Complexation of Copper(II) Ion with Tetraglycine as Followed by Electronic Absorption Spectroscopy

The complexation of Cu(II) ion with tetraglycine (TetraGly) is studied by electronic absorption spectroscopy as a function of pH. In an equimolar solution, tetraglycine forms a sequence of 1:1 chelated species: [CuL]+ around pH 5.4, [CuLH-1] around pH 6.4, [CuLH-2]- around pH 8.0, and [CuLH-3]2- above pH 11.0. They are characterized, respectively, by the equatorial donor sets [(NH2, CO); H2O; H2O], [(NH2, N-, CO); H2O], [NH2, N-, N-, CO], and [NH2, N-, N-, N-], where NH2 is the amino, CO the carbonyl, and N- the deprotonated amide group. The electronic absorption spectra exhibit a single band in the visible region, which becomes sharper and shifts to higher energy values as the complexation proceeds and the water molecules are replaced by stronger donors. The experimental λmax value can be calculated through Billo's equation, which assigns a different contribution to the equatorial water, carbonyl-O, carboxylate-O, amino-N, and deprotonated amide-N. Two biological applications, the interaction of Cu(II) with prion and the transport of copper by albumin, are presented.