Redox potentials of carotenoids associated with type-II photosynthetic reaction centers

Carotenoids play dual roles in photosynthesis, extending light absorption and providing photoprotection through radical cation formation. Their ability to act as electron donors is defined by their redox potential for one-electron oxidation (Em). Here, we determine Em for carotenoids whose Em values are missing or inconsistently established in type-II reaction centers or associated antenna complexes, using a quantum chemical approach. The highest occupied molecular orbital (HOMO) energy levels correlated strongly with reported Em values in CH2Cl2, enabling the determination of reliable Em values. All-trans isomers were found to be slightly stronger electron donors than their 15-cis counterparts. As this Em difference is small, the biological significance of cis isomerization appears to lie in pigment orientation rather than intrinsic redox properties. For xanthophyll-cycle pigments, a stepwise increase in Em from zeaxanthin to antheraxanthin to violaxanthin, attributable to their substituents, was revealed. Em values measured in micelles deviated markedly not only from those in CH2Cl2 but also from HOMO energy levels in water, questioning their relevance as a reference value in protein environments. These results provide a unified framework for understanding carotenoid redox chemistry in photosynthesis.