A Hidden State in Light-Harvesting Complex II Revealed By Multipulse Spectroscopy

Area of Science:

• Photosynthesis research
• Plant photobiology
• Biophysics of light-harvesting complexes

Background:

• Light-harvesting complex II (LHCII) is essential for capturing solar energy and photoprotection via nonphotochemical quenching (NPQ).
• LHCII aggregation is a model for NPQ, but the nature of the quencher and model validity are debated.
• Understanding NPQ mechanisms is critical for optimizing photosynthesis and preventing photodamage.

Purpose of the Study:

• To investigate the nature of the quenching mechanism in unaggregated LHCII using advanced spectroscopic techniques.
• To identify the molecular components and dynamics of the transient quenched state.
• To evaluate the role of this state in NPQ and fluorescence phenomena.

Main Methods:

• Ultrafast multipulse spectroscopy was employed to induce and probe a quenched state in LHCII.
• Spectroscopic analysis involved monitoring excited states populated by re-excitation with varied wavelengths and pulse durations.
• The study examined both trimeric and monomeric forms of LHCII.

Main Results:

• A transient, quenched state was populated in unaggregated LHCII, distinct from aggregation-induced quenching.
• This state exhibits spectral features indicative of excitonic coupling between lutein and chlorophyll.
• The quenched state has a lifetime of approximately 10 picoseconds and is observed in both monomeric and trimeric LHCII.

Conclusions:

• The identified excitonically coupled state is a potential quencher present even in unaggregated LHCII.
• This transient state may act as a photoprotective channel under high light intensities, possibly via conformational changes.
• The transient state could explain fluorescence blinking observed in single-molecule studies of LHCII.