A study of protein-carotenoid interactions in the astaxanthin-protein crustacyanin by absorption and Stark spectroscopy; evidence for the presence of three spectrally distinct species

Molecular mechanisms underlying the peculiar spectral properties of the carotenoid astaxanthin in alpha-crustacyanin, the blue carotenoprotein isolated from the exoskeleton of the lobster Homarus gammarus, were investigated by comparing the basic electrooptical parameters of astaxanthin free in vitro with those of astaxanthin in the complex. Absorption and electroabsorption (Stark effect) spectra were obtained for alpha-crustacyanin in low-temperature glasses to provide information about the molecular interactions that lead to the large bathochromic shift of the spectra resulting from this complexation. The low-temperature spectra reveal the presence of at least three spectral forms of alpha-crustacyanin, with vibronic (0-0) transitions at 14000 cm(-1), 13500 cm(-1) and 11600 cm(-1) (corresponding to approximately 630, 660 and 780 nm, respectively, at room temperature) and with relative aboundance 85%, 10% and 5%. The longer wavelength absorbing species have not previously been detected. The changes in polarizability and in permanent dipole moments associated with the S0-->S2 electronic transition for all these forms are about 1.5 times larger than for isolated astaxanthin. The results are discussed with reference to the symmetric polarization model for astaxanthin in alpha-crustacyanin.