Detection of binuclear copper-caffeine complexes under electrospray ionization condition

A number of copper salts, Cu(OOCCH(3))(2), Cu(ClO(4))(2), Cu(NO(3))(2), CuCl(2) and CuSO(4) have been tested for their ability to form binuclear copper-caffeine complexes. The electrospray ionization (ESI) mass spectra of methanol solution containing caffeine and CuCl(2) or CuSO(4) show signals of two copper atom containing ions, so the signals correspond to binuclear complexes: 2Caf + Cu(2)SO(4)](+), 2Caf + Cu(2)](+), 2Caf + Cu(2)Cl](+), 2Caf + Cu(2)Cl(2)](+) and 2Caf + Cu(2)Cl(3)](+). Sulfate and chloride anion are characterized by charge densities higher than those of the carboxylate, nitrate and perchlorate anion. Thus, due to the electrostatic forces, the binuclear complexes containing SO(4)(2-) or Cl(-) can survive the transfer from solution to the gas phase and then can successfully be observed on ESI mass spectra. The ion 2Caf + Cu(2)Cl(3)](+) is present in solution and could be detected when using methanol/chloroform as solvent. The ions 2Caf + Cu(2)](+), 2Caf + Cu(2)Cl](+) and 2Caf + Cu(2)Cl(2)](+) are formed from the 2Caf + Cu(2)Cl(3)](+) ion (by subsequent loss of Cl atoms) on transfer from the solution to the gas phase or in the gas phase. The ion 2Caf + Cu(2)](+) does not contain a bridging agent, thus it is reasonable to assume that it contains a Cu-Cu bond.