ESI-MS study of self-assembly-formed tetraglycine macrocyclic ligand complex of Cu(II)

• Copper–tetraglycine (Cu–G4) complexes were characterized by ESI-MS at various pH and Cu:G4 molar ratios.
• G4 mobilizes a large amount of copper ions from insoluble salts on increasing pH.
• Increased pH and copper ions induce oligomerization of G4.

Copper–tetraglycine system is considered as a suitable candidate for the investigation of metal–peptide complexes as those related to the origin of life on Earth. Besides, the interaction of Cu(II) with prion protein as well as the transport of copper by albumin assumes copper reaction with glycine residues. Here, we investigated copper–tetraglycine (Cu–G4) complexes by electrospray ionization mass spectrometry (ESI-MS) under various pH and concentration conditions. Tandem mass spectrometric (MS/MS and MS/MS/MS) study of Cu(II)–G4 structure revealed copper- and pH-induced oligomerization of G4. ESI-MS has provided direct information on pH- and copper-induced oligomerization of G4 and brought about evidence that G4 mobilizes a large amount of copper ions from insoluble salts at high pH values. Copper ion showed a high affinity toward tetraglycine and this interaction is clearly demonstrated by the presence of copper adducts of b2 and b3 fragments of G4. Gas-phase MS measurements proved to afford information on the metal–peptide complexes, which may be complementary to that given by X-ray diffraction studies in solid state.

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