The largely uncharted complexation chemistry of the veterinary polyether ionophores, monensic and salinomycinic acids (HL) with metal ions of type M⁴⁺ and the known antiproliferative potential of antibiotics has provoked our interest in exploring the coordination processes between MonH/SalH and ions of Ce⁴⁺. (1) Methods: Novel monensinate and salinomycinate cerium(IV)-based complexes were synthesized and structurally characterized by elemental analysis, a plethora of physicochemical methods, density functional theory, molecular dynamics, and biological assays. (2) Results: The formation of coordination species of a general composition [CeL₂(OH)₂] and [CeL(NO₃)₂(OH)], depending on reaction conditions, was proven both experimentally and theoretically. The metal(IV) complexes [CeL(NO₃)₂(OH)] possess promising cytotoxic activity against the human tumor uterine cervix (HeLa) cell line, being highly selective (non-tumor embryo Lep-3 vs. HeLa) compared to cisplatin, oxaliplatin, and epirubicin.

Two new asymmetric monomethine cyanine dyes, featuring dimethoxy quinolinium or methyl quinolinium end groups and benzothiazole or methyl benzothiazole end groups were synthesized. The chemical structures of the two dyes—(E)-6,7-dimethoxy-1-methyl-4-((3-methylbenzo[d]thiazol-2(3H)-ylidene)methyl)quinolin-1-ium iodide (3a) and (E)-4-((3,5-dimethylbenzo[d]thiazol-2(3H)-ylidene)methyl)-1,2-dimethylquinolin-1-ium iodide (3b)—were confirmed through NMR spectroscopy and MALDI-TOF mass spectrometry. A new methodology was developed to study monocationic dyes in the absence of a matrix and cationizing compounds in MALDI-TOF mass experiments. The newly synthesized dyes contain hydrophobic functional groups attached to the chromophore, enhancing their affinity for the hydrophobic regions of nucleic acids within the biological matrix. The dyes’ photophysical properties were investigated in aqueous solutions and DMSO, as well as in the presence of nucleic acids. The dyes exhibit notable aggregachromism in both pure aqueous and buffered solutions. The observed aggregation phenomena were further elucidated using computational methods. Fluorescence titration experiments revealed that upon contact with nucleic acids, the dyes exhibit bioaggregachromism–aggregachromism on the surfaces of the respective biomolecular matrix (RNA or DNA). This bioaggregachromism was further confirmed by CD spectroscopy. Given the pronounced aggregachromism detected, we conclude that the dyes investigated in this study are highly suitable for use as fluorogenic probes in biomolecular recognition techniques. The unique absorption and fluorescence spectra of these dyes make them promising fluorogenic markers for various bioanalytical methods related to biomolecular recognition.

Two trinuclear oxo-centred iron(III) coordination compounds of monensic and salinomycinic acids (HL) were synthesized and their spectral properties were studied using physicochemical/thermal methods (FT–IR, TG–DTA, TG–MS, EPR, Mössbauer spectroscopy, powder XRD) and elemental analysis. The data suggested the formation of [Fe₃(µ₃–O)L₃(OH)₄] and the probable complex structures were modelled using the DFT method. The computed spectral parameters of the optimized constructs were compared to the experimentally measured ones. In each complex, three metal centres were joined together at the axial position by a μ₃–O unit to form a {Fe₃O}⁷⁺ core. The antibiotics monoanions served as bidentate ligands through the carboxylate and hydroxyl groups located at the termini. The carboxylate moieties played a dual role bridging each two metal centres. Hydroxide anions secured the overall neutral character of the coordination species. Mössbauer spectra displayed asymmetric quadrupole doublets that were consistent with the existence of two types of high-spin iron(III) sites with different environments—two Fe[O₅] and one Fe[O₆] centres. The solid-state EPR studies confirmed the +3 oxidation state of iron with a total spin S_t = 5/2 per trinuclear cluster. The studied complexes are the first iron(III) coordination compounds of monensin and salinomycin reported so far.

In the present study, we report the solid-state isolation and structural characterization of novel iron(III) complexes of the veterinary antibiotic monensin. Monensic acid (MonH × H₂O) forms a dinuclear complex of composition with FeCl₃ [FeCl(Mon)₂]₂ (1), while its interaction with FeSO₄ leads to the isolation of a triangular oxo-ferric coordination species [Fe₃O(Mon × H₂O)₆(H₂O)₂(OH)] (2). During the procedure resulting in 2, oxidation of the Fe(II) ions by atmospheric oxygen was observed. In the presence of organic bases, both complexation reactions proceeded to successfully deprotonate the carboxylic function of the ligand. Iron(III) complexes 1 and 2 were characterized by IR, EPR, NMR, and Mössbauer spectroscopies as well as with thermal (TG-DTA/MS) and elemental analyses. In addition, the structures of the two coordination compounds were modelled and selected calculated parameters were compared with the experimental results. The biological assay revealed the enhanced antibacterial potential of the newly obtained complexes against the Gram-positive aerobic microorganisms Bacillus cereus and Bacillus subtilis.

The commercial veterinary antibiotic sodium monensinate (MonNa) binds mercury(II) or zinc(II) cations as thiocyanate [Hg(MonNa)₂(SCN)₂] (1) or isothiocyanate [Zn(MonNa)₂(NCS)₂] (2) neutral coordination compounds. The structure and physicochemical properties of 1 and 2 were evaluated by the methods of single crystal and/or powder X-ray diffraction, infrared, nuclear magnetic resonance, X-ray photoelectron spectroscopies, and electrospray-mass spectrometry. The primary cores of the two complexes comprise HgS₂O₂ (1) and ZnN₂O₂ (2) coordination motifs, respectively, due to the ambidentate binding modes of the SCN–ligands. The directly bound oxygen atoms originate from the carboxylate function of the parent antibiotic. Sodium cations remain in the hydrophilic cavity of monensin and cannot be replaced by the competing divalent metal ions. Zinc(II) binding does not influence the monensin efficacy in the case of Bacillus cereus and Staphylococcus aureus whereas the antimicrobial assay reveals the potential of complex 2 as a therapeutic candidate for the treatment of infections caused by Bacillus subtilis, Kocuria rhizophila, and Staphylococcus saprophyticus.

In this study we present the preparation and characterization of novel thallium(I) coordination compound of the polyether ionophorous antibiotic salinomycin (SalH). The complex [TlSal(H2O)] exists as two subunits SalTl1 and SalTl2 which differ slightly in their structural parameters. Sali-nomycin acts in a pentadentate coordination mode through oxygen donor atoms and the six-fold arrangement around the metal centers is completed by interaction with water molecule. In the overall complex structure, the two mononuclear species SalTl1 and SalTl2 are connected via hy-drogen bond network by third water molecule. The inclusion of the heavy metal ion into the structure of the polyether ionophore reduces its biological activity against Gram-positive microor-ganisms and cervical cancer cells at in vitro conditions.