Quenching of bacteriochlorophyll a triplet state by carotenoids in the chlorosome baseplate of green bacterium Chloroflexus aurantiacus

Abstract

To capture weak light fluxes, green photosynthetic bacteria have unique structures – chlorosomes, consisting of 10^4–5 molecules of bacteriochlorophyll (BChl) c, d, e. Chlorosomes are attached to the cytoplasmic membrane through the baseplate, a paracrystalline protein structure containing BChl a and carotenoids (Car). The most important function of Car is the quenching of triplet states of BChl, which prevents the formation of singlet oxygen and thereby provides photoprotection. In our work, we studied the dynamics of the triplet states of BChl a and Car in the baseplate of Chloroflexus aurantiacus chlorosomes using picosecond differential spectroscopy. BChl a of the baseplate was excited into the Q_y band at 810 nm, and the corresponding absorption changes were recorded in the range of 420–880 nm. It was found that the formation of the Car triplet state occurs in ∼1.3 ns, which is ∼3 times faster than the formation of this state in the peripheral antenna of C. aurantiacus according to literature data. The Car triplet state was recorded by the characteristic absorption band T₁ → T_n at ∼550 nm. Simultaneously with the appearance of absorption T₁ → T_n, there was a bleaching of the singlet absorption of Car in the region of 400–500 nm. Theoretical modeling made it possible to estimate the characteristic time of formation of the triplet state of BChl a as ∼0.5 ns. It is shown that the experimental data are well described by the sequential scheme of formation and quenching of the BChl a triplet state: BChl a* → BChl a^T → Car^T. Thus, carotenoids from green bacteria effectively protect the baseplate from possible damage by singlet oxygen.