Spectroscopic characterization of alpha-crustacyanin.
alpha-Crustacyanin exhibits two CD extrema with negative and positive bands at 690 and 583 nm in phosphate buffer, respectively. The CD spectrum is interpreted in terms of dipoledipole coupling between the transition moments of the two astaxanthin molecules on each subunit. The CD splitting yields an exciton bandwidth of 2800 cm-1, corresponding to an inter-chromophore distance of ca. 13 A in which the two astaxanthin molecules exist in a dimeric array with a mutual orientation angle of ca. 90 degrees. The SDS denaturation completely abolishes the long wavelength CD splitting, while 2 M NaCl reduces the overall CD intensity without destroying the split CD spectrum. It is suggested that the characteristic CD extrema at 583 and 690 nm arise from the intra-subunit astaxanthin-astaxanthin coupling. The binding site for astaxanthin contains one or more tryptophan residues, as the tryptophan fluorescence is quenched by energy transfer from the tryptophan to the prosthetic group in the native form of the carotenoprotein. The strong red shift (lambda max 487 nm) is probably due to interactions between tryptophan residue(s) and the chromophore molecules through dipole-dipole or partial charge transfer forces.