Investigating electrostatic pigment-protein interactions and energy transfer

As scientists seek to understand how photosynthetic light-harvesting complexes work, the Fenna-Matthew-Olson (FMO) protein serves as an important model system. FMO proteins mediate energy transfer between an outer antenna system in green sulfur bacteria, known as the chlorosome, and the reaction center complex.

Klinger et al. analyzed the vibrations of the FMO protein and demonstrated the importance of electrostatic pigment-protein interactions, and validated critical approximations in the description of energy transfer.

“We developed a simple and accurate multiscale method for the structure-based description of energy transfer in pigment-protein complexes,” said author Thomas Renger.

When proteins move, they tune the optical transition energies or the color of the pigments. During these processes, proteins take the excess energy of excited states and allow the excitation energy to relax towards low-energy states that are located close to the reaction center.

The authors found that low energy vibrations of the complex were delocalized over the entire FMO trimer, and were responsible for correlated fluctuations in the optical transition energies of the pigments. These correlations, however, had practically no influence on energy transfer, an effect that is explained by the different frequency dependencies of the diagonal and off-diagonal parts of spectral density. They were surprised to find that quantum mechanical delocalization of excited states in the FMO monomers leads to a delay of intermonomer transfer. However, they reckon that green sulfur bacteria should be able to tolerate this delay due to other transfer steps with longer timescales.

They hope their results will help future research on energy transfer in artificial light-harvesting systems and electrostatic transition energy shifts of biological photoreceptors.

Source: “Normal mode analysis of spectral density of FMO trimers: Intra- and intermonomer energy transfer,” by Alexander Klinger, Dominik Lindorfer, Frank Müh, and Thomas Renger, Journal of Chemical Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0027994 .

This paper is part of the Excitons: Energetics and Spatio-temporal Dynamics, learn more here .