The reactions of the second-generation platinum anticancer drug carboplatin with L-selenomethionine (L-Se-MetH) and L-methionine (L-MetH) (abbreviations used: L-Se-MetH: L-selenomethionine; L-Se-Met: deprotonated L-selenomethionine; L-MetH: L-methionine; L-Met: deprotonated L-methionine) were studied by ESMS and 2D [1H,15N] HSQC NMR spectroscopy. The combination of the two techniques provided the unambiguous assignment of [Pt(NH3)2(CBDCA-O)(L-Se-MetH-Se)] (1), the long-lived ring-opened adduct, [Pt(NH3)(CBDCA-O)(L-Se-Met-Se,N)] (2), [Pt(CBDCA-O,O)(L-Se-MetH-Se,N)] (3) and [Pt(L-Se-Met-Se,N)2] (4). The reaction of carboplatin with L-MetH is very similar to that with L-Se-MetH, except for a slower reaction rate. Interestingly, we observed the dimer and polymer forms of carboplatin in solution by electrospray mass spectrometry. This work demonstrates that carboplatin differs from cisplatin in that both reactions with L-Se-MetH and L-MetH form [Pt(L-Se-Met-Se,N)2] or [Pt(L-Met-S,N)2] as the dominant adducts. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)