Electrochemical Study of Astaxanthin and Astaxanthin n-Octanoic Monoester and Diester: Tendency to Form Radicals

The carotenoid astaxanthin known for its powerful antioxidant activity was electrochemically investigated along with the synthesized astaxanthin n-octanoic monoester and astaxanthin n-octanoic diester. Cyclic voltammograms (CVs) revealed a two-electron transfer oxidation for all three carotenoids with a difference in the two oxidation potentials (Delta E = E-2(0) - E-1(0)) slightly increasing from astaxanthin to the monoester to diester. Minimal or no exposure to water prevented the formation of carotenoid neutral radicals from dications and radical cations, causing near absence of the fifth peak in the CVs. This makes the CVs almost reversible and enables a more precise simulation of the redox potentials and the equilibrium constants for the formation of radical cations. The first oxidation potential (E-1(0)) of 0.767(8), 0.773(8), and 0.775(3) V versus SCE and the second oxidation potential (E-2(0)) of 0.982(8), 0.993(1) and 0.996(6) V versus SCE for astaxanthin, monoester, and diester, respectively, have been standardized to the potential of ferrocene of 0.528 V vs SCE given in a previous study. Reduction potentials (E-3(0)) for formation of carotenoid neutral radicals from dications after proton loss from the three studied carotenoids are presented and compared to those of other carotenoids. According to our DFT calculations, the most favorable sites for deprotonation of radical cations and dications are found on the cyclohexene rings. These measurements provide insight into important properties of these carotenoids like radical scavenging of (OH)-O-center dot, (CH3)-C-center dot, and (OOH)-O-center dot by proton abstraction from the carotenoid or the formation of carotenoid neutral radicals from radical cations which can quench photoexcited states. There is no essential difference in first oxidation potentials for the three carotenoids, which suggests a similar scavenging rate of the esters of astaxanthin toward (OH)-O-center dot, (CH3)-C-center dot, and (OOH)-O-center dot radicals when compared to astaxanthin itself. The large equilibrium constants K-com (102.4, 409.6, and 204.8 for astaxanthin, monoester, and diester) derived from simulation indicate a preference for radical cation formation for both astaxanthin and its esters, while electron transfer to form dications will be unlikely. Proton transfer from the radical cations, which are weak acids, to the neighboring proton acceptors will form neutral radicals, which allows quenching of excited states.