Utilization of blue-green light by chlorosomes from the photosynthetic bacterium Chloroflexus aurantiacus: Ultrafast excitation energy conversion and transfer

• Published in: Biochimica et biophysica acta. Bioenergetics 2021-06, Vol.1862 (6), p.148396-148396, Article 148396
• Main Authors: Yakovlev, Andrei G., Taisova, Alexandra S., Fetisova, Zoya G.
• Format: Article
• Language: English
• Subjects: Carotenoid; Chloroflexus aurantiacus; Energy relaxation; Excitation energy transfer; Photosynthesis; Pump-probe spectroscopy
• ISSN: 0005-2728; 1879-2650
• DOI: 10.1016/j.bbabio.2021.148396

Chlorosomes of photosynthetic green bacteria are unique molecular assemblies providing efficient light harvesting followed by multi-step transfer of excitation energy to reaction centers. In each chlorosome, 104–105 bacteriochlorophyll (BChl) c/d/e molecules are organized by self-assembly into high-ordered aggregates. We studied the early-time dynamics of the excitation energy flow and energy conversion in chlorosomes isolated from Chloroflexus (Cfx.) aurantiacus bacteria by pump-probe spectroscopy with 30-fs temporal resolution at room temperature. Both the S2 state of carotenoids (Cars) and the Soret states of BChl c were excited at ~490 nm, and absorption changes were probed at 400–900 nm. A global analysis of spectroscopy data revealed that the excitation energy transfer (EET) from Cars to BChl c aggregates occurred within ~100 fs, and the Soret → Q energy conversion in BChl c occurred faster within ~40 fs. This conclusion was confirmed by a detailed comparison of the early exciton dynamics in chlorosomes with different content of Cars. These processes are accompanied by excitonic and vibrational relaxation within 100–270 fs. The well-known EET from BChl c to the baseplate BChl a proceeded on a ps time-scale. We showed that the S1 state of Cars does not participate in EET. We discussed the possible presence (or absence) of an intermediate state that might mediates the Soret → Qy internal conversion in chlorosomal BChl c. We discussed a possible relationship between the observed exciton dynamics and the structural heterogeneity of chlorosomes. •EET from Cars to BChl c proceeds within ~100 fs.•Energy relaxation from the Soret to the Qy band of BChl c proceeds within ~40 fs.•Exciton and vibrational relaxation in the Qy band of BChl c proceeds within 100–270 fs.•The S1 state of Cars does not participate in EET.