Structures of complexes of benzyl and allyl esters of monensin A with Mg2+, Ca2+, Sr2+, Ba2+ cations studied by ESI-MS and PM5 methods

Benzyl ester of monensin A (MON3) and allyl ester of monensin A (MON4) were synthesized and their ability of forming complexes with divalent cations was studied by ESI mass spectrometry and by PM5 semi empirical calculations. The ESI-MS spectra indicate that MON3 as well as MON4 form stable 1:1 complexes with the divalent cations used in this study. The complexes of MON3 are stable to cv = 50 V and after this cv value no m/z signals are more observed in the spectra. In contrast, with increasing cone voltage values, a fragmentation process of the MON4 complexes is simultaneously observed, where dehydration processes are the main events. The structures of the complexes are discussed in detail on the basis of conformational analysis together with the PM5 semiempirical calculations.