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Being both hydrophilic and hydrophobic makes opsins and other membrane proteins hard to crystallize. Unable to make crystals of opsin, Wald turned instead to crustacyanin, the pigment in lobster shell. Although crustacyanin resembles rhodopsin in that it consists of a protein wrapper and astaxanthin, a vitamin-A-like molecule, it is not a membrane protein. [Extracted from the article]