Dynamic Stark effect in β and γ carotenes induced by photoexcitation of bacteriochlorophyll c in chlorosomes from Chloroflexus aurantiacus

Self-aggregation of 132,132-disubstituted bacteriochlorophyll-d analog

Zinc methyl 132,132-disubstituted 3-hydroxymethyl-pyropheophorbides-a were prepared as models of bacteriochlorophyll-d, which self-aggregated in the main light-harvesting antenna (chlorosome) of photosynthetic green bacteria. The synthetic zinc 31-hydroxy-131-oxo-chlorins possessing methyl and methoxycarbonyl groups at the 132-position could not self-aggregate in an aqueous Triton X-100 solution. However, another model compound bearing an ethane-1,2-diyl group at the 132-position did self-aggregate under the same conditions to give red-shifted and broadened Qy and Soret absorption bands. The spiro-cyclopropane condensation slightly suppressed the chlorosome-like self-aggregation due to an increase in the steric hindrance around the 13-carbonyl group. The red-shifted and broadened values of these bands by the self-aggregation were dependent on the 132-substituents. The 132-substitution substantially controlled the aqueous J-aggregation.

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

To capture weak light fluxes, green photosynthetic bacteria have unique structures - chlorosomes, consisting of 104-5 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 Qy 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 → Tn at ∼550 nm. Simultaneously with the appearance of absorption T1 → Tn, 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 aT → CarT. Thus, carotenoids from green bacteria effectively protect the baseplate from possible damage by singlet oxygen.