Structural Elucidation of Cisplatin and Hydrated cis-Diammineplatinum(II) Complex Conjugated with Cyanocobalamin by Liquid Chromatography with Electrospray Ionization–Mass Spectrometry and Multistage Mass Spectrometry

Pt(II)-based derivatives bearing a cyanocobalamin (CNCbl) unit were synthesized in aqueous solutions, and the reaction mixtures were examined by reversed-phase liquid chromatography with electrospray ionization and linear ion trap mass spectrometry (MS). Isotopic pattern analysis, multistage mass-spectra (MS/MS and MS 3 ) interpretation, and differential isotopic labeling were used to establish the chemical composition and to suggest the chemical structures of reaction products. When cisplatin ( cis -[PtCl 2 (NH 3 ) 2 ]) was used as a Pt(II) drug derivative, a coordination bond between diamminemonochloroplatinum(II) and the cyano group of CNCbl, in turn linked covalently to the vitamin Co(III) ion, occurred. The resulting conjugate with a Co III -CN-Pt II bridge was MS detected as a doubly positive charged ion with the prevailing isotopologue at m / z 810.26 (empirical formula [C 63 H 95 ClCo III N 16 O 14 PPt] 2+ ). Likewise, a peak signal centered at m / z 811.26 was observed when 15 N-labeled cisplatin cis -[PtCl 2 ( 15 NH 3 ) 2 ] was used as Pt(II) complex, thus confirming the presence of both the cisplatin amino groups in the conjugate. A bifunctional conjugate was obtained between CNCbl and the cis -diamminediaquaplatinum(II), that is, cis -[Pt(NH 3 ) 2 (H 2 O) 2 ] 2+ ; in this case, the planar coordination complex of Pt(II) was also involved in a covalent bond with the oxygen atom of one of the CNCbl amide moieties. The peak signal detected at m / z 792.26 (empirical formula [C 63 H 94 Co III N 16 O 14 PPt] 2+ ) changed to m / z 793.26 when the labeled cis -[Pt( 15 NH 3 ) 2 (H 2 O) 2 ] 2+ complex was adopted for conjugation. Comparison between MS/MS spectra allowed an extended structural characterization of both conjugates, as such or 15 N-labeled. Two-dimensional heteronuclear ( 1 H- 15 N) single quantum correlation NMR spectroscopy, applied to 15 N-labeled conjugates, supported the hypotheses made on the Pt(II) coordination in both cases.