Electrochemistry of Blue Copper Proteins

Blue (or type 1 ) copper proteins (or cupredoxins) are important components of biological electron transfer processes in many organisms ranging from bacteria to animals, from fungi to plants.56 They are characterized by  

The electrochemical reversibility of the Cu(II)/Cu(I) process E° = + 0.36 V vs. NHE) is in agreement with the X-ray structure of reduced plastocyanin, which shows that the active site maintains its trigonally distorted tetrahedral geometry with a slight lengthening (of about 0.1 A) of the Cu-N distances. 

In this case also, the Cu(II) ion is coordinated to two histidine nitrogen atoms, a cysteine sulfur atom and a methionine sulfur atom with a distorted tetrahedral geometry. Since this protein has an overall positive charge (p/ = 10.6), it gives a well formed response for the Cu(II)/Cu(I) reduction ( ,o =+0.32 V vs. NHE) simply using a glassy carbon 

Another interesting blue protein is stellacyanin (FW = 20 000) from the Japanese lacquer tree Rhus vernicifera, in which, with respect to the other cupredoxins, glutamine replaces the methionine ligand.64 Stellacyanin also bears an overall positive charge (p/=9.9). It, therefore, gives a reversible Cu(II)/Cu(I) response at a glassy carbon electrode in aqueous solution (pH 7.6).61 The formal electrode potential of the Cu(II)/Cu(I) reduction (E01 = + 0.18 V vs. NHE) is the lowest among cupredoxins. 

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