[3] Structures of the protein from poplar, algae, parsley, spinach, and French bean plants have been characterized crystallographically.

Another way to rephrase the function of plastocyanin is that it can facilitate the electron transfer reaction by providing a small reorganization energy, which has been measured to about 16–28 kcal/mol (67–117 kJ/mol).

This unusual geometry is induced by the rigid “pre-organized” conformation of the ligand donors by the protein, which is an entatic state.

A feature of the entatic state is a protein environment that is capable of preventing ligand dissociation even at a high enough temperature to break the metal-ligand bond.

In the case of plastocyanin, it has been experimentally determined through absorption spectroscopy that there is a long and weak Cu(I)-SMet bond that should dissociate at physiological temperature due to increased entropy.

[11] In ordinary copper complexes involved in Cu(I)/Cu(II) redox coupling without a constraining protein environment, their ligand geometry changes significantly, and typically corresponds to the presence of a Jahn-Teller distorting force.

However, the Jahn-Teller distorting force is not present in plastocyanin due to a large splitting of the dx2-y2 and dxy orbitals (See Blue Copper Protein Entatic State).

Additionally, the structure of plastocyanin exhibits a long Cu(I)-SMet bond (2.9Å) with decreased electron donation strength.

It was surprising to find these organisms containing the protein plastocyanin because the concentration of copper dissolved in the ocean is usually low (between 0.4 – 50 nM).