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

Abstract

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 (ΔE = E2(0) - E1(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 (E1(0)) of 0.767₈, 0.773₈, and 0.775₃ V versus SCE and the second oxidation potential (E2(0)) of 0.982₈, 0.993₁, and 0.996₆ 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 (E3(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, (•)CH3, and (•)OOH 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, (•)CH3, and (•)OOH 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.

Research Progress of Astaxanthin on Contrast agent induced acute kidney injury

“…That decreasing scavenging rate of free radicals decreases with decreasing oxidation potential. The oxidation potential of ATX being significantly higher than that of β-Carotene, thus the scavenging rate of ATX is much higher than that for β-Carotene and exhibits Pro-oxidative character which includes reduction of Fe3+ to Fe2+ 15,16 . The antioxidative activity of ATX on cells is greater than that of β-carotene, vitamin C, vitamin E, lutein, lycopene, and other catechins 17,18 .…”

Electrochemical Oxidation of Astaxanthin on Glassy‐carbon Electrode and its Amperometric Determination Using Batch Injection Analysis (BIA)

“…Astaxanthin is a carotenoid xanthophyll (3,3′‐Dihydroxy‐β,β‐carotene‐4,4′‐dione) that presents antioxidant activity ten times higher than other carotenoids, such as lutein and canthaxanthin, , 40 times higher than β‐carotene, 100 times more effective than vitamin E , and 550 times higher than α‐tocopherol .…”

“…Recently, astaxanthin has aroused considerable interest because of these antioxidant properties. Clinical studies show many benefits of astaxanthin in cardiovascular health, fertility, diabetes , antitumor effects , and immunomodulatory activity . Astaxanthin can be found in various marine microorganisms and animals .…”

“…Astaxanthin can be found in various marine microorganisms and animals . The Haematococcus pluvialis algae is a common natural source of astaxanthin , which is responsible for the pigment on salmon and shrimp as this algae is the main source of food for these animals .…”

“…On the other hand, electrochemical methods can be an interesting tool to monitor astaxanthin because they are simple, sensitive, provide fast response (high sample throughput), and require portable equipment. Electrochemistry of astaxanthin on platinum electrode was investigated and the proposed mechanism involves a two‐electron transfer oxidation with the formation of a neutral carotenoid radical .…”