Cu ch3cn 4 pf6

Peptide stock solutions were prepared by dissolving lyophilized peptide in 1 mL of nanopure water. Concentration of stock solutions was determined using the Edelhoch method.^{61 (link)} In short, 4–6 µL of peptide stock were diluted into 400 µL of 8 mM urea to obtain an absorbance at 280 nm between 0.1 and 1 absorbance unit. The concentration of the peptide solution was determined from the A_278nm readings using an extinction coefficient of 1450 M⁻¹cm⁻¹ for each tyrosine.^{62 (link)} Peptide stock solutions were stored in sealed cryogenic storage vials. Copper(II) stock solutions were prepared by dissolving copper sulfate (CuSO₄, Sigma) in nanopure water and standardized by EDTA titration in an ammonium buffer to a murexide endpoint. Copper(I) solutions were prepared by dissolving [Cu(CH₃CN)₄]PF₆ (Aldrich) in anhydrous acetonitrile (Fisher) and subsequently standardized with the use of the chromophoric ligand bicinchoninate (BCA). The concentration of Cu(I) solutions was determined from the absorption at 563 nm due to the Cu(I)(BCA)₂ complex (ε = 7900 M⁻¹ cm⁻¹).^{63 (link)}