Carotenoid blues: Structural studies on carotenoproteins

a-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 a-crustacyanin complexes have been prepared from a range of natural and synthetic carotenoids. Only normal C ~ O 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 with the main polyene chain. Circular dichroism shows that the carotenoid chromophore experiences a chiral twist, but this is not a major factor in the spectral shift, and that the two astaxanthin molecules in the P-crustacyanin dimer are close together and show some interaction in the excited state. Resonance Raman and NMR spectroscopy of complexes containing I3C-labelled astaxanthins shows that the blue colour can be attributed to perturbation of the ground-state electronic structure of the carotenoid, caused by polarization of the chromophore. The results are consistent with protonation of the C(4) and C(4’) keto groups, but the magnitude of the polarization effect is not the same in the two halves of the molecule.