µL/min, which was maintained by a syringe pump. The ESI source was operated with a voltage of +4000 V (HV end plate offset: -500 V) using N2 as nebulizer (0.3 bar) and drying gas (4 L/min, temperature: 453 K). The experiments were typically carried out in negative mode ESI(-)-MS, with a scan range of m/z = 200-2000. Accuracies of <10 ppm were obtained for the measured m/z ratios after external calibration using a low concentration tuning mix solution (mixture of CF3COOH and phosphazenes in H2O/CH3CN, Agilent Technologies). Theoretical exact m/z ratios and isotope patterns were calculated with the DataAnalysis software package (Bruker Daltonics).