Understanding the blue copper proteins

The blue copper proteins, exemplified by plastocyanin, ceruloplasmin, azurin, and stellacyanin, are intriguing and have been subjects of investigation over many years. Some have only Type I copper sites; others have other types of copper, as well. The blue color reminds one of the blue color of many square-planar copper (II) chelates but is much more intense; the optical transition responsible for it is attributed to a sulfur-metal charge transfer band. The EPR technique is excellent for probing such copper sites, but may suffer from lack of resolution. With the picture of electronic structure at the copper site considerably improved, Antholine et al., 1993, now have revisited the conventional EPR spectroscopy of Pseudomonas aeruginosa azurin.