13C Magic Angle Spinning NMR Analysis and Quantum Chemical Modeling of the Bathochromic Shift of Astaxanthin in α-Crustacyanin, the Blue Carotenoprotein Complex in the Carapace of the Lobster Homarus gammarus†

Selective isotope enrichment, 13C magic angle spinning (MAS) NMR, and semiempirical quantum chemical modeling, have been used to analyze ligand−protein interactions associated with the bathochromic shift of astaxanthin in α-crustacyanin, the blue carotenoprotein complex from the carapace of the lobster Homarus gammarus. Spectra of α-crustacyanin were obtained after reconstitution with astaxanthins labeled with 13C at positions 4,4‘, 12,12‘, 13,13‘, or 20,20‘. The data reveal substantial downfield shifts of 4.9 and 7.0 ppm at positions 12 and 12‘ in the complex, respectively. In contrast, at the 13 and 13‘ positions, small upfield shifts of 1.9 ppm were observed upon binding to the protein. These data are in line with previously obtained results for positions 14,14‘ (3.9 and 6.8 ppm downfield) and 15,15‘ (0.6 ppm upfield) and confirm the unequal perturbation of both halves after binding of the chromophore. However, these results also show that the main perturbation is of symmetrical origin, since the chemi...