Citrus carotenoids—VI.: Carotenoid pigments in the flavedo of Sinton citrangequat

Abstract

The flavedo of the fruit of the trigeneric hybrid Sinton citrangequat contains new and unusual carotenoid ketones (apo-carotenones). These pigments are unique in the carotenoid series in that they contain the terminal methyl ketone group in the side chain. Thus far, the methyl ketone carotenoids with nonaeneone and decaeneone chromophores have been isolated and characterized. They are sintaxanthin, citranaxanthin and reticulataxanthin. A minor carotenoid ketone, probably 3-OH-sintaxanthin, was also present. A methyl ketone carotenoid containing an in-chain hydroxyl group, 8′-OH-7′8′-dihydrocitranaxanthin, was also isolated. The remainder of the carbonyl carotenoids consisted of β-apo-10′-carotenal, β-apo-8′carotenal, β-citraurin, and probably 3-OH-β-apo-10′-carotenal. β-Zeacarotene was isolated for the first time from citrus. Neurosporene, γ-carotene and β-carotene in minor amounts were also detected. The rich red color of the flavedo is due mainly to the methyl ketone carotenoids.