Sample purification and characterization of the α- and β-crustacyanin pigments from the American lobster for crystallographic and cryo-EM studies

The striking blue hue of live Homarus americanus, known as the American lobster, arises from the interaction of astaxanthin with the carotenoprotein alpha-crustacyanin. The mechanism underlying the bathochromic shift of the chromophore from its unbound red form (lambda(max) = 472 nm) to the blue protein-bound form (lambda(max) = 631 nm) is the subject of attention from the food and nutraceutical industries for the development of versatile food colourants. Here, we present sample purification and characterization of the alpha- and beta-crustacyanin pigments from the American lobster for crystallographic studies and cryo-EM. Moving from H. gammarus to H. americanus for complex isolation, together with an integrated biophysical characterization, resulted in the production, for the first time, of alpha-crustacyanin crystals that diffracted to 6.3 angstrom resolution at < I/sigma(I)> = 1.0 and high-quality negative-stain and cryo-EM images.