Study of metal complexation of cardenolides with divalent metal ions by Electrospray Ionization Mass Spectrometry

Abstract

Cardenolides are natural products with positive inotropic and cytotoxic activity that are able to interact with metals, although the possible role that these interactions may play in their biological activity is not known. Mixtures of the following cardenolides: digoxigenin (DgG), gitoxigenin (GxG), digitoxigenin (DxG), uzarigenin (UzG) and a butenolide, 2(5H)-furanone (Fur), with different metal cations, namely Ca2+, Mg2+, Cu2+, Co2+ and Zn2+,were studied by Electrospray Ionization Mass Spectrometry in a Quadrupole-Time of Flight. The relative stability of the most important adducts was studied by threshold collision induced dissociation, E1/2. Computational modeling of the observed complexes with calcium was performed using DFT B3LYP/6-31G+(d,p) level of theory. Complexes of stoichiometry [nM+Me]2+, with n = 4 to 6 ligands and Me a metal cation, were observed for all studied compounds. The adducts [4M+Me]2+ corresponded to the most intense peaks in most of the mass spectra and showed the highest E1/2. GxG showed a higher tendency to form complexes with low coordination numbers. Calculations showed that the carbonyl oxygen of the butenolide moiety is the most important site of coordination and allowed the proposal of different binding modes to explain the differences observed in the GxG MS spectra. A direct relationship was observed between experimental and computational data, which allowed to predict the MS behavior of these or similar compounds. The analysis can be extrapolated to other compounds with a furanone ring, and used as an analytical tool to characterize furanone compounds, or for the differentiation of DgG and GxG.