CAROTENOID BLUES - STRUCTURAL STUDIES ON CAROTENOPROTEINS

alpha-Crustacyanin, the 320 kDa astaxanthin-protein from the carapace of the lobster, Homarus gammarus, is the best known of the blue-purple carotenoproteins found in marine invertebrate animals. Reconstituted alpha-crustacyanin complexes have been prepared from a range of natura l and synthetic carotenoids. Only normal C-40 carotenoids in the all-E configuration fit into the binding site, though some variation in the ring size, shape and methylation pattern is tolerated. The C(20) and C(20') methyl groups must be present; presumably these are involved in essential steric interactions. The main structural requirement is the presence of keto groups at C(4) and C(4'); these must be conjugated w ith the main polyene chain. Circular dichroism shows that the caroteno id chromophore experiences a chiral twist, but this is not a major fac tor in the spectral shift, and that the two astaxanthin molecules in t he beta-crustacyanin dimer are close together and show some interactio n in the excited state. Resonance Raman and NMR spectroscopy of comple xes containing C-13-labelled astaxanthins shows that the blue colour c an be attributed to perturbation of the ground-state electronic struct ure of the carotenoid, caused by polarization of the chromophore. The results are consistent with protonation of the C(4) and C(4') keto gro ups, but the magnitude of the polarization effect is not the same in t he two halves of the molecule.