The circular dichroism of ovoverdin and other carotenoproteins from the lobster Homarus americanus

The circular dichroic (CD) spectra of the α-, β′-, and γ-crustacyanins, ovoverdin, and the yellow lobster-shell protein were measured in the region 200–750 nm, for comparison with the CD spectrum of the free carotenoid astaxanthin. The two carotenoid chromophores of ovoverdin gave a CD spectrum with a series of bands of alternating sign and ellipticities up to 1.9 × 105 degree∙cm2∙dmol−1, comparable to the low temperature CD spectrum of astaxanthin in the UV region. The visible region of ovoverdin also contained strong CD bands where astaxanthin itself has very weak ones. The blue (640 nm) chromophore of ovoverdin gave a broad negative CD feature quite different from the blue chromophores of the three crustacyanins. The crustacyanins have a broad positive feature between 400 and 610 nm with ellipticities up to 2 × 105 degree∙cm2∙dmol−1, followed at higher wavelengths by a negative band with an ellipticity up to 1 × 105 degree∙cm2∙dmol−1. Gaussian curve-fitting procedures showed the positive features to consist of a minimum of two or three CD bands. In addition to the exciton bands at the main visible absorption, the yellow lobster-shell protein had a pair of CD bands of equal but opposite ellipticity associated with an absorption band between 250 and 340 nm. The UV regions of the CD spectra of the five carotenoproteins have bands from both carotenoid and protein chromophores, and possible assignments of these bands to one or the other of the two types of chromophore are proposed.