Protonated canthaxanthins as models for blue carotenoproteins

Abstract

It has been suggested that astaxanthin (3,3′-dihydroxy-β,β-carotene-4,4′-dione) in the carotenoprotein α-crustacyanin occurs in the diprotonated form. As a model system for protonated astaxanthin in α-crustacyanin the reactions of canthaxanthin (β,β-carotene-4,4′-dione) with Brønsted acids (CF₃COOH and CF₃SO₃H) and the Lewis acid BF₃–etherate have been investigated. Structures of C-5 protonated, C-7 protonated, enolised O-4 protonated and O-4,4′, 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 ¹³C 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′, C-7 triprotonated canthaxanthin relative to C-7 protonated canthaxanthin is considered a relevant model for O-4,4′ diprotonated canthaxanthin, in comparison with neutral canthaxanthin. The positive charge was mainly located at C-6/6′ ≫ C-8/8′ > C-10/10′ > C-12/12′ > C-14/14′ ∼ C-15/15′ 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 ¹³C labelled astaxanthin in α-crustacyanin and recent X-ray analysis of β-crustacyanin.