Blue oxonium ions

A classical colour test for carotenoid 5,6-epoxides (e.g. violaxanthin (37)) is the formation of a dark blue colour upon treatment with strong acid [78,79], The chemistry behind this reaction was elucidated in our laboratory, as illustrated for neoxanthin (38) in Fig. 16 [80]. Furanoxides such as neochrome (39) are intermediates in this reaction. The elimination of a hydride ion from C-8 of the furanoxide is a key step in the formation of the blue oxonium ion 40 and is effected by strong mineral acid (HC1), CF3COOH or by the hydride acceptor triphenylcarbenium tetrafluoro-borate. 

In CF3COOH the allenic end group of neoxanthin (38) is converted to the acetylenic end group [80] and with concentrated HC1, a chloride is added to the allene [10]. The blue oxonium ion 40 is converted to a yellow, cyclic hemiketal 41 by treatment with KOH. The hemiketal 41 is... 

The VIS spectrum of the yellow alkali treated product of the blue oxonium salt also contained products with octaene-, heptaene- and hexaene like chromophores [80], compatible with conjugate addition and charge delocalisation in the blue oxonium ion. 

Fucoxanthin (42), lacking a double bond in the 7,8-position is an atypical 5,6-epoxide. However, the formation of blue oxonium ions with... 

Fig. (17). Formation of blue oxonium ions via cyclic hemiketals upon treatment of fucoxanthin (42) with strong acid, and conversion of the blue oxonium ion to yellow hemiketal with base Reprinted with permission from [10], Copyright 1994 Acta Chemica Scandinavica.

The striking colour change observed upon treatment of the yellow deacetylated fucoxanthin hemiketal (43) with acid, providing the blue oxonium ion (10, )imax 690 nm), followed by reversion to the yellow fucoxanthinol hemiketal (43) by treatment with base is illustrated in Fig. 18. The hemiketals exhibit characteristic fragmentation pattern by EIMS. 

Well resolved H NMR spectra have not yet been achieved of the blue oxonium ions, and the structural proof relies on the chemical reactions and characterisation of the hemiketals. 

Consequently, the current picture concerning the bathochromic shift in carotenoproteins, such a crustacyanin (9), involves partial positive charge caused by hydrogen bonding of the astaxanthin (17) keto groups, cf. the blue oxonium ions discussed above. 

Haugan, J.-A. and Liaaen-Jensen, S. (1994a) Blue carotenoids. Part 1. Novel oxonium ions derived from fucoxanthin. Acta Chem. Scand., 48, 68-75. 

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