Light-harvesting complex II (LHCII) is the most abundant membrane protein on Earth, responsible for capturing sunlight and funneling it to photosynthetic reaction centers. Under high light, LHCII aggregates to dissipate excess energy as heat — a photoprotective mechanism called non-photochemical quenching. The aggregation quenches fluorescence, and measuring the fluorescence decline has been the standard way to study the protection mechanism.
Conradie et al. (arXiv:2603.11752) show that the fluorescence quenching during LHCII aggregation has two distinct sources that have been conflated. Intrinsic quenching — the formation of energy-dissipating states within the protein complex — develops gradually as aggregates grow. But singlet-triplet annihilation (STA) — where a long-lived triplet excited state eliminates a newly formed singlet excitation — emerges at moderate excitation intensities and rapidly dominates the quenching signal even in small aggregates.
The distinction matters because the two mechanisms have different size dependencies. Intrinsic quenching grows steadily with aggregate size, consistent with the standard picture of protein conformational changes upon aggregation. STA kicks in abruptly once the aggregate is large enough that triplet states accumulate — because triplets live microseconds while singlets live nanoseconds, even a few triplets in a connected aggregate can quench many singlets.
Most fluorescence quenching experiments use excitation intensities where STA is significant, meaning the measured quenching conflates the two mechanisms. The intrinsic quenching — the biologically relevant photoprotective process — is smaller than the total measured quenching suggests. Previous estimates of photoprotective efficiency based on total fluorescence decline are overestimates, because part of the measured quenching is a photophysical artifact rather than a biological adaptation.