Pigment interactions in light-harvesting complex II in different molecular environments

Area of Science:

• Photosynthesis research
• Plant molecular biology
• Biophysics of light-harvesting complexes

Background:

• Extraction of plant light-harvesting complex II (LHCII) using surfactants alters its excitonic circular dichroism (CD) spectrum and fluorescence.
• Understanding these changes is crucial for elucidating the roles of trimer-trimer contacts and surfactant-induced structural perturbations.

Purpose of the Study:

• To differentiate spectral changes caused by LHCII-LHCII interactions from those induced by detergents or lipids.
• To investigate the role of the carotenoid neoxanthin in LHCII spectral properties and function.
• To correlate spectral changes with fluorescence quenching and light-harvesting efficiency.

Main Methods:

• Comparative analysis of CD spectra and fluorescence kinetics of LHCII in various states: aggregates, artificial and native lipid membranes, detergent-solubilized forms, and polymer gels.
• Utilizing neoxanthin-deficient thylakoid membranes to identify neoxanthin-specific spectral bands.
• Reconstitution of LHCII into proteoliposomes with varying lipid:protein ratios.

Main Results:

• Identified specific CD spectral bands associated with LHCII-LHCII interactions (at -437 and +484 nm) and detergent/lipid interactions (at +447 and -494 nm).
• Attributed the -494 nm band to conformational changes in the carotenoid neoxanthin, which is sensitive to its molecular environment.
• Found no direct correlation between aggregation- or surfactant-specific CD bands and fluorescence quenching; quenching could occur independently.
• Observed no significant quenching in reconstituted LHCII proteoliposomes, despite high energetic connectivity.

Conclusions:

• Differentiates spectral signatures of LHCII aggregation from environmental influences (detergents, lipids).
• Highlights the conformational sensitivity of neoxanthin to its molecular surroundings within LHCII.
• Suggests that fluorescence quenching is not solely dependent on specific aggregation or surfactant-induced spectral changes.
• Demonstrates that reconstituted LHCII proteoliposomes can achieve efficient light harvesting with high energetic connectivity, independent of significant quenching.