Molecular spectroscopy. Computer artwork of light shining through a molecule of the plant enzyme catechol oxidase. This represents molecular spectroscopy; the determination of a molecule's composition by studying the wavelengths of light it absorbs. Each atom has a characteristic absorption. The light is shining through two copper ions at the centre of the molecule. The structure of catechol oxidase is shown by the ribbons and rods. The enzyme causes damaged plant material to turn brown by oxidising phenols in plant cells. The products of this reaction are often converted into anti-fungal compounds.