Nanosecond Laser Photolysis Studies of Chlorosomes and Artificial Aggregates Containing Bacteriochlorophyll e: Evidence for the Proximity of Carotenoids and Bacteriochlorophyll a in Chlorosomes from Chlorobium phaeobacteroides strain CL1401¶

Abstract

Time‐resolved, laser‐induced changes in absorbance, ΔA(λ; t), have been recorded with a view to probing pigment–pigment interactions in chlorosomes (control as well as carotenoid‐depleted) and artificial aggregates of bacteriochlorophyll e (BChle). Control chlorosomes were isolated from Chlorobium phaeobacteroides strain CL1401, whose chromophores comprise BChle, bacteriochlorophyll a (BChla) and several carotenoid (Car) pigments; Car‐depleted chlorosomes, from cells grown in cultures containing 2‐hydroxybiphenyl. Artificial aggregates were prepared by dispersing BChle in aqueous phase in the presence of monogalactosyl diglyceride. In chlorosomes ΔA(λ; t) shows, besides a signal attributable to triplet Car (with a half‐life of about 4 μs), signals in the Qy regions of both BChl. The BChla signal decays at the same rate as the Car signal, which is explained by postulating that some Car are in intimate contact with some baseplate BChla pigments, and that when a ground‐state Car changes into a triplet Car, the absorption spectrum of its BChla neighbors undergoes a concomitant change (termed transient environment‐induced perturbation). The signal in the Qy‐region of BChle behaves differently: its amplitude falls, under reducing conditions, by more than a factor of two during the first 0.5 μs (a period during which the Car signal suffers negligible diminution), and is much smaller under nonreducing conditions. The BChle signal is also attributed to transient environment‐induced perturbation, but in this case the perturber is a BChle photoproduct (probably a triplet or a radical ion). The absence of long‐lived BChle triplets in all three systems, and of long‐lived BChla triplets in chlorosomes, indicates that BChle in densely packed assemblies is less vulnerable to photodamage than monomeric BChle and that, in chlorosome, BChla rather than BChle needs, and receives, photoprotection from an adjacent Car.

Triplet Exciton Formation as a Novel Photoprotection Mechanism in Chlorosomes of Chlorobium tepidum

“…For the oxygen tolerance experiment, cells were grown to late exponential phase, diluted 100-fold in aerobic phosphate buffer, and exposed to an air-saturated atmosphere under dark conditions or variable low lightintensity conditions (20,30, and 40 mmol m À2 s À1 ) for two days. Cells were later diluted 10,000-fold and plated to allow single colonies to develop.…”

“…This is also consistent with the recent x-ray scattering and electron cryomicroscopy data (27) that suggest that carotenoids are localized between the planes of BChl c and drive assembly of chlorosomal antenna by augmenting hydrophobic interactions. The photoprotection realized by the quenching of the BChl triplet states ( 3 BChl*) in chlorosomes or scavenging singlet oxygen has also been tested, and triplet energy transfer to carotenoids was detected using different experimental techniques (23,(28)(29)(30). The structural role played by carotenoids in chlorosomes is also manifested by decreased levels of the BChl a when carotenoid levels in chlorosomes are reduced (31,32), although this could also be a consequence of lessened photoprotection.…”

Bacteriochlorophyll e Monomers, but Not Aggregates, Sensitize Singlet Oxygen: Implications for a Self-photoprotection Mechanism in Chlorosomes¶

“…A xenon arc lamp (250 W) and a mechanical chopper (10 Hz) provided the analyzing beam. For other details, the reader is requested to refer to our earlier publication (16).…”

“…3 contain no discernible contribution from Car , the dominant feature of the corresponding spectra of hexanol-treated chlorosomes (Fig. 4) is the TmS spectrum of Car, which is known to have (in the case of unperturbed chlorosomes) a positive peak around 530 nm arising from T n ‹ T 1 absorption (16). Although the initial Car formation cannot be time resolved by the TmS setup, it is unlikely to be a consequence of the direct photoexcitation of the Car because intersystem crossing in Car is inefficient; the initial Car signal is therefore attributed to triplet energy transfer from BChl .…”

“…Car molecules in chlorosomes are supposed to participate in photoprotection, mainly by quenching the electronic excitation of BChl † , a BChl molecule in the lowest triplet state (9,10), to function as light‐harvesting pigments (11–13) and to play a structural role (14,15). Spectroscopic investigations, based on nanosecond laser photolysis (16) and optically detected magnetic resonance (17), have indicated that Car molecules photoprotect BChl a rather than BChl x ; no evidence was found for triplet–triplet energy transfer from BChl x † to the Car ground state (Car 0 ) or for a long‐lived BChl x † . These results imply that Car molecules might be close to the baseplate to photoprotect the vulnerable BChl a monomers.…”