Vibronic Enhancement of Algae Light Harvesting

doi:10.1016/j.chempr.2016.11.002

Chem

Volume 1, Issue 6, 8 December 2016, Pages 858-872

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Highlights

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Phycobiliproteins such as PC645 efficiently harvest and down-convert solar energy
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2D spectroscopy reveals vibronic coherence between remote pigments
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Vibronic coupling leads to enhancement of the energy-transfer rate

The Bigger Picture

Light harvesting is the crucial first step in photosynthesis and operates at extraordinary efficiencies granted by the specific molecular architectures of antenna proteins. Cryptophyte algae utilize antennas with pigments that absorb in the middle of the solar spectrum, where plants and most algae lack absorptions, allowing them survival flexibility in a green world. This advantage comes at a cost because the absorbed energy must be down-converted and made compatible with chlorophyll complexes, where charge separation finally occurs. Even so, cryptophyte antennas can prepare an excitation for transfer in under a picosecond. In this work, the occurrence of coherence between two remote pigments in a cryptophyte antenna is found to be linked through a vibrational resonance, transiently delocalizing the excitation and modifying energy transfer. This vibronic interaction unveils a molecular design principle probably utilized by cryptophytes to enhance transfer efficiency.

Summary

Plants, algae, and photosynthetic bacteria use surprisingly sophisticated optimizations at the quantum mechanical level to harvest the sun's energy. The observation of coherence phenomena within light-harvesting complexes after short laser-pulse excitation has inspired advances in our understanding of light-harvesting optimization, highlighting the interplay of electronic excitations and vibrations. However, it remains unclear how these vibronic effects change or optimize the function of light-harvesting complexes—in other words, what is the design principle we could learn? Here, we use two-dimensional electronic spectroscopy to quantify the vibronic mixing among the light-absorbing molecules of a light-harvesting complex from cryptophyte algae. These data reveal a striking reallocation of absorption strength that, in turn, provides a robust increase in the rate of energy transfer of up to 3.5-fold. The realization of how absorption-strength redistribution, induced by vibronic coupling, provides a multiplicative increase in the rate of energy funneling establishes a bioinspired design principle for optimal light-harvesting systems.

Graphical Abstract

Keywords

photosynthesis

light-harvesting complexes

cryptophyte algae

ultrafast spectroscopy

two-dimensional electronic spectroscopy

UN Sustainable Development Goals

SDG7: Affordable and clean energy

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