Monitoring Silver(I)-Insulin Complexes with Electrospray Ionization Quadrupole Ion Trap Mass Spectrometry

Ag(I)-insulin complex formation was investigated using electrospray quadrupole ion trap mass spectrometry (ESI-QIT-MS), and Ag(I) ion binding to an insulin molecule was evaluated. The Ag(I) binding ratios were measured in the range of pH 3-8. The highest binding ratio of the Ag(I) ions was obtained at pH 7. Spectrometric titration was carried out at varied molar ratios of Ag(I) ions to insulin from 20/1 to 250/1. It was observed that four Ag(I) ions were bound effectively to an insulin molecule to form Ag(I)_1-4-insulin complexes. The formation equilibrium constants of Ag(I)_1-4-insulin complexes were calculated from the ESI-QIT-MS peak intensities. The equilibrium constants were found as K_f1 = (2.92 ± 0.18) × 10⁴ M^-1, K_f2 = (1.03 ± 0.07) × 10⁴ M^-1, K_f3 = (6.67 ± 0.46) × 10³ M^-1, and K_f4 = (2.00 ± 0.16) × 10³ M^-1. The tandem MS/MS spectroscopies were studied to evaluate the stability of the Ag(I) complexes. The different flow rates with nano-ESI were performed to determine the binding of Ag(I) ions in solution or gas phase. In conclusion, it was observed that the Ag(I) ion forms stable Ag(I)_1-4-complexes with high formation equilibrium constants.