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Blue copper proteins are relatively small copper-containing proteins. The two most studied are Azurin and Plastocyanin, which are extracted from bacteria and Poplar leaves, respectively. The protein shown here is Plastocyanin. These proteins are blue because of a sulfur-to-Cu2+charge- transfer transition (LMCT). All atoms, including hydrogen atoms, are shown in this display. In order to omit the hydrogens, press the right mouse button, click on "Style" and then "Atoms" and uncheck "Show Hydrogens"
Plastocyanin is of the beta-barrel type. These proteins carry out reversible electron-transfer
Cu2+ + e- Cu+
When Cu2+ is reduced to Cu+ the electronic configuration changes from d9 to d10, and the colour of the complex changes from blue to colourless.
2+ was buried deep inside the protein.
2+) of Plastocyanin is shown here. The two nitrogens and the thiolate sulfur define an approximately trigonal planar coordination about the copper while the thioether sulfur lies above this triangle with a long Cu-S distance of 2.82 Å. This distance is very long, suggesting a very weak Cu-S interaction. The Cu-S(Cys) bond length is 2.07 Å which is short and therefore this bond is strong.
+) of the protein. There is very little change upon reduction. The Cu-S(Met) distance changes from 2.82 to 2.87 Å while the Cu-S(Cys) distance goes from 2.07 to 2.17 Å. Clearly the Cu is held in an entatic state by the protein in order to minimize structural and electronic change during the reaction.
This table shows that in the oxidized form all the distances are shorter, in comparison with the reduced form. This is because Cu2+ is smaller and forms stronger electrostatic bonds than Cu+. Therefore Cu2+ will pull the ligands inward. In the apo form there are three distances which are very large. This is due to His 87 which re-orientates itself when the copper ion is not bonded.