收的光能，并将其转移到光合作用反应中心。除了LHCII，还有两个被称为CP26和CP29的次要捕光复合体。除了这一作用外，LHCII已被发现在调节传递到反应中心的能量方面发挥重要作用。这是通过消散在强烈光照期间吸收的多余能量来实现的，这一过程通常被称为光保护。最近我们发现，目前可用的LHCII结构对应于该络合物的光保护构象的结构。这一发现具有特别重要的意义，因为它为类囊体膜如何感知和响应过度光照，从而光合作用系统如何保护自己免受光损伤提供了独特的结构见解。也有人提出，负责光保护的是CP26和CP29，而不是LHCII。这项拟议工作的目的是了解被称为叶黄素的光合色素如何调节主要和次要天线复合体传递给反应中心的光能。这项工作分为两个交配部分，每个部分都将使用来自实验生物物理、理论物理和量子化学领域的独特方法组合来完成。首先，了解LHCII的特定叶黄素互补如何影响天线复合体中的能量耗散率是至关重要的。这将通过天线复合体的光谱测量和激发能量传递的理论模型来实现。其次，我们将调查LHCII、CP26或CP29中的特定位点是否负责多余能量的耗散。这将需要对光合色素的电子性质进行详细的理论建模，对天线复合体内的能量传递和耗散进行理论模拟，并对突变体和自然标本的天线复合体内的能量传递路径进行详细的光谱映射。

The biological process of photosynthesis is the foundation upon which all life on Earth is supported. Since the advent of oxygenic photosynthesis, oxygen-evolving organisms have provided all heterotrophic organisms with both food and the oxygen required to utilize it. More recently, photosynthetic organisms have provided humans with a huge variety of useful compounds, including fuels, pharmaceuticals, fibres, pigments, and animal feeds and it has become apparent that plant-derived commodities will only increase in importance in the future green economy. The proliferation of oxygenic photosynthesis on Earth may be attributed to the efficient evolutionary design of the molecular machinery of photosynthesis, along with their adaptability with respect to changing environmental conditions. The photosynthetic membranes of the chloroplasts, commonly known as the thylakoid membranes, are the most complex of all biological membranes, being greatly enriched in a diverse collection of various protein complexes. These protein complexes represent the necessary molecular machinery of complex, multi-step process of photosynthesis, carrying out such diverse tasks as light-harvesting, electron transport and the synthesis of vital biochemical compounds. In order to fulfil these roles the various membrane proteins bind a number of cofactors such as chlorophylls, carotenoids, lipids, water, and various metal ions. One of the major pigment-lipoprotein complexes found within the thylakoid membrane is the light-harvesting complex of photosystem-II, LHCII. This complex, which is a trimer of three identical proteins, each binding 18 photosynthetic pigment molecules, collects light energy received by the thylakoid membrane and transfers it to the photosynthetic reaction centres. In addition to LHCII there are two minor light-harvesting complexes known as CP26 and CP29. In addition to this role LHCII, has been found to play an important role in regulating the amount of energy that is delivered to the reaction centre. This is achieved via the dissipation of excess energy absorbed during periods of intense illumination, a process commonly referred to as photoprotection. Recently we have discovered that the currently available structure of LHCII corresponds to the structure of a photoprotective conformation of the complex. This finding is of particular importance since it offers unique structural insights into how the thylakoid membrane senses and responds to excessive illumination, and hence how photosynthetic systems protect themselves against photodamage. It has also been suggested that the minor antennas, CP26 and CP29, rather than LHCII are responsible for photoprotection. The aim of this proposed work is to understand how the photosynthetic pigments known as xanthophylls regulate the amount of light energy delivered to the reactions centres by the major and minor antenna complexes. This work is divided into two copuled parts, each of which will employ a unique combination of methods from the fields of experimental biophysics, theoretical physics, and quantum chemistry to complete. First, it is essential to understand how varying the specific xanthophyll compliment of LHCII affects the rate of energy dissipation in the antenna complex. This will be achieved via spectroscopic measurements of the antenna complexes and theoretical modelling of the transfer of excitation energy. Second, we will investigate whether specific sites in LHCII, CP26, or CP29 are responsible for the dissipation of excess energy. This will require detailed theoretical modelling of the electronic properties of the photosynthetic pigments, theoretical simulation of the transfer and dissipation of energy within the antenna complexes, and detailed spectroscopic mapping of the energy transfer pathways within the antenna complexes of both mutants and natural specimens.