Protonated canthaxanthins as models for blue carotenoproteins.

It has been suggested that astaxanthin (3,3'-dihydroxy-beta,beta-carotene-4,4'-dione) in the carotenoprotein alpha-crustacyanin occurs in the diprotonated form. As a model system for protonated astaxanthin in [small alpha]-crustacyanin the reactions of canthaxanthin ([small beta],[small beta]-carotene-4,4[prime or minute]-dione) with Bronsted acids (CF(3)COOH and CF(3)SO(3)H) and the Lewis acid BF(3)-etherate have been investigated. Structures of C-5 protonated, C-7 protonated, enolised O-4 protonated and O-4,4[prime or minute], C-7 triprotonated canthaxanthin have been established by VIS-NIR and NMR spectroscopy. The charge distribution in the cations has been considered by comparison of the (13)C chemical shift difference relative to neutral relevant carotenoid models. The experimental evidence for protonated canthaxanthins differs significantly from previous AM1 calculations. Experimental data for O-4,4[prime or minute], C-7 triprotonated canthaxanthin relative to C-7 protonated canthaxanthin is considered a relevant model for O-4,4[prime or minute] diprotonated canthaxanthin, in comparison with neutral canthaxanthin. The positive charge was mainly located at C-6/6[prime or minute][dbl greater-than] C-8/8[prime or minute] > C-10/10[prime or minute] > C-12/12[prime or minute] > C-14/14[prime or minute][similar] C-15/15[prime or minute] in the polyene chain. Moreover, it was inferred that only 14% of the positive charge is delocalised to the polyene chain, the remaining charge must therefore be located at the protonated carbonyl moiety. The results are discussed in relation to previous solid state NMR studies of (13)C labelled astaxanthin in [small alpha]-crustacyanin and recent X-ray analysis of [small beta]-crustacyanin.