The effect of pH on Cu(I) and Cu(II) forms of the isolated soluble domain of the stellacyanin from Rhus vernicifera (SCu), the Japanese lacquer tree, has been studied by electronic and NMR spectroscopy and using direct electrochemical measurements. A pK(a) value of 10.1-10.4 is observed for the alkaline transition in this oxidized phytocyanin and results in a slightly altered active-site struct...