Complexation of the lithium cation with beauvericin: Experimental and DFT study
Highlights
► Water-LiCl–nitrobenzene-beauvericin-dicarbollide system was investigated. ► Stability constant of the Li+⋅beauvericin complex in nitrobenzene was determined. ► Quantum mechanical DFT calculations were carried out. ► Structures of the beauvericin⋅Li+ and beauvericin Li+⋅3H2O complexes were predicted.
Introduction
Beauvericin (abbrev. 1; see Scheme 1) is a cyclic hexadepsipeptide with alternating methyl-phenylalanyl and hydroxy-iso-valeryl residues [1]. For the first time, beauvericin was isolated from the fungus Beauveria bassiana. It was further found that the antibiotic beauvericin was produced by many other fungi (e.g. by some Fusarium species) [2] so that this compound, which belongs to the enniatin family, can occur very often in grain contaminated with these fungi [2], [3], [4]. Its ion-complexing capability allows beauvericin to transport alkaline earth metal and alkali metal ions across cell membranes evidently in the same way as in the case of other related antibiotics [5].
The dicarbollylcobaltate anion [6] and some of its halogen derivatives are very useful reagents for the extraction of various metal cations (especially Cs+, Sr2+, Ba2+, Eu3+ and Am3+) from aqueous solutions into a polar organic phase, both under laboratory conditions for purely theoretical or analytical purposes [7], [8], [9], [10], [11], [12], [13], and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive waste [14], [15].
In the current communication, the stability constant of the beauvericin·Li+ complex (1·Li+) in nitrobenzene saturated with water was determined. Furthermore, applying DFT calculations, the most probable structures of the mentioned cationic complex species were derived. At this point it is necessary to emphasize that the main reasons for our research work were expected, very interesting structures of these complex species. It is apparent that the considered structures may be an important contribution to the theoretical study of antibiotics as well.
Section snippets
Experimental
Beauvericin (1; see Scheme 1) was purchased from Aldrich. Cesium dicarbollylcobaltate (CsDCC) was synthesized by means of the method published by Hawthorne et al. [16]. The other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity. A nitrobenzene solution of hydrogen dicarbollylcobaltate (HDCC) [6] was prepared from CsDCC by the procedure described elsewhere [17]. The equilibration of the nitrobenzene solution of HDCC with stoichiometric NaOH, which was dissolved in an
Extraction experiments
Regarding the results of previous papers [6], [18], [19], [20], [21], the two-phase water-LiCl–nitrobenzene-NaDCC extraction system can be described by the following equilibrium:with the corresponding exchange extraction constant Kex (Li+, Na+); aq and nb denote the presence of the species in the aqueous and nitrobenzene phases, respectively. For the constant Kex (Li+, Na+) one can write [18]:where and are
Conclusions
In summary, we have demonstrated that a complementary theoretical and experimental approach can provide important information on the beauvericin ligand (1) complexation with the lithium cation. From the experimental investigation of the resulting complex 1·Li+ in the two-phase water–nitrobenzene system, the strength of the considered 1·Li+ cationic complex species in nitrobenzene saturated with water was characterized quantitatively by the stability constant, log βnb (1·Li+) = 5.6 ± 0.2 (for a
Acknowledgements
This work was supported by the Grant Agency of Faculty of Environmental Sciences, Czech University of Life Sciences, Prague, Project No. 42900/1312/3114 “Environmental Aspects of Sustainable Development of Society”, by the Czech Ministry of Education, Youth and Sports (Project MSM 6046137307) and by the Czech Science Foundation (Project P 205/10/2280). The computer time at the MetaCentrum (Project LM 2010005), as well as at the Institute of Physics (computer Luna/Apollo), Academy of Sciences of
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