Carotenoid-mediated light harvesting in green plants uncovered by ultrabroadband two-dimensional electronic spectroscopy

Abstract

Plants absorb across the visible solar spectrum and rapidly funnel the energy downhill to power growth. In excess sunlight, they dissipate harmful energy as heat to protect against photodamage. Previous measurements have been limited to the two lowest-energy, exclusively chlorophyll transitions of their light-harvesting machinery, leaving the carotenoid-mediated pathways unexplored. I will discuss the development of an ultrabroadband two-dimensional (2D) electronic spectrometer that enables mapping of the energy flow in the major light-harvesting protein of plants, LHCII, across the visible range. I will then discuss two previously inaccessible pathways of light harvesting as well as dissipation in LHCII, both mediated by carotenoids, uncovered by this apparatus. By analyzing the vibrational wavepackets in the 2D spectra, I identified a debated dark state (S_X) specific to a single carotenoid, lutein 2, that serves as a key mediator for efficient light harvesting. On a second carotenoid, lutein 1, I resolved a dissipative energy transfer from the chlorophyll to its dark S₁ state. The distinct photophysics revealed for these two chemically identical pigments highlight the capability of the protein binding pocket to control the electronic structure, and in turn, function of carotenoids in photosynthesis.

Speaker

Minjung Son, Ph.D. (Department of Chemistry, University of Wisconsin-Madison)

Minjung Son earned her B.S. (2013) and M.S. (2015) in Chemistry from Yonsei University, Seoul, South Korea, where she investigated ultrafast exciton dynamics of artificial light-harvesting molecular assemblies working in the laboratory of Prof. Dongho Kim. In 2015, she began her doctoral studies at the Massachusetts of Institute of Technology as a Robert T. Haslam Presidential Fellow. Under the supervision of Prof. Gabriela Schlau-Cohen, she developed ultrabroadband two-dimensional electronic spectroscopy, and applied it to the photosynthetic antenna complex of green plants to elucidate previously hidden photophysical pathways of light harvesting as well as photoprotective dissipation therein. Her work also contributed insights into the physiological relevance of the measured dynamics by implementing a model membrane that mimics the native plant membrane environment. After obtaining her Ph.D. in Chemistry in September 2020, she joined the group of Prof. Martin Zanni at the University of Wisconsin-Madison as a postdoctoral researcher. Her current research interests lie on the interface of materials science, physical chemistry, and optical spectroscopy, where she focuses on understanding and manipulating the photophysics of energy transport in organic photovoltaic materials and exciton-polaritons towards next-generation solar cell devices using two-dimensional white-light spectroscopy and microscopy.

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