Copper(II) Complexes of Neuropeptide Gamma with Point Mutations (S8,16A) Products of Metal-Catalyzed Oxidation

To obtain the information about the influence of the serine residues (S8,S16) on the acid–base properties of the neuropeptide gamma, the peptide with point mutations (S8,16A) and its N-acetyl derivative were synthesized. Any additional deprotonations were not observed. It means that the presence of serine residues is necessary in the amino acid sequence of the neuropeptide gamma to have its acid–base properties. The stability constants, stoichiometry and solution structures of copper(II) complexes of the neuropeptide gamma mutants D1AGH4GQIA8H9KRH12KTDA16FVGLM21-NH2 (S8,16A) 2ANPG and its N-acetyl derivative Ac-2ANPG were determined in aqueous solution. The equilibrium and structural properties of copper(II) complexes have been characterized by pH-metric, spectroscopic (UV–visible, CD, EPR) and mass spectrometric (MS) methods. At physiological pH7.4 the 2ANPG forms the CuH2L and CuHL complexes in equilibrium with 3N {NH2,βCOO−-D1,2NIm} and 4N {NH2,N−,2NIm} binding sites, respectively. The exchange Ser on Ala residues does not alter the coordination mode of the peptide. To elucidate the products of the copper(II)-catalyzed oxidation of 2ANPG and Ac-2ANPG the liquid chromatography–mass spectrometry method (LC–MS) and the Cu(II)/H2O2 as a model oxidizing system were employed. For solutions containing a 1:4 peptide–hydrogen peroxide molar ratio oxidation of the methionine residue to methionine sulphoxide was observed. For the 1:1:4 Cu(II)–2ANPG–H2O2 system oxidation of two His residues and cleavage of the G3H4 peptide bond was observed, while for the 1:1:4 Cu(II)–Ac-2ANPG–H2O2 system oxidation of three histidine residues to 2-oxohistidines was also observed.