Complexes of monensin A methyl ester with Mg2+, Ca2+, Sr2+, Ba2+ cations studied by electrospray ionization mass spectrometry and PM5 semiempirical method

Monensin A methyl ester (MON1) was synthesized and its ability to complex formation with divalent cations was studied by the ESI mass spectrometry and PM5 semiempirical calculations. The ESI MS spectra indicated that MON1 forms stable 1:1 complexes with the divalent cations studied. Additionally also complexes of MON1 with divalent cations and perchlorate anions were also detected. With increasing cone voltage values the fragmentation process of the complexes with the formation of respective fragmentary cations occured. The structures of the complexes are discussed in detail on the basis of conformational analysis together with the PM5 semiempirical calculations.