Monensin A benzyl ester and its complexes with monovalent metal cations studied by spectroscopic, mass spectrometry and semiempirical methods

Monensin A benzyl ester (MON3) was synthesized and its ability to form complexes with Li+, Na+ and K+ cations was studied by ESI MS, 1H and 13C NMR, FT-IR and PM5 semiempirical methods. MON3 has been found to preferentially form complex with Na+ cations. The formation of stable complexes of 1:1 stoichiometry up to cv = 70 V is indicated in the electrospray ionisation mass spectra. With increasing cone voltage value, the fragmentation of the respective complexes is detected and is connected primary with the dehydration process. The structures of the complexes are stabilized by intramolecular hydrogen bonds in which the OH groups are always involved. The structures of MON3 and its complexes with Li+, Na+ and K+ cations are visualized and discussed in detail. It is shown that in the structure of MON3 the oxygen atom of this CO ester group is involved in very weak bifurcated hydrogen bonds with two hydroxyl groups. Within the complexes of MON3 with Li+ and K+ cations, this CO ester group is not hydrogen bonded, whereas in the structure of MON3 with Na+ it can also coordinate this cation. Such a structure is, however, not dominant in acetonitrile solution. It is demonstrated that the formation of a pseudo-crown ring structure formed by MON3 prefers the complexation of Na+ cations.