β-Crustacyanin, the blue–purple carotenoprotein of lobster carapace: consideration of the bathochromic shift of the protein-bound astaxanthin

The crystal structure of a β-crustacyanin allows an analysis of the various proposals for the mechanism of the bathochromic shift from orange to purple–blue of astaxanthin in this lobster carotenoprotein. Structural and previous chemical and biophysical studies suggest that extension of conjugation by coplanarization of the β-­ionone rings with the polyene chain and polarization resulting from hydrogen bonding at the C(4) and C(4′) keto groups may be mainly responsible for the bathochromic shift. Additional contributions may arise from medium effects and possibly from bowing of the polyene chain on binding. Previous biophysical data revealing a somewhat symmetrical polarization of astaxanthin in crustacyanin are thereby also accounted for. A puzzling feature remains unexplained: the bathochromic shifts, larger than that of astaxanthin, shown by some cyclopentenedione carotenoids in reconstituted carotenoproteins. This mini review enlarges on the original analysis and conclusions of Cianci et al. [(2002), Proc. Natl Acad. Sci. USA, 99, 9795–9800].