23Na NMR and FT-IR studies of sodium complexes with the ionophore lasalocid in solution

Journal of Molecular Structure 516 (2000) 91–98 www.elsevier.nl/locate/molstruc

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Na NMR and FT-IR studies of sodium complexes with the ionophore lasalocid in solution

G. Schroeder a, B. Gierczyk a, B. Brzezinski a, B. Ro´z˙alski a, F. Bartl b, G. Zundel c,*, J. Sos´nicki d, E. Grech d b

a Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan´, Poland Institute of Medical Physics and Biophysics, Universita¨tsklinikum Charite´, Humboldt University, D-10098 Berlin, Germany c Institute of Physical Chemistry, University of Munich Theresienstr. 41, 80333 Munich, Germany d Institute of Fundamental Chemistry, Technical University of Szczecin, Al. Piasto´w 41, 71-065 Szczecin, Poland

Received 8 March 1999; accepted 6 April 1999

Abstract Lasalocid forms 1:1 or 2:2 complexes with sodium ions. The process of complexation was studied in different solvents at various temperatures by 23Na NMR. The formation constants and DG values were determined. The nature of the complex between lasalocid and Na 1 ions was also studied by FT-IR spectroscopy. In chloroform, a 2:2 complex of lasalocid and Na 1 ions is formed. A continuous absorption is observed in the far FT-IR spectrum of this complex. It indicates the large Na 1 polarizability due to fast fluctuations of the Na 1 ions in multiminima potentials, in the dimeric structure. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Lasalocid; Na 1 –lasalocid complexes; 23Na NMR; FT-IR; Cation fluctuation; Hydrogen bonds; Cation polarizability

1. Introduction Recently, we have studied the behaviour of H 1 and Li ions in the cation channel of gramicidin A, gramicidin C and their model compounds [1,2]. In contrast to gramicidin, lasalocid does not transport ions through channels formed by its dimers, but lasalocid forms ion-transporting complexes. If lasalocid is used, both complexation and transport of the cations are significantly affected by the structure of the ligand [3–12]. In the past several years the antibiotic lasalocid has been the subject of many biochemical investigations 1

* Corresponding author. Bruno-Walter-Str. 2, A-5020 Salzburg, Austria. Tel.: 1 43-662-6423-11; fax: 1 43-662-6423-1176.

because of its ability to transport metal cations as carriers across natural and artificial membranes [3– 12]. The molecular structure of lasalocid— carboxylic-1-polycyclic polyether consists of a salicylic acid group attached to a hydrocarbon backbone terminating in a tetrahydropyran ring as shown in Scheme 1. The spectroscopic and X-ray studies of lasalocid have demonstrated that its structure is very similar both in solutions and in solid state [9–21]. The same methods were used to determine the structure of lasalocid complexes with monovalent and divalent cations [13,22–27] as well as with several asymmetric amine salts [28] and amine acid ester salts [29–32]. Dimeric R2M (R ˆ lasalocid, M ˆ cation) type structures have generally been observed for the complexes of lasalocid with divalent cations

0022-2860/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00139-8

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Scheme 1.

[33–35]. Dimeric R2M2 type structures were also found for monovalent cation complexes of lasalocid in non-polar solutions [10,11,36] as well as in crystals [19,34]. The monomeric RM-type structures of complexes of lasalocid with monovalent cations were observed in crystals grown from methanol [34–36]. Encouraged by these results we have undertaken 23 Na NMR studies of complexes of lasalocid with Na 1 cations in methanol solution. This NMR technique was used also in the studies of ion transport across phospholipid vesicles by lasalocid [37]. In this paper we report 23Na NMR studies of the complex formation of lasalocid with Na 1 cations in methanol solution as well as in other solvents. The behaviour of the Na 1 cations in the far FT-IR is also described. 2. Experimental Lasalocid and its Na 1 salt were purchased from Aldrich and were used after recrystallization from methanol. All solvents were spectroscopic grade and ˚ molecular sieve. were dried over 3 A 2.1. 23Na NMR measurements 23

Na NMR measurements were carried out at opening frequency 105.8 MHz using a Bruker DPX400 spectrometer, equipped with 5 mm 1H/BBinverse probe head. The 23Na NMR spectra were obtained using 21231.4 Hz spectral width, 4/2K data points with and 17.5 ms pulse width (908 flip angle). Line broadening of 15 Hz was used. Chemical shifts were referenced to the measured at 300 K the resonance peak of 3 mol dm 23 NaCl aqueous solution. The external acetone-d6 was used for the frequency lock. The complexes of lasalocid–Na 1 were prepared

by mixing different amounts of 1 mol dm 23 solutions of lasalocid tetrabuthylammonium salt in methanol with sodium tetraphenylborate. The concentration of the lasalocid anion was always 0.1 mol dm 23. Complex formation constants of the sodium cation with lasalocid were determined from the 23Na NMR data. The formation constants of the 1:1 (lasalocid– Na 1) complexes were calculated by fitting the 23Na NMR chemical shifts (d obs) at various lasalocid–Na 1 mole ratios at the fixed concentration of the sodium salt to the following expression [38,39]:

dobs ˆ

d0 1 d1 ‰LŠK 1 1 K‰LŠ

…1†

where d 0 and d l are the chemical shifts of the free solvated sodium cation and the limiting chemical shift of the 1:1 complex, respectively. K is the formation constant of the 1:1 complex and [L] is the free ligand concentration. The value for [L] was obtained according to the equation:  ÿ
 …2† K‰LŠ2 1 K CL 2 CNa 1 1 ‰LŠ 2 CL ˆ 0 where CNa and CL are the analytical concentrations of the salt and the ligand, respectively. The values of DG were calculated using the equation 2DG ˆ RT ln K. 2.2. FT-IR measurements A cell with Si windows and a wedge-shaped layer was used to avoid interferences (mean layer thickness 0.176 mm). The IR spectra were taken with the FT-IR spectrophotometer IFS 113v from Bruker, using an MCT detector in the middle infrared region (125 scans, resolution 2 cm 21) and a helium-cooled bolometer in the far infrared region (125 scans, resolution 1 cm 21). The concentration of the samples was

G. Schroeder et al. / Journal of Molecular Structure 516 (2000) 91–98 Table 1 23 Na chemical shifts (ppm) and line widths (Hz) of lasalocid–Na 1 (1:1) complexes in different solvents at 300 K Solvents

Chemical shifts

Line widths

Pyridine Methanol DMF Chloroform Acetic acid THF

2 5.56 2 5.74 2 5.81 2 6.21 2 6.69 2 8.17

470 150 450 710 320 300

0.1 mol dm 23 in chloroform. All preparations and transfers of solutions were carried out in a carefully dried glovebox under nitrogen atmosphere. The solid state FT-IR spectra of lasalocid samples were measured in KBr pellets.

3. Results and discussion 3.1. 23Na NMR studies 23

Na chemical shift and line width of lasalocid– Na 1 (1:1) in various solvents at 300 K are collected in Table 1. It demonstrates that the 23Na NMR chemical shifts are strongly dependent on the nature of the solvent. However, no correlation between the empirical solvent parameters, chemical shift and line width was found. For the sake of comparison the 23Na NMR chemical shifts of sodium tetraphenylborate in methanol in which Na 1 ions occur in solvated free form are Table 2 23 Na chemical shifts (ppm) and line widths (Hz) of sodium tetraphenylborate in methanol at various temperatures Temp. (K)

Chemical shifts

Line widths

210 220 230 240 250 260 270 280 290 300 310

2 2.12 2 2.26 2 2.42 2 2.56 2 2.72 2 2.89 2 3.05 2 3.21 2 3.36 2 3.51 2 3.66

330 250 190 130 120 100 90 75 70 65 60

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given at various temperatures in Table 2. The data in this table illustrate the temperature dependence of the chemical shifts as well as the line width of the 23 Na NMR signals. With increasing temperature a decrease of both values is observed due to the various interaction effects between the Na 1 cation and the solvent. At 300 K the chemical shift and line width are 23.51 ppm and 65 Hz. The chemical 23Na NMR signals of sodium tetraphenylborate and lasalocid–Na 1 salt, both measured in methanol at 300 K, are different. The chemical shift (25.74 ppm) and the line width in the complex indicate a strong interaction between lasalocid and the Na 1 cation in agreement with the literature [34,36], on the formation of the 1:1 complex. Still larger differences between the chemical shifts of free Na 1 ion and complexed Na 1 cation are observed in apolar solvents such as chloroform, acetic acid and especially in THF. This result is also in agreement with the literature [10,11,36]. These data suggest formation of R2M2-type complexes. 23 Na NMR chemical shifts and line width for the various ratios of mixtures of lasalocid with Na 1 cations at various temperatures in methanol are summarized in Table 3. These data indicate that the chemical shifts and line widths for various ratios lasalocid:Na 1 depend on temperature. The largest negative chemical shifts are observed in the case of 1:0.33 mixture of lasalocid:Na 1, i. e. when the concentration of lasalocid ions is three times larger than the concentration of Na 1 cations. With increasing concentration of Na 1 cations the chemical shift of 23Na NMR signals slightly decreases. This correlation is observed for Na 1 cation concentrations up to the ratio of 1:1.5 of lasalocid to Na 1 ions. If the concentration of Na 1 increases further the 23Na NMR chemical shift is almost constant. Following the equations (1) and (2) the formation constants and termodynamical parameters were calculated and summarized in Table 4. The data given in Table 4 demonstrate that K is almost independent of temperature. The value of K of about 10 indicates that the following equilibrium between the complexed and not complexed Na 1 cation by the lasalocid ion is present in methanol. Las2 Na1 N Las2 1 Na1

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Ratio

1:0.33

1:0.4

1:0.5

1:0.66

1:1

1:1.5

1:1.75

1:2

1:2.25

1:2.5

Temp. (K)

Chemical shifts

Line widths

Chemical shifts

Line widths

Chemical shifts

Line widths

Chemical shifts

Line widths

Chemical shifts

Line widths

Chemical shifts

Line widths

Chemical shifts

Line widths

Chemical shifts

Line widths

Chemical shifts

Line widths

Chemical shifts

Line widths

240 250 260 270 280 290 300 310

25.70 25.76 25.88 25.94 26.06 26.11 26.27 26.33

1200 950 800 600 580 470 320 280

25.55 25.63 25.79 25.93 25.98 26.00 26.21 26.23

1200 900 800 720 500 400 340 260

25.43 25.55 25.74 25.82 25.89 25.99 26.12 26.19

1100 1000 900 800 500 450 340 260

25.23 25.45 25.67 25.70 25.72 25.88 25.99 26.04

1100 900 800 600 480 400 320 200

25.13 25.29 25.37 25.42 25.51 25.61 25.71 25.82

1000 800 500 440 260 220 150 120

23.96 24.24 24.61 24.89 25.09 25.22 25.33 25.39

820 540 450 260 230 200 130 100

23.48 23.83 23.91 24.09 24.24 24.37 24.49 24.59

540 400 300 230 180 130 120 100

23.38 23.45 23.73 23.89 23.95 24.02 24.29 24.39

480 370 280 220 180 170 100 90

23.35 23.43 23.66 23.82 23.96 24.09 24.23 24.35

520 380 280 230 170 140 120 100

23.34 23.44 23.66 23.82 23.96 24.09 24.21 24.31

460 350 280 220 150 135 120 90

G. Schroeder et al. / Journal of Molecular Structure 516 (2000) 91–98

Table 3 23 Na chemical shifts (ppm) and line widths (Hz) of various ratios of lasalocid–Na 1 mixture in methanol

G. Schroeder et al. / Journal of Molecular Structure 516 (2000) 91–98 Table 4 Formation constants and thermodynamic parameters for complexation of Na 1 by lasalocid in methanol at different temperatures Temp. (K)

K [dm 3mol 21]

2 DG (kJ/mol)

240 250 260 270 280 290 300 310

9.70 ^ 0.05 9.47 ^ 0.05 10.16 ^ 0.05 10.94 ^ 0.05 11.16 ^ 0.05 11.13 ^ 0.05 10.32 ^ 0.05 9.58 ^ 0.05

4.53 ^ 0.02 4.44 ^ 0.02 5.01 ^ 0.02 5.37 ^ 0.02 5.62 ^ 0.02 5.81 ^ 0.02 5.82 ^ 0.02 5.82 ^ 0.02

95

3.2. FT-IR studies The spectra of lasalocid and lasalocid–Na 1 salt both in solid state and in chloroform are compared in Fig. 1(a) and (b), respectively. In Fig. 2(a) and (b) these spectra are shown with extended scale in the region 1800–1100 cm 21. Lasalocid and lasalocid–Na 1 salt occur in crystals as monomeric forms according to the X-ray studies [34,36]. In chloroform solution lasalocid probably also occurs as monomeric form. This fact is demonstrated by the spectra of lasalocid in solid state, and in chloroform, given in Fig. 2(a). These two spectra are comparable. Important differences are, however, observed in the region 3700–1800 cm 21 given in Fig. 1(a). The

Fig. 1. FT-IR spectra of: (—) the compounds in KBr, and (- - - -) the compounds in chloroform: (a) lasalocid and (b) 1:1 complex of lasalocid with Na 1 ions.

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Fig. 2. FT-IR spectra in the region 1800–1100 cm 21 of (—) the compounds in KBr, and (- - - -) the compounds in chloroform: (a) lasalocid and (b) (—) 2:2 complex of lasalocid with Na 1 ions.

intensity of the band of n (OH) at 3587 cm 21 increases in comparison to that at 3481 cm 21, indicating an increasing number of weakly hydrogen bonded OH groups. In the region 3000–1800 cm 21 a continuous

Scheme 2.

absorption with band-like structure—maxima at 2500 and 1920 cm 21 —also strongly increases in the solution, proving that the intramolecular collective hydrogen bonds O–H…OyC–OH…O–H in the circular structure of lasalocid are slightly polarizable due to proton shifts [40] in liquid state (Scheme 2). A comparison of the spectra of lasalocid–Na 1 in solid state and in chloroform (Fig. 2(b)) reveal some important differences. The n (CyO) stretching vibration of the ketone group shifts from 1702 cm 21 in the solid to 1711 cm 21 in chloroform indicating that this carbonyl group is weakly hydrogen bonded in chloroform than in the solid state where it is a monomer. Similar shifts are observed in the region 1350– 1250 cm 21 where the n as and n s etheric stretching

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Fig. 3. Far infrared spectra of (…) sodium perchlorate, (- - - -) lasalocid and (—) the 2:2 complex of lasalocid with Na 1 ions.

vibrations occur. These results suggest that the lasalocid–Na 1 salt in chloroform occurs in the dimeric form as was previously proposed in the literature [10,11,16,36]. 3.3. FIR spectrum and Na 1 polarizability In Fig. 3 the far infrared spectra of lasalocid and its Na 1 salt are compared. For the sake of comparison also the spectrum of sodium perchlorate is given. In the latter the Na 1 ion motion band at about 195 cm 21 is observed. In the spectrum of the 2:2 complex of lasalocid with Na 1

ions this band vanishes completely and a continuous absorption in the region 150–50 cm 21 appears. This continuous absorption demonstrates that Na 1 cations fluctuate very fast in multiminima Na 1 potentials in the dimeric structure of the lasalocid–Na 1 salt as shown in Scheme 3, and that the Na 1 bonds in this complex are highly polarizable [41,42]. This large polarizability of the Na 1 bonds occur due to a high mobility of the Na 1 ions in the complex. This mobility is crucial for the function of the antibacterial agent. Knowing the properties of such cation complexes it should be possible in future to compose antibiotics

Scheme 3.

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