Excitons in the LH3 complexes from purple bacteria.

Area of Science:

• Photosynthetic pigment-protein complexes
• Bioenergetics
• Spectroscopy of light-harvesting systems

Background:

• Light-harvesting complex 2 (LH2) and LH3 contain bacteriochlorophyll a chromophores.
• These complexes are crucial for efficient excitation energy transfer in photosynthetic purple bacteria.
• Variability in exciton spectra (820-850 nm) is linked to chromophore arrangement and interactions.

Purpose of the Study:

• To investigate the molecular origin of excitonically coupled B820 bacteriochlorophylls in LH3.
• To understand the spectroscopic differences between LH3 and LH2.
• To elucidate the role of intermolecular interactions and protonation states in LH3 function.

Main Methods:

• Femtosecond transient absorption spectroscopy
• Deconvolution of steady-state absorption spectra
• Modeling of electrostatic intermolecular interactions using charge density coupling

Main Results:

• The upper excitonic level in LH3 is red-shifted (755 to 790 nm) compared to LH2.
• A ~2-fold decrease in B820 intrapigment coupling was observed in LH3.
• Accurate modeling of LH3 absorption requires adjusting interpigment coupling scaling and considering histidine protonation states.

Conclusions:

• The unique absorption properties of LH3 are not solely due to B850 site energy.
• Histidine protonation patterns significantly influence the B820 ring's intradimer resonance interaction (162-173 cm⁻¹ at 100 K).
• These findings provide molecular insights into efficient light harvesting in purple bacteria.