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A series of new Monensin A dimers linked by diurethane moiety were synthesised and their molecular structures were studied using ESI-MS, FT-IR, 1 H and 13 C NMR and PM5 methods. The results showed that the compounds form a pseudo-cyclic structure stabilized by three intramolecular hydrogen bonds and the sodium cation was coordinated by five oxygen atoms of polyether skeleton of Monensin moiety. The NMR and FT-IR data of complexes of Monensin urethane sodium salts demonstrated that within the pseudo-cyclic structure the carbonyl oxygen atom of the urethane group did not coordinate the sodium cation. Monensin urethanes were tested in vitro for the activity against Gram-positive and Gram-negative bacteria and fungi as well as against a series of clinical isolates of Staphylococcus : methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-sensitive S. aureus (MSSA). The most active compound against MRSA and MSSA was 1,4-phenylene diurethane of Monensin with MIC 10.4–41.4 μmol/L).

Two new complexes of the ionophore antibiotic Lasalocid acid (LAS) with phenylamine (PhA) and butylamine (BuA) were synthesized and their molecular structures were studied using single crystal X-ray diffraction and spectroscopic methods. In the solid state both amines are protonated and all NH 3 + protons are hydrogen bonded to etheric, hydroxyl and carboxylic oxygen atoms of the LAS anion. In chloroform solutions the structure observed in the crystal of LAS–BuA complex is preserved and an equilibrium between the LAS–PhA complex and dissociated Lasalocid acid and phenylamine is observed. In vitro antimicrobial tests of the complexes showed a significant activity towards some strains of Gram-positive bacteria. For the first time Lasalocid acid and its complexes with amines were tested in vitro for cytotoxic activity against human cancer cell lines: A-549 (lung), MCF-7 (breast), HT-29 (colon) and mouse cancer cell line P-388 (leukemia). We found that LAS and its complexes are strong cytotoxic agents towards all tested cell lines. The cytostatic activity of the compounds studied is greater than that of cisplatin, indicating that Lasalocid and its complexes are promising candidates for new anticancer drugs.

The unexpectedly stable benzotriazole ester of salinomycin (SAL-HOBt) – an intermediate product of the amidation reaction of salinomycin has been isolated and structurally characterised (using a single crystal) by X-ray, FT-IR, NMR and semiempirical methods. The results of the X-ray and spectroscopic studies demonstrated that this intermediate ester exist in the solid state and in solution exclusively as the stable O–acyl form. The molecular structure of SAL-HOBt is stabilised by relatively weak intramolecular hydrogen bonds. The PM5 calculation of possible structures of SAL-HOBt has shown that the O–acyl form is more energetically favourable than its N–oxide–N–acyl isomers. The antimicrobial tests show that SAL-HOBt is active against Gram-positive bacteria and clinical isolates methicillin-resistant Staphylococcus aureus (MIC = 1–2 μg/ml).

A new complex of lasalocid acid with allylamine (LAS–AM) is synthesised and studied by X-ray, FT-IR, 1 H NMR and 13 C NMR, ESI MS methods. In the solid state allylamine is protonated and all protons of NH 3 + are hydrogen bonded. We show that in the gas, liquid and solid states lasalocid forms 1:1 complexes with allylamine and that its structures of all states are comparable, which indicates that the complex LAS–AM is very stable. The stability of the LAS–AM complex is achieved by some intra-molecular hydrogen bonds. Due to these interactions the outside of the complex is hydrophobic enabling its transport across the biological membranes. This property of the complex is reflected in its anti-microbial activity, which is discussed.

A complex between a new N-phenylamide of monensin A (M-AM1) and sodium chloride has been synthesised and studied by X-ray diffraction and FT-IR spectroscopy. The crystal structure of the complex between M-AM1 and sodium chloride with acetonitrile was examined using X-ray diffraction and discussed in detail. Its structure is stabilised by coordination of the Na+ cation with oxygen atoms. The Na–O bond lengths are between 2.382(2) and 2.562(2) A. The chloride anion is involved in a weak intermolecular hydrogen bond between different species forming a supramolecule. The ESI-MS spectra indicate that the amide forms stable complexes of exclusively 1:1 stoichiometry with Na+ cations. The FT-IR spectrum of the crystal is consistent with the results obtained by the X-ray study and provides spectroscopic evidence for the complex formation. Due to its specific structural properties N-phenylamide of monensin A efficiently binds sodium chloride. The result of the PM5 semiempirical calculation is in agreement with the spectroscopic data and allows visualisation of the structure of the M-AM1–Na+ complex. The new amide of monensin A has been additionally tested in view of its antimicrobial properties. It shows great activity towards some strains of Gram-positive bacteria (MIC = 6.25–12.5 μg/ml).

A new lasalocid 1-naphthylmethyl ester (NAFA) has been synthesised and studied by X-ray, 1 H NMR, 13 C NMR, FT-IR, UV–vis, fluorescence as well as by PM5 semiempirical methods. The crystals of NAFA belong to the monoclinic system with the space group P2 1 with a = 13.4251(5) A, b = 17.1064(7) A, c = 18.5454(7) A, β = 98.924(4)° and Z = 4. Two conformers of NAFA have been observed for two symmetry-independent molecules in different crystal environments. The molecular conformation of NAFA is partially stabilized by three intramolecular hydrogen bonds, in which the keto group is not involved. The FT-IR spectrum of NAFA in chloroform indicates that in this solvent the equilibrium between two structures of NAFA is realized. In one of the structures, the keto group is hydrogen bonded while in the other one this group is not involved in any hydrogen bond. The two structures of NAFA are discussed in detail. The new ester which has been additionally tested for its antimicrobial properties shows a certain activity against Gram-positive bacteria, however no activity against Gram-negative bacteria and Candida .