ESI MS, NMR and PM5 semiempirical studies of oligomycin A and its complexes with Li+ and Na+ cations

The ability to complex formation of oligomycin A (OLA) with monovalent cations was studied by the ESI mass spectrometry, 1H, 13C, 7Li and 23Na NMR spectroscopic and PM5 semiempirical methods. The ESI MS spectra indicated that OLA formed stable 1:1 complexes with all monovalent cations irrespective of the stoichiometry and cone voltage values used. With increasing cone voltages only the degradations of the OLA-Li+ and OLA-Na+ complexes with formation of fragmentary metal cation complexes were found. For OLA complexes with K+, Rb+ and Cs+ cations, with increasing cone voltage the fragmentation of these complexes occurred without the formation of fragmentary metal cation complexes. On the basis of the NMR studies the assignment of the signals of hydroxyl groups in the spectrum of OLA was made and the spectra of its complexes evidenced the involvement of certain oxygen atoms in the complexation process. The PM5 semiempirical calculations allowed visualizations of all structures of the OLA-monovalent cation complexes as well as the fragmentary cations. The most interesting structure was found for OLA-Li+ complex, in which the Li+ cation is fully isolated from the interactions with the environment.