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

Molecular mechanisms underlying the peculiar spectral properties of the car otenoid astaxanthin in alpha -crustacyanin, the blue carotenoprotein isolat ed from the exoskeleton of the lobster Homarus gammarus, were investigated by comparing the basic electrooptical parameters of astaxanthin free in vit ro with those of astaxanthin in the complex. Absorption and electroabsorpti on (Stark effect) spectra were obtained for a-crustacyanin in low-temperatu re glasses to provide information about the molecular interactions that lea d to the large bathochromic shift of the spectra resulting from this comple xation. The low-temperature spectra reveal the presence of at least three s pectral forms of alpha -crustacyanin, with vibronic (0 0) transitions at 14 000 cm(-1), 13 500 cm(-1) and 11 600 cm-(1) (corresponding to approximatel y 630, 660 and 780 nm, respectively, at room temperature) and with relative aboundance 85%, 10% and 5%. The longer wavelength absorbing species have n or previously been detected. The changes in polarizability and in permanent dipole moments associated with the S-0 -S-2 electronic transition for all these forms are about 1.5 times larger than for isolated astaxanthin. The r esults are discussed with reference to the symmetric polarization model for astaxanthin in alpha -crustacyanin. (C) 2001 Elsevier Science B.V. All rig hts reserved.