Journal of Chromatography A, 1216 (2009) 3660–3665 Contents lists available at ScienceDirect Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma Application of capillary affinity electrophoresis and density functional theory to the investigation of valinomycin–lithium complex Sille Ehala a , Jiˇ rí Dybal b , Emanuel Makrlík c , Václav Kaˇ siˇ cka a,∗ a Institute of Organic Chemistry and Biochemistry, v.v.i., Academy of Sciences of the Czech Republic, Flemingovo 2, 166 10 Prague 6, Czech Republic b Institute of Macromolecular Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Heyrovského 2, 162 06 Prague 6, Czech Republic c Faculty of Applied Sciences, University of West Bohemia, Husova 11, 306 14 Pilsen, Czech Republic article info Article history: Available online 10 February 2009 Keywords: Capillary affinity electrophoresis Binding constant DFT Valinomycin–lithium complex Valinomycin abstract Capillary affinity electrophoresis (CAE) and quantum mechanical density functional theory (DFT) have been applied to the investigation of interactions of valinomycin (Val), a macrocyclic dodecadepsipeptide antibiotic ionophore, with lithium cation Li + . Firstly, from the dependence of effective electrophoretic mobility of Val on the Li + ion concentration in the background electrolyte (BGE) (methanolic solution of 50 mM chloroacetic acid, 25 mM Tris, pH MeOH 7.8, 0–40mM LiCl), the apparent binding (stability) con- stant (K b ) of Val–Li + complex in methanol was evaluated as log K b = 1.50 ± 0.24. The employed CAE method include correction of the effective mobilities measured at ambient temperature, at different input power (Joule heating) and at variable ionic strength of the BGEs to the mobilities related to the reference temper- ature 25 ◦ C and to the constant ionic strength 25 mM. Secondly, using DFT calculations, the most probable structures of the non-hydrated Val–Li + and hydrated Val–Li + ·3H 2 O complex species were predicted. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Structures and substances mediating transfer of ions across bio- logical membranes are extremely important and they still attract a large scientific interest [1–8]. Among them, valinomycin (Val), an antibiotic dodecadepsipeptide composed of 12 alternating amino and hydroxy acids forming a macrocycle molecule, has been one of the first to be recognized as an ion carrier or ionophore [9,10]. It consists of three identical fragments, each of them composed of l-valine–d--hydroxyisovaleric acid–d-valine–l-lactic acid, and its 36-membered ring contains six amide and six ester bonds, see Fig. 1. The ability of Val to carry ions across a membrane is primarily based on the formation of its molecular complexes with these ions, and secondarily on the lipophilic nature of the outer rim of its depsipep- tide ring, which secures its embedding into the membrane. It had been originally believed that a complex is formed selectively with potassium cation but further studies have shown that Val binds other metal and non-metal ions as well [1,2,9–12]. In the present work, the interaction of Val with lithium ion Li + was investigated. The binding (stability) constant of the Val–Li + complex in methanolic background electrolyte (BGE) was deter- mined by capillary affinity electrophoresis (CAE). Due to the Val insolubility in water, the CAE analyses were carried out in methano- ∗ Corresponding author. Tel.: +420 220 183 239; fax: +420 220 183 592. E-mail address: kasicka@uochb.cas.cz (V. Kaˇ siˇ cka). lic BGEs. CAE mode of capillary electrophoresis (CE) has become in the recent years an attractive tool for the investigation of biomolec- ular interactions [13–18] because of its high speed of analysis, high resolving power, low amount of sample required and the ability to employ non-pure samples provided that CE can separate the analyte of interest from the impurities. As such, this work is an advanced and complement study to our previous investigation dealing with the determination of the bind- ing constants of Val complexes with various alkali metal ions and ammonium ion [12]. The main improvements of the current work include (i) the correction of the effective electrophoretic mobilities of Val–Li + complex measured by CAE at variable ambient temper- ature, at different input power (Joule heating) and at variable ionic strength of the BGEs to the mobilities related to the reference tem- perature 25 ◦ C and to the constant ionic strength 25 mM, and (ii) the change of the BGE, resulting in an increased separation effi- ciency and shortened analysis time. In addition, a prediction of the most probable structures of Val and its non-hydrated and hydrated complexes with Li + ion by quantum mechanical density functional theory (DFT) calculations have been implemented to this study. 2. Experimental 2.1. Chemicals All chemicals used were of analytical reagent grade with the exception of LiCl, which was of chemically pure quality. Val was 0021-9673/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2009.02.003