Final answer:
In the Coomassie Assay, the colors red and blue bind to specific ligands coordinated to the metal center of proteins. Blue copper proteins have copper ions coordinated by specific ligands, which causes them to appear intense blue. Other coordination complexes can appear blue-green or pale yellow based on the ligands coordinated to the metal center.
Step-by-step explanation:
In the Coomassie Assay, the colors red and blue bind to specific ligands coordinated to the metal center. For example, blue copper proteins such as azurin have a copper ion coordinated by four ligands in a roughly tetrahedral arrangement, including histidine imidazole nitrogen atoms and sulfur atoms from cysteine and methionine. These specific ligands cause the protein to absorb light in the visible range, resulting in the intense blue color. On the other hand, coordination complexes like iron(II) complexes can appear blue-green or pale yellow depending on the ligands coordinated to the metal center.