The progress in design of molecular assemblies for energy transport is critically dependent on both predictive modeling and experimental characterization of structure and energy transfer dynamics in synthesized systems. The steady state circular dichroism (CD) spectroscopy has been providing a wealth of information on the structures of complex molecules. The use of time-resolved circular dichroism (TRCD) spectroscopy was, however, limited due to insufficient sensitivity. We report here the design of a new TRCD spectrometer whose sensitivity is orders of magnitude higher than that of the previously described setups, which makes that technique suitable to study the dynamics of a wide range of molecular systems in both femtosecond and nanosecond-to-millisecond time scales.<br/>This technique is applied to characterize a model energy transfer system - the Fenna-Matthews-Olson (FMO) pigment-protein complex, which is a part of photosynthetic light harvesting machinery of green sulfur bacteria that channels light excitation energy from chlorosomal antenna complex to the reaction center. A single FMO contains eight strongly coupled BChl molecules leading to excitonic delocalization of the excited states. Due to excitonic nature, the FMO was the first protein complex, for which quantum coherences between the excitonic states were observed in 1997 by the author and it was later proposed that entanglement between excitonic states can substantially enhance the energy transfer efficiency in such systems. In addition, a number of mutations have been generated in this complex controllably re-engineering its internal structure. In spite of extensive study of FMO by numerous groups, there is still a debate about the energies of the individual pigments in this system – more than 10 different Hamiltonians have been proposed to model its properties. Unlike conventional absorption, which is sensitive primarily to the spectral positions of the individual pigments and interactions between them, the spectral shape and sign of CD depends strongly on the mutual orientation of the interacting pigments, making TRCD an excellent tool to test the model simulations and refine the excitonic model as well as computational methods. In this work we report the first TRCD measurement of both triplet and singlet energy transfer dynamics in a photosynthetic complex and refinements of computational models based on the new data.

Energy transport in a strongly coupled photosynthetic system revealed by novel time resolved circular dichroism spectroscopy

2019 Fall National Meeting

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