Nanodiscoidal Lipid-Protein Assembly Modulated Ultrafast Excitation Relaxation in an Algal LHCII Complex

Area of Science:

• Photosynthesis research
• Marine biology
• Biophysics

Background:

• Bryopsis (Bry.) corticulans is a marine green alga adapted to intertidal zones.
• Its light-harvesting antenna complex II (LHCII) contains chlorophyll b and keto-carotenoids (siphonin and siphonaxanthin) for blue-green light utilization.
• Understanding pigment-pigment energy transfer dynamics is crucial for comprehending light-harvesting efficiency.

Purpose of the Study:

• To investigate ultrafast excitation transfer dynamics within Bryopsis LHCII under blue-light excitation.
• To elucidate the influence of a biomimicking lipid environment on these dynamics.
• To determine the specific roles of carotenoids in light harvesting and photoprotection within LHCII.

Main Methods:

• Comparison of Bryopsis LHCII in aqueous solution versus embedded in lipid-membrane nanodiscs (mnd-BryLHCII).
• Femtosecond time-resolved absorption spectroscopy to analyze excitation transfer pathways.
• Comprehensive spectral analysis to identify pigment-specific relaxation dynamics.

Main Results:

• Singlet excitation transfer (SET) efficiency decreased by 5-13% in mnd-BryLHCII compared to aqueous BryLHCII.
• Lipid environment altered carotenoid-to-chlorophyll a SET pathways, deactivating some while accelerating others.
• Specific carotenoids (siphonin at L1, siphonaxanthin at L2) showed distinct roles in light harvesting and photoprotection.

Conclusions:

• The lipid environment plays a critical role in optimizing the light-harvesting and photoprotective functions of algal LHCII.
• Altered excitation transfer pathways in the lipid nanodisc environment lead to reduced overall SET efficiency.
• Findings provide insights into the molecular mechanisms governing pigment cofactor function in algal light-harvesting complexes.