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

Abstract

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.