Spectrophotometric studies of the copper(II)–D-o-tyrosine complex. Assignment of the 330-nm dichroic band in copper(II) and iron(III) transferrins

Abstract

The separation of both enantiomers of racemic o-tyrosine by means of binapthylphosphoric acid has been undertaken. Copper(II) interacts with D-o-tyrosine to form two complexes. The first, obtained at pH 6, displays a circular-dichroism (c.d.) spectral pattern characteristic of metal co-ordination through amino-nitrogens and carboxylate oxygens. The second complex starts to form at pH 8 and is fully defined at pH 10.5. Its c.d. spectrum, in the region below 400 nm, is very similar to those of iron(III) and copper(II) transferrins and displays two well defined bands at ca. 400 and 330 nm. So far, only the origin of the higher-frequency peak has been well established as arising from a phenolate-oxygen-to-metal charge-transfer transition, whereas that of the lower-frequency peak remains uncertain. Resonance-Raman measurements upon excitation into the envelope of the two absorptions at 400 and 330 nm clearly indicate a common origin to both bands, namely: phenolate oxygen → Cu^II charge-transfer transition.