On the structure of carotenoid iodine complexes

Previous work on carotenoid–iodine complexes is briefly reviewed. The formation of iodine complexes of β,β-carotene and of (3R,3′R)-β,β-carotene-3,3′-diol (zeaxanthin) has been studied by modern methods including UV/VIS/NIR, IR MS, EPR, ENDOR and NMR (¹H, ¹H–¹H COSY, TOCSY, 2D ROESY, ¹H–¹³C HSQC and ¹H–¹³C HMBC) spectroscopy, and chemical reactions monitored by HPLC, TLC and spectral analysis (VIS, MS, ¹H NMR). β,β-Carotene formed a solid complex C₄₀H₅₆ × 4I with iodine in hexane and a solvent complex with λ_max 1010 nm in chlorinated solvents. Iodine was not covalently bound to the carotene. Spectroscopic and chemical evidence is consistent with the representation of the β,β-carotene–iodine complex containing iodine in a π complex with cationic/radical cationic properties. Extensive E/Z isomerisation was noted for all quenching products obtained in acetone, with thiosulfate, by dilution, or by reaction with nucleophile (MeOH). Key products obtained from the β,β-carotene–iodine complex were 4′,5′-didehydro-4,5′-retro-β,β-carotene (isocarotene) and 4-methoxy-β,β-carotene. The zeaxanthin–iodine complex was not suitable for a practical synthesis of (3S,3′S)-4′,5′-didehydro-4,5′-retro-β,β-carotene-3,3′-diol (eschscholtzxanthin).