ζ-Carotene: Generation and Quenching of Singlet Oxygen, Comparison with Phytofluene

It is known that C₄₀ carotenoids with a short chain of conjugated double bonds (CDB) (5 and 7, respectively) are universal precursors in the biosynthesis of colored carotenoids in plant cells. Previously, using mainly stationary measurements of photosensitized phosphorescence of singlet oxygen (¹O₂), we discovered that phytofluene efficiently generates ¹O₂ in aerated solution and therefore, can serve as a source of the UV photodynamic stress in living cells [Ashikhmin et al., Biochemistry (Moscow), 2020, 85, 773]. In the present paper, by using novel pulsed light emitting diodes (LEDs), aerated hexafluorobenzene as a solvent and time-resolved measurements of ¹O₂ phosphorescence we confirmed that phytofluene efficiently photosensitizes ¹O₂ formation. The quantum yield of this process according to the novel experiments is about 0.4. An ability to generate ¹O₂ was also found in aerated solutions of ζ-carotene although the quantum yield of this process is 30-fold lower that in phytofluene solutions. Both carotenoids were found to quench ¹O₂ in the dark with the quenching rate constants equal to (3.6 ± 0.9)×10⁷ and (2.1 ± 0.2)×10⁸ M^-1s^-1, respectively. To our knowledge, the rate constant of ¹O₂ quenching by ζ-carotene has been reported in the present paper for the first time. It follows from the data obtained that the rate constants of ¹O₂ quenching by both carotenoids are much (by 2-3 orders of magnitude) smaller than the rate constant of the diffusion-limited biomolecular reactions. Hence, both carotenoids are weak protectors against ¹O₂ oxidative activity. It is more likely that they are potential promoters of photodynamic stress in living cells.