Spectroscopic, semi-empirical and antimicrobial studies of a new amide of monensin A with 4-aminobenzo-15-crown-5 and its complexes with Na+ cation at 1:1 and 1:2 ratios

A new amide of monensin A with 4-aminobenzo-15-crown-5 (M-AM3) was synthesised and its ability to form complexes with Na+ cations was studied by ESIMS, 1H, 13C and 23Na NMR, FTIR and PM5 semi-empirical methods. ESI mass spectrometry indicates that in the gas phase M-AM3 amide forms complexes of 1:1 and 1:2 stoichiometry with Na+ cations. The formation of such complexes is also confirmed in the acetonitrile solution, in which the existence of equilibrium between two structures A and B is found, of which B structure is dominant. The structures of M-AM3 and its 1:1 and 1:2 complexes with Na+ cations are stabilised by various intramolecular hydrogen bonds, which are discussed in detail. The in vitro biological tests have demonstrated that the new M-AM3 amide shows good activity towards some strains of Gram-positive bacteria (MIC 25–50 μg/ml).