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

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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.