Inefficient Carotenoid-Mediated Quenching of Chlorophyll a Triplet: A Hypothetical Cause for the Evolution of Cys-Mediated Quenching in IsiA.

Iron-stress-induced protein A (IsiA) is a chlorophyll (Chl) a-containing protein in cyanobacteria. Under low-iron growth, it is the most abundant intracellular Chl a-binding protein. IsiA and the CP43 protein are suggested to have coevolved from a common ancestral protein, sharing similar spectroscopic properties. When expressed, IsiA forms IsiA-only and photosystem I (PSI)-IsiA complexes. The IsiA-only complex shows a cysteine-mediated photoprotective mechanism that partially mitigates the risk of photo-oxidative damage by quenching Chl a singlet excitation. However, the effectiveness of longer-time-scale photoprotection, such as Chl a → carotenoid triplet-triplet sensitization (TTS), remains unknown. Our evaluation suggests an extraordinarily slow TTS with a time constant of 100 ns. This is comparable to the concurrent sensitization of singlet oxygen. From this perspective, the photoprotective role of carotenoids in IsiA is underperforming. Therefore, a cysteine-based Chl a quenching could be seen as a supplementary photoprotective mechanism, developed specifically to complement underperforming Chl a → carotenoid TTS.