Colour, Magnetic Resonance and Symmetry of the Cupric Site in Blue Protein

Abstract

THE blue copper-proteins are characterized by very intense optical absorption near 600 mµ and unusually low cupric hyperfine splitting (A_∥) as observed by electron paramagnetic resonance. The nature of the copper-protein binding and the relationship between the colour and the electron paramagnetic resonance spectrum, while the subject of much investigation and speculation, have remained obscure^1–4. However, almost all investigations have centred on ceruloplasmin and laccase, which contain eight and four atoms of copper per protein molecule, respectively, with only about half this number in the cupric state. It seemed, therefore, that these two proteins are not best suited for further investigation, particularly since it has been shown that blue proteins which contain only one cupric ion per molecule also have intense optical absorption near 600 mµ and small A_∥ values^5,6. It was with the foregoing in mind that an investigation of the blue protein from Pseudomonas aeruginosa was undertaken. This protein has a broad absorption with a maximum at 625 mµ (molar extinction coefficient of approximately 2,000) and an A_∥ value of 60 × 10⁻⁴ cm⁻¹. The protein was isolated according to the method of Horio⁷, with some modifications, and investigated in solution at room temperature and in frozen solution at −160° C by optical, electron paramagnetic resonance and optical rotatory dispersion methods.