THE MECHANISM OF THE COLOR SHIFT OF ASTAXANTHIN IN ALPHA-CRUSTACYANINAS INVESTIGATED BY C-13 MAS NMR AND SPECIFIC ISOTOPE ENRICHMENT

By selective isotope enrichment of astaxanthin, MAS NMR and semi-empir ical modelling, ligand-protein interactions associated with the red sh ift in alpha-crustacyanin, the major blue astaxanthin binding caroteno protein complex from the carapace of the lobster Homarus gammarus, hav e been analysed. C-13 Magic Angle Spinning (MAS) NMR spectra were obta ined after reconstitution with astaxanthins labelled in the centre of the molecule or at the two keto groups. The MAS data reveal electrosta tic polarizations of the conjugated chain. In addition, solid state NM R results for pure unlabelled astaxanthin can be compared with natural abundance C-13 MAS data for canthaxanthin and beta-carotene, to addre ss the effect of the ring functionalities on the electronic properties of the polyene chain. Quantum chemical calculations were performed to reconcile the MAS data with one of several simple and straightforward mechanisms for the colour shift. The results point towards a colour s hift mechanism in which the astaxanthin may be doubly charged, possibl y by a double protonation of the two ring keto groups.