Blue-Green Light Harvesting in the Ocean: Theory meets Experiment

全球大约40%的光合作用太阳能转换发生在被称为硅藻的海洋微藻和鞭毛藻中。这些生物采用聚光器的水下光照条件和振子强度转移到蓝色和绿色光谱区域采用叶绿素c(排名c)和墨角藻黄素(Fx)硅藻和peridinin(每)鞭毛藻类作为额外的色素无处不在的背影。最近报道的高分辨率的晶体结构的聚光fucoxanthin-chlorophyll a / c蛋白(FCP)硅藻的高分辨率叶绿素蛋白（PCP）的结构是基于结构理解海洋光捕获的关键。我们的中心假设是，与植物相比，海洋生物使用类胡萝卜素作为主要的光收集色素，以适应水下不同的光条件。开放的研究问题涉及光收集过程的分子细节。我们的目的是确定Fx和Per的不同光谱形式以及Chl a和c颜料的局部激发能。基于这种参数化，将有可能研究色素间能量传递的细节。我们面临的一个关键问题是，从Fx和Per的吸光S2状态到其光学暗S1状态的激发能的分子内快速衰减，并从那里（很可能通过分子内电荷转移（ICT）状态）进入电子基态。如何通过激发能向Chl a和c的超快转移来绕过这个损失通道是一个悬而未决的问题，我们将在理论上和实验上进行研究。结合量子化学计算和静电/分子力学/非绝热激发态分子动力学模拟，采用多尺度方法对FCP和PCP的Frenkel激子哈密顿量进行了参数化。利用非平衡量子统计的微扰和非微扰方法计算的光谱，与文献和合作伙伴的实验数据进行了比较。关键的创新在于将前沿的基于结构的理论与高端的超快光谱技术相结合，如基于二维电子能谱的波包干涉测量。

About 40 percent of the global photosynthetic solar energy conversion takes place in marine microalgae termed diatoms and in dinoflagellates. These organisms have adopted their light-harvesting apparatus to the light conditions under water and have shifted oscillator strength into the blue and green spectral regions by employing chlorophyll c (Chl c) and fucoxanthin (Fx) in diatoms and peridinin (Per) in dinoflagellates as additional pigment to the ubiquitous Chl a. The recently reported high-resolution crystal structure of the light-harvesting fucoxanthin-chlorophyll a/c protein (FCP) of diatoms together with the high-resolution structure of the peridinin-chlorophyll protein (PCP) hold the key for a structure-based understanding of marine light-harvesting. Our central hypothesis is that marine organisms in contrast to plants use carotenoids as major light-harvesting pigments in order to adopt to the different light conditions under water. Open research questions concern the molecular details of the light-harvesting process. Our objective is to identify the different spectral forms of Fx and Per as well as the local excitation energies of the Chl a and c pigments. Based on this parameterization it will be possible to study the details of the interpigment energy transfer. A key problem that we face is the fast intramolecular decay of excitation energy from the light-absorbing S2 state of Fx and Per into its optically dark S1 state and from there (most likely via an intramolecular charge transfer (ICT) state) into the electronic ground state. How this loss channel is circumvented by ultrafast transfer of excitation energy to Chl a and c is an open question that we will investigate, both theoretically and experimentally. The Frenkel exciton Hamiltonian of FCP and PCP is parameterized with multiscale methods combining quantum chemical calculations with electrostatic/molecular mechanics/non-adiabatic excited state molecular dynamics simulations. The spectra calculated, using perturbative and non-perturbative methods from non-equilibrium quantum statistics, are compared with experimental data from the literature and from our collaboration partner. The key innovation lies in the combination of cutting-edge structure-based theory and high-end ultrafast spectroscopic techniques such as wavepacket interferometry based on 2D electronic spectroscopy.