Coordination properties of the ACE inhibitor captopril towards Me2Sn(IV)2+ in aqueous solution, and biological aspects of some dialkyltin(IV) derivatives of this ligand

The coordination of Me2Sn(IV)2+ (M) to captopril {N-[(S)-3-mercapto-2-methylpropionyl]-l-proline, H2(cap), H2L} in aqueous solution was studied by means of potentiometric titration, electrospray mass spectrometry, 1H-NMR spectroscopy and Mössbauer spectroscopy in the pH range 2–11 (I=0.1 mol dm−3 NaClO4, 298 K). The results obtained with these methods proved that only monomeric complexes are formed in solution. In the acidic pH range, species with a metal-to-ligand ratio of 1:1 exist. The neutral complex ML, similarly to the complex Me2Sn(cap) crystallized in the same pH range, adopts a tbp structure with eq S− and ax COO−, while, instead of the coordination of the amide CO, observed in the solid state, the other ax position is occupied by a H2O molecule. With increasing pH, in the neutral and weakly basic pH range the complexes MLH−1 and ML2 are formed, in which the COO− group is displaced from the coordination sphere by an OH− and an S− of another ligand, respectively. Biological activity tests on Me2Sn(cap) and three further R2Sn(IV) captopril complexes showed that n-Bu2Sn(cap) and t-Bu2Sn(cap) exert toxic activity towards the embryos of Ciona intestinalis, while the ligand itself does not affect the development of the embryos to any significant extent; Me2Sn(cap) and Et2Sn(cap) produced normal swimming larvae.