On Excitation Energy Transfer within the Baseplate BChl a-CsmA Complex of Chloroflexus aurantiacus.

Abstract

Recently, a hybrid approach combining solid-state NMR spectroscopy and cryo-electron microscopy showed that the baseplate in the green sulfur bacterium Chlorobaculum (Cb.) tepidum is a 2D lattice of BChl a-CsmA dimers [Nielsen, J. T.; et al., Nat. Commun. 2016, 7, 12454-12465]. While the existence of the BChl a-CsmA subunit was previously known, the proposed orientations of the BChl a pigments had only been elucidated from spectral data up to this point. Regarding the electronic structure of the baseplate, two models have been proposed. 2D electronic spectroscopy data was interpreted as revealing that at least four excitonically coupled BChl a might be in close contact. Conversely, spectral hole burning data suggested the lowest energy state was localized, yet additional state are sometimes observed due to the presence of the Fenna-Matthews-Olson (FMO) antenna protein. To solve this conundrum, this work studies the chlorosome-baseplate complex from Chloroflexus (Cfx.) aurantiacus, which does not contain the FMO protein. The results confirm that in both Cb. tepidum and Cfx. aurantiacus excitation energy is transferred to a localized low-energy trap state near 818 nm with similar rates, most likely via exciton hopping.