How Purple Bacteria Capture Light Without Perfectly Tuned Antenna Complexes

Greg Howard
22nd June, 2024

How Purple Bacteria Capture Light Without Perfectly Tuned Antenna Complexes

Image Source: Natural Science News, 2024

Key Findings

  • The study focused on light-harvesting complexes (LH2) from purple bacteria, specifically comparing two variants from different species
  • Despite structural differences, the energy transport and relaxation dynamics in both LH2 variants were remarkably similar
  • This suggests that the light-harvesting functionality of LH2 complexes is highly robust to structural changes and does not rely on finely tuned electronic conditions
The ring-like peripheral light-harvesting complex 2 (LH2) expressed by many phototrophic purple bacteria has long been a model system in biological light-harvesting research. Known for its robustness, small size, and well-documented crystal structure, LH2 complexes are instrumental in understanding how structural variations influence electronic structures and optical properties. A recent study by the Technical University of Munich[1] delves deeper into these relationships by comparing the photo-induced dynamics in two structurally distinct LH2 variants. The study employed polarization-controlled 2D electronic spectroscopy at cryogenic temperatures to access information on dynamic and static disorder within the complexes. This technique allowed the researchers to characterize ultrafast energy relaxation and exciton transport within the complexes with both optimal spectral and temporal resolution. Despite clear differences in electronic structure and disorder due to molecular structural variations, the energy-transport and relaxation dynamics remained remarkably similar between the two LH2 variants. This finding suggests that the light-harvesting functionality of purple bacteria within a single LH2 complex is highly robust to structural perturbations and does not rely on finely tuned electronic- or electron-vibrational resonance conditions. Earlier studies have contributed to the understanding of light-harvesting complexes. Nonlinear spectroscopy has revealed long-lasting oscillations in the optical response of various photosynthetic complexes, which are thought to arise from coherent coupling of electronic (excitonic) or electronic-vibrational (vibronic) degrees of freedom[2]. These oscillations have been observed in fluorescent traces from time-delayed two-pulse single-molecule experiments. The current study's findings align with these earlier observations by indicating that the LH2 complex's functionality is robust and does not depend on finely tuned resonances, thus supporting the idea that both excitonic and vibronic interactions play complementary roles in photosynthetic systems. Additionally, femtosecond fluorescence anisotropy measurements on cyclic porphyrin arrays have shown that the excitation energy hopping process between porphyrin units can be well described by Förster incoherent excitation hopping[3]. These porphyrin arrays, with their structural and chromophoric similarities to natural light-harvesting arrays, have proven useful in understanding energy migration processes. The current study's findings extend this understanding by demonstrating that despite structural variations, the energy transport and relaxation dynamics in LH2 complexes remain consistent, further underscoring the robustness of these biological systems. The 2.4 Ångström resolution structure of the LH2 complex from Marichromatium purpuratum, determined by cryogenic electron microscopy, revealed a unique heptameric ring structure that controls the resonant coupling of the long-wavelength energy absorption band[4]. This structural insight complements the current study by providing a detailed understanding of the assembly and oligomeric form of purple bacterial LH2 complexes. The present research builds on this structural knowledge to explore how variations in molecular structure impact electronic properties and energy dynamics, ultimately showing that these complexes maintain their functionality despite such variations. In summary, the study from the Technical University of Munich demonstrates that the light-harvesting functionality of LH2 complexes in purple bacteria is highly resilient to structural changes. This robustness suggests that precise electronic- or electron-vibrational resonance conditions are not critical for their function, which aligns with and expands upon earlier findings in the field[2][3][4]. This research provides valuable insights into the interplay between structure and function in biological light-harvesting systems, emphasizing the robustness and adaptability of these complexes.

BiotechGeneticsBiochem

References

Main Study

1) Light harvesting in purple bacteria does not rely on resonance fine-tuning in peripheral antenna complexes

Published 21st June, 2024

https://doi.org/10.1007/s11120-024-01107-4

Related Studies

2) Theory of Excitonic Delocalization for Robust Vibronic Dynamics in LH2.

https://doi.org/10.1021/acs.jpclett.8b00933

3) Excitation energy transport processes of porphyrin monomer, dimer, cyclic trimer, and hexamer probed by ultrafast fluorescence anisotropy decay.

Journal: Journal of the American Chemical Society, Issue: Vol 125, Issue 19, May 2003

4) The 2.4 Å cryo-EM structure of a heptameric light-harvesting 2 complex reveals two carotenoid energy transfer pathways.

https://doi.org/10.1126/sciadv.abe4650


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