Experimental and theoretical study on the complexation of the strontium cation with nonactin

Article abstract of DOI:10.1016/j.molstruc.2012.07.058

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Sr²⁺(aq)+2A ^-(aq)+1(nb)?1·Sr²⁺(nb)+2A^-(nb) taking place in the two-phase water-nitrobenzene system (A^- = picrate, 1 = nonactin; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K_ex(1·Sr²⁺, 2A^-) = -2.0 ± 0.1. Further, the stability constant of the 1·Sr²⁺ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log β_nb (1·Sr²⁺) = 7.1 ± 0.1. Finally, by using DFT calculations, the most probable structure of the cationic complex species 1·Sr²⁺ was derived. In the resulting complex having S₄ symmetry, the central cation Sr ²⁺ is bound by eight relatively strong bond interactions to eight oxygen atoms of the parent nonactin ligand 1.

Full text of DOI:10.1016/j.molstruc.2012.07.058

Journal of Molecular Structure 1032 (2013) 155–158
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Experimental and theoretical study on the complexation of the strontium
cation with nonactin
Emanuel Makrlík , Petr Toman , Petr Va nˇ ura c
a,
⇑
b
a
Faculty of Environmental Sciences, Czech University of Life Sciences, Prague, Kam y´ cká 129, 165 21 Prague 6, Czech Republic
Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovského sq. 2, 162 06 Prague 6, Czech Republic
Department of Analytical Chemistry, Institute of Chemical Technology, Prague, Technická 5, 166 28 Prague 6, Czech Republic
b
c
h i g h l i g h t s
"
"
"
Stability constant of the Sr2+. nonactin complex (abbrev. 1ꢀSr2+) in nitrobenzene was determined as log bnb (1ꢀSr2+) = 7.1 ± 0.1.
2
+
By using DFT calculations, the structure of the 1ꢀSr complex having S
4
symmetry was predicted.
2
+
In the resulting complex, the ‘‘central’’ cation Sr is bound by eight bond interactions to eight oxygens of the nonactin ligand 1.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 May 2012
Received in revised form 31 July 2012
Accepted 31 July 2012
Available online 10 August 2012
From extraction experiments and c-activity measurements, the extraction constant corresponding to the
2þ
ꢁ
2þ
ꢁ
equilibrium Sr ðaqÞ þ 2A ðaqÞ þ 1ðnbÞ () 1 ꢀ Sr ðnbÞ þ 2A ðnbÞ taking place in the two-phase water-
ꢁ
nitrobenzene system (A = picrate, 1 = nonactin; aq = aqueous phase, nb = nitrobenzene phase) was
2+
ꢁ
2+
evaluated as log Kex(1ꢀSr , 2A ) = ꢁ2.0 ± 0.1. Further, the stability constant of the 1ꢀSr complex in nitro-
2+
benzene saturated with water was calculated for a temperature of 25 °C: log bnb (1ꢀSr ) = 7.1 ± 0.1.
2+
Finally, by using DFT calculations, the most probable structure of the cationic complex species 1ꢀSr
Keywords:
Strontium
Nonactin
Complexation
2+
was derived. In the resulting complex having S
relatively strong bond interactions to eight oxygen atoms of the parent nonactin ligand 1.
Ó 2012 Elsevier B.V. All rights reserved.
4
symmetry, the ‘‘central’’ cation Sr is bound by eight
Extraction and stability constants
DFT calculations
Complex structure
1
. Introduction
stants have been determined from polarographic kinetic currents.
The macrotetrolides gave diffusion controlled currents. The stabil-
ity of their complexes increases with the degree of substitution
from nonactin to trinactin [11]. The very large differences in stabil-
ities of the ammonium and the other alkali metal complexes of
nonactin are the cause of the high ammonium selectivity of the
nonactin-based ion-selective electrode [12]. Recently, experimen-
tal evidences for a valinomycin – proton complex and for some
unusual divalent metal cation complexes of valinomycin have been
reported in detail [13,14]. Furthermore, the theoretical structures
Macrotetrolides (nonactin, monactin, dinactin, trinactin, and
tetranactin) [1–6], isolated from a variety of Streptomycetes species,
are cyclotetralactones derived from nonactic acid and homononac-
tic acid as the building units [4] of ionophoretic character [7]. With
the exception of nonactin, they exhibit, in addition to antibacterial
and antifungal activity, also remarkable acaricidal, insecticidal,
coccidiostatic, and anthelminthic effects [5,8,9]. Besides, the mac-
rotetrolide antibiotics are very potent uncouplers of oxidative
phosphorylation in mitochondria [10].
+
+
þ
2+
of the valinomycin complexes with Li , K , NH , and Mg have
4
The stability and selectivity of complex formation of natural
macrocyclic ionophores (valinomycin, nonactin, monactin, dinac-
tin, and trinactin) with alkali metal ions and monovalent thallium
cation have been studied by polarography [11]. With valinomycin,
both stability constants and homogenous dissociation rate con-
been solved as well [15–18].
In the current work, the stability constant of the complex of
nonactin (abbrev. 1; see Scheme 1) with the strontium cation (ab-
2
+
brev. 1ꢀSr ) was determined in nitrobenzene saturated with water.
Besides, applying DFT calculations, the most probable structure of
2
+
the 1ꢀSr cationic complex species was predicted on the basis of
the thorough conformational analysis (i.e., different mutual posi-
⇑
Corresponding author. Tel.: +420 376 594 672.
E-mail address: makrlik@centrum.cz (E. Makrlík).
2
+
tions of the ligand 1 and the Sr cation were considered during
0
022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.07.058

1
56
E. Makrlík et al. / Journal of Molecular Structure 1032 (2013) 155–158
Sr ðaqÞ þ 2A ðaqÞ þ 1ðnbÞ () 1 ꢀ Sr2þðnbÞ
2
þ
ꢁ
ꢁ
2þ
ꢁ
þ 2A ðnbÞ;
K
exð1 ꢀ Sr ; 2A Þ
ð3Þ
ð4Þ
to which the equilibrium extraction constant
2
þ
ꢁ 2
½
1 ꢀ Sr ꢃ ½A ꢃ
2þ
ꢁ
nb
nb
K
exð1 ꢀ Sr ; 2A Þ ¼
2þ
ꢁ 2
½
Sr ꢃ ½A ꢃ ½1ꢃ
aq
aq
nb
corresponds. It is necessary to emphasize that 1 is a considerably
Scheme 1. Structural formula of nonactin (abbrev. 1).
hydrophobic ligand, practically present in the nitrobenzene phase
2
+
only, where this ligand forms – with the Sr cation – the relatively
2
+
stable complex 1ꢀSr , as given below.
the geometry optimization) and the respective vibrational fre-
quency calculations.
Applying the conditions of electroneutrality in the organic and
aqueous phases of the system under study, the mass balances of
1
2
and SrA at equal volumes of the nitrobenzene and aqueous
phases, as well as the measured equilibrium distribution ratio of
2
. Experimental
2
+
2+
strontium, DSr = [1ꢀSr ]nb/[Sr ]aq, combined with Eq. (4), we gain
2+
ꢁ
the final expression for the extraction constant Kex(1ꢀSr , 2A ):
Nonactin (purum, P98%; abbrev. 1; see Scheme 1) was pur-
ꢀ
ꢁ
chased from Aldrich. The other chemicals used (Lachema, Brno,
Czech Republic) were of reagent grade purity. A solution of stron-
tium picrate (abbrev. SrA
stoichiometric amount of picric acid in an aqueous solution of
Sr(OH)
D
þ DSr
Sr
2
þ
ꢁ
3
Sr
in;nb
1
in;aq
SrA2
K
exð1 ꢀ Sr ; 2A Þ ¼ D = C
ꢁ 1
C
ð5Þ
2
) in water was prepared by dissolving
in;aq
SrA
where C
2
is the initial concentration of SrA in the aqueous phase
and C1 denotes the initial concentration of 1 in the organic phase
2
85
2+
in;nb
2
.
The radionuclide
Sr
was supplied by DuPont,
Belgium.The extraction experiments were carried out in 10 mL
polypropylene test-tubes with polypropylene stoppers: 2 mL of
of the system under consideration.
In this study, from the extraction experiments and
measurements (see Experimental) by means of Eq. (5), the follow-
c-activity
-4
-4
an aqueous solution of SrA
amounts of Sr were added to 2 mL of a nitrobenzene solution
2
(1 ꢂ 10 to 3 ꢂ 10 M) and micro
8
5
2+
2+
ꢁ
ing value of the constant Kex(1ꢀSr , 2A ) was determined: log
-2
2+
ꢁ
of 1, the initial concentration of which varied from 1 ꢂ 10 to
K
ex(1ꢀSr , 2A ) = ꢁ2.0 ± 0.1 (see Table 1).
-
2
3
ꢂ 10 M (in all experiments, the initial concentration of 1 in
Moreover, with respect to previous results [22,23], for the
in;nb
1
2+
ꢁ
2+
ꢁ
nitrobenzene, C
tion of SrA
, was always higher than the initial concentra-
extraction constants Kex(Sr , 2A ) and Kex(1ꢀSr , 2A ) defined
in;aq
2+
in water, CSrA2 ). The test-tubes filled with the solutions
above, as well as for the stability constant of the complex 1ꢀSr
2
2+
were shaken for 2 h at 25 ± 1 °C, using a laboratory shaker. Then
the phases were separated by centrifugation. Afterwards, 1 mL
in nitrobenzene saturated with water, denoted by bnb (1ꢀSr ),
one gets
samples were taken from each phase and their
measured by means of a well-type NaI(Tl) scintillation detector
connected to a -analyzer NK 350 (Gamma, Budapest, Hungary).
c-activities were
log bnb ð1 ꢀ Sr2þÞ ¼ log Kexð1 ꢀ Sr2þ; 2A Þ ꢁ log KexðSr2þ; 2A Þ
ꢁ
ꢁ
ð6Þ
2
+
ꢁ
2+
ꢁ
c
Using the constants log K (Sr , 2A ) and log K (1ꢀSr , 2A ) gi-
ex
ex
The equilibrium distribution ratios of strontium, DSr, were
ven above, and employing Eq. (6), we obtain the stability constant
2
+
determined as the ratios of the corresponding measured radioac-
of the 1ꢀSr complex in water-saturated nitrobenzene at 25 °C as
8
5
2+
2+
nb
tivities of Sr in the nitrobenzene and aqueous samples.
log b (1ꢀSr ) = 7.1 ± 0.1. In this context it should be noted that
2
+
the stability constant of the complex species 2ꢀSr , where 2 de-
notes valinomycin (see Scheme 2), in nitrobenzene saturated with
3
. Results and discussion
2
+
water is log bnb (2ꢀSr ) = 5.4 [24]. This means that in the men-
2
+
tioned nitrobenzene medium, the stability of the 1ꢀSr complex
3.1. Extraction experiments
under study is somewhat higher than that of the cationic complex
2
+
species 2ꢀSr (2 = valinomycin).
Regarding the results of previous papers [19–21], the two-phase
ꢁ
water-SrA
2
(A = picrate) – nitrobenzene extraction system can be
3.2. DFT calculations
described by the following equilibrium
2
þ
ꢁ
2þ
ꢁ
exðSr2þ; 2A Þ ð1Þ
ꢁ
Sr ðaqÞ þ 2A ðaqÞ () Sr ðnbÞ þ 2A ðnbÞ;
K
The theoretical calculations were carried out at the density
functional level of theory (DFT, B3LYP) [25,26], using the Gaussian
3 suite of programs [27], analogously as in our previous papers
28–35]. The LanL2DZ basis set was used in the same way as in
our recent article [35], studying interaction of the cesium cations
with an electroneutral ligand, and the optimizations were uncon-
strained. In order to increase the numerical accuracy and to reduce
with the corresponding extraction constant Kex(Sr2+, 2A ); aq and
nb denote the presence of the species in the aqueous and nitroben-
zene phases, respectively. For the constant Kex(Sr , 2A ) one can
ꢁ
0
[
2+
ꢁ
write [19,21]
2
þ
ꢁ
i
i
log KexðSr ; 2A Þ ¼ log K 2þ þ 2 log K
where K 2þ and KA are the individual extraction constants for Sr2+
ꢁ
ð2Þ
Sr
A
i
i
Sr
ꢁ
Table 1
and A , respectively, in the water–nitrobenzene system [19,21].
2+
ꢁ
Experimental data concerning determination of log Kex(1ꢀSr , 2A ) on the basis of Eq.
i
i
A
ꢁ
Knowing the values log K 2þ ¼ ꢁ10:7 [21] and log K
ꢁ
¼ 0:8 (A =
Sr
(5).
2
+
ꢁ
picrate) [19], the extraction constant Kex(Sr , 2A ) was simply
in;aq
in;nb
1
D
Sr
log Kex(1ꢀSr , 2A )
2+
ꢁ
2+
ꢁ
C
(M)
C
(M)
calculated from Eq. (2) as log Kex(Sr , 2A ) = ꢁ9.1.
SrA
2
Previous results [22,23] indicated that the two-phase water-
ꢁ4
ꢁ4
ꢁ2
ꢁ2
ꢁ2
ꢁ2
ꢁ2
1
.0 ꢂ 10
1.0 ꢂ 10
1.5 ꢂ 10
2.0 ꢂ 10
2.5 ꢂ 10
3.0 ꢂ 10
0.050
0.055
0.062
0.065
0.059
ꢁ1.9
ꢁ2.0
ꢁ1.9
ꢁ2.0
ꢁ2.2
ꢁ
SrA
2
(A = picrate)-nitrobenzene-1 (nonactin) extraction system,
chosen for determination of the stability constant of the complex
1.5 ꢂ 10
ꢁ
4
4
4
2
.0 ꢂ 10
ꢁ
2
+
2.5 ꢂ 10
1
ꢀSr in nitrobenzene saturated with water (see Experimental),
3
.0 ꢂ 10ꢁ
can be characterized by the main chemical equilibrium

E. Makrlík et al. / Journal of Molecular Structure 1032 (2013) 155–158
157
Scheme 2. Structural formula of valinomycin (abbrev. 2).
2+
Fig. 2. Two projections of the DFT optimized structure of the 1ꢀSr complex
B3LYP/LanL2DZ): (a) side view and (b) top view.
(
4
S . These findings were confirmed by the respective vibrational fre-
quency calculations.
Although a possible influence of a polar solvent on the detailed
2
+
structures of 1 and the 1ꢀSr complex species could be imagined,
our theoretical calculations in similar cases, performed in an anal-
ogous way, showed very good agreement of experiment with the-
ory [28–35].
In the model calculations, we optimized the molecular geome-
tries of the parent nonactin ligand 1 and its complex with Sr2+
.
4
The optimized structure of the free ligand 1 with S symmetry is
illustrated in Fig. 1.
Fig. 1. Two projections of the DFT optimized structure of free ligand 1 (B3LYP/
LanL2DZ): (a) side view and (b) top view.
4
In Fig. 2, the S symetric structure obtained by the full DFT opti-
2
+
mization of the 1ꢀSr complex having a tennis-ball-seam confor-
mation is depicted, together with the lengths of the
2+
oscillations during the molecular geometry optimization, two-
electron integrals and their derivatives were calculated by using
the pruned (99,590) integration grid, having 99 radial shells and
corresponding bonds (in Å; 1 Å = 0.1 nm). In the 1ꢀSr cationic
complex species, which is most energetically favoured, the ‘‘cen-
2
+
tral’’ cation Sr is bound by eight relatively strong bond interac-
tions to four carbonyl oxygen atoms from ester groups (2.56,
2.56, 2.56, and 2.56 Å) and to four oxygens from tetrahydrofuran
rings (2.81, 2.81, 2.81, and 2.81 Å) of the parent nonactin ligand
1 (see Fig 2.).
5
90 angular points per shell, which was requested by means of
the Gaussian 03 keyword ‘‘Int = UltraFine’’. Further, it was found
that the most probable structures of the free ligand 1 and the con-
2
+
sidered complex 1ꢀSr have 4-fold rotation-reflectional symmetry

1
58
E. Makrlík et al. / Journal of Molecular Structure 1032 (2013) 155–158
2
+
Finally, the interaction energy, E(int), of the 1ꢀSr complex (cal-
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[
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to be ꢁ1125.5 kJ/mol, which confirms the formation of the consid-
(
[
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(
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2
+
resulting complex 1ꢀSr in the two-phase water-nitrobenzene
[
[
2+
extraction system, the strength of the considered 1ꢀSr cationic
complex species in nitrobenzene saturated with water was charac-
2
+
terized quantitatively by the stability constant, log bnb(1ꢀSr ) =
[
[
7
.1 ± 0.1 (for a temperature of 25 °C). By using theoretical DFT
2+
calculations, the structural details of the 1ꢀSr complex, such as
2
+
position of the Sr cation with regard to the parent nonactin
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Acknowledgments
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 Sustain-
able 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 Meta Centrum (Project LM 2010005), as well as at the Insti-
tute of Physics (computer Luna/Apollo), Academy of Sciences of the
Czech Republic, is gratefully acknowledged.
[
[
[
[
[
[
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2012.
7.058.
[
[
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