Singlet oxygen quenching (1O2)

Two quenching mechanisms exist. Physical quenching, dominant in carotenoids, transfers the singlet oxygen's energy to the carotenoid's conjugated polyene chain; this energy is delocalized along the conjugated double bonds and dissipated as heat, returning the carotenoid to its ground state without degradation. Chemical quenching involves direct oxidation of the molecule, which is consumed in the process. Physical efficiency is measured by the quenching rate constant (kq) and depends on the number of conjugated double bonds.

In the classic work by Di Mascio, Kaiser and Sies (1989), lycopene proved the most efficient biological quencher (kq ≈ 31 × 10⁹ M⁻¹ s⁻¹), well above beta-carotene (≈ 14 × 10⁹) and alpha-tocopherol (≈ 0.3 × 10⁹). Astaxanthin, a xanthophyll bearing keto and hydroxyl groups, stands out for its polar configuration and transmembrane orientation: its polar end groups anchor to both faces of the lipid bilayer, protecting the membrane interior and surface simultaneously. This positioning explains why astaxanthin is a particularly potent membrane antioxidant against lipid peroxidation.