Abstract
β-carotene spontaneously copolymerizes with molecular oxygen to form a β-carotene–oxygen copolymer compound (“copolymer”) as the main product, together with small amounts of many apocarotenoids. Both the addition and scission products are interpreted as being formed during progression through successive free radical β-carotene–oxygen adduct intermediates. The product mixture from full oxidation of β-carotene, lacking both vitamin A and β-carotene, has immunological activities, some of which are derived from the copolymer. However, the copolymer’s chemical makeup is unknown. A chemical breakdown study shows the compound to be moderately stable but nevertheless the latent source of many small apocarotenoids. GC–MS analysis with mass-spectral library matching identified a minimum of 45 structures, while more than 90 others remain unassigned. Newly identified products include various small keto carboxylic acids and dicarboxylic acids, several of which are central metabolic intermediates. Also present are glyoxal and methyl glyoxal dialdehydes, recently reported as β-carotene metabolites in plants. Although both compounds at higher concentrations are known to be toxic, at low concentration, methyl glyoxal has been reported to be potentially capable of activating an immune response against microbial infection. In plants, advantage is taken of the electrophilic reactivity of specific apocarotenoids derived from β-carotene oxidation to activate protective defenses. Given the copolymer occurs naturally and is a major product of non-enzymatic β-carotene oxidation in stored plants, by partially sequestering apocarotenoid metabolites, the copolymer may serve to limit potential toxicity and maintain low cellular apocarotenoid concentrations for signaling purposes. In animals, the copolymer may serve as a systemic source of apocarotenoids.
Highlights
The propensity of oxygen to preferentially add polymerically to highly unsaturated hydrocarbon compounds was demonstrated in the pioneering styrene oxidation model study published by Miller and Mayo in 1956.1 Yet, despite the presence of the highly unsaturated conjugated polyene backbone and many studies carried out over several decades,[2,3,4] only relatively recently has it been reported that the spontaneous oxidation of b-carotene is dominated by the formation of a b-carotene–oxygen copolymer product (“copolymer”).[5]
The material recovered in the ethyl acetate extracted fraction of the reaction product mixture from each experiment was evaluated by GPC and HPLC to identify any changes in the copolymer and by GC– mass spectrometry (MS) to identify, where possible, any degradation products that were formed
The copolymer’s UV–vis spectrum is characterized by a loss of absorption from much of the conjugated polyene structure and the appearance of a peak at 205 nm and shoulder absorptions at 235 nm (a,b-unsaturated carbonyl) and 280 nm.[21]
Summary
The propensity of oxygen to preferentially add polymerically to highly unsaturated hydrocarbon compounds was demonstrated in the pioneering styrene oxidation model study published by Miller and Mayo in 1956.1 Yet, despite the presence of the highly unsaturated conjugated polyene backbone and many studies carried out over several decades,[2,3,4] only relatively recently has it been reported that the spontaneous oxidation of b-carotene is dominated by the formation of a b-carotene–oxygen copolymer product (“copolymer”).[5] Previously, the oxidation. Published at www.cdnsciencepub.com/cjc on 8 April 2021.
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