Light sizes up the internal structure of chlorosomes

Chlorosomes, the photosynthetic organelles of green bacteria, are nature’s most efficient light-harvesting systems. The size of tubular aggregates of bacteriochlorophyll molecules inside of chlorosomes affects their efficiency. To potentially incorporate this high efficiency into biomimetic applications, researchers must understand the structure of these aggregates using techniques capable of probing single chlorosomes.

In a theoretical study, Kunsel et al. demonstrated a combination of single-chlorosome spectroscopies, using linearly and circularly polarized light, can assess size variations of tubular bacteriochlorophyll aggregates inside chlorosomes of green photosynthetic bacteria.

Measuring the linear dichroism spectrum, as a function of polarization angle, and circular dichroism spectrum, as a function of angle of incidence, a single chlorosome can provide the average radius and average length of its bacteriochlorophyll aggregates. This combination of spectroscopy techniques could be used to explore the variations between individual chlorosomes that occur due to different growth stages, growth conditions, or bacterial strains.

“This is an important result, because excitation energy transport efficiencies in these light-harvesting systems depend on these quantities, which means that their light-harvesting functionality depends on them,” said author Jasper Knoester. “An important ingredient here is that we investigated all this as function of the angle of incidence of the circularly polarized light.”

The results suggested a 45-degree angle of incidence can retrieve size variation if the circular dichroism spectrum cannot be measured for a wide range of angles of incidence.

The authors will experimentally test the predictions made in this work on single tubular molecular aggregates. They hope the concept of combining spectroscopic techniques can be extended to the measurement of structural variation in other types of molecular aggregates.

Source: “Probing size variations of molecular aggregates inside chlorosomes using single-object spectroscopy,” by T. Kunsel, L. M. Günther, J. Köhler, T. L. C. Jansen, and J. Knoester, Journal of Chemical Physics (2021). The article can be accessed at https://aip.scitation.org/doi/full/10.1063/5.0061529 .