Monensin A benzyl ester and its complexes with monovalent metal cations studied by spectroscopic, mass spectrometry and semiempirical methods

Article abstract of DOI:10.1016/j.molstruc.2006.03.014

Monensin A benzyl ester (MON3) was synthesized and its ability to form complexes with Li⁺, Na⁺ and K⁺ cations was studied by ESI MS, ¹H and ¹³C NMR, FT-IR and PM5 semiempirical methods. MON3 has been

Full text of DOI:10.1016/j.molstruc.2006.03.014

Journal of Molecular Structure 797 (2006) 99–110
www.elsevier.com/locate/molstruc
Monensin A benzyl ester and its complexes with monovalent
metal cations studied by spectroscopic, mass spectrometry
and semiempirical methods
a
a
a
a,
*
´
Adam Huczynski , Dominik Michalak , Piotr Przybylski , Bogumil Brzezinski
,
b
Franz Bartl
a
Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
Institute of Medical Physics and Biophysics Charite´ – Universita¨tsmedizin Berlin Campus Charite´ Mitte, Ziegelstr. 5/9, 10117 Berlin, Germany
b
Received 10 February 2006; accepted 1 March 2006
Available online 24 April 2006
Abstract
Monensin A benzyl ester (MON3) was synthesized and its ability to form complexes with Li+, Na+ and K+ cations was studied
by ESI MS, 1H and 13C NMR, FT-IR and PM5 semiempirical methods. MON3 has been found to preferentially form complex
with Na+ cations. The formation of stable complexes of 1:1 stoichiometry up to cv = 70 V is indicated in the electrospray ionisation
mass spectra. With increasing cone voltage value, the fragmentation of the respective complexes is detected and is connected pri-
mary with the dehydration process. The structures of the complexes are stabilized by intramolecular hydrogen bonds in which
the OH groups are always involved. The structures of MON3 and its complexes with Li+, Na+ and K+ cations are visualized
and discussed in detail. It is shown that in the structure of MON3 the oxygen atom of this C@O ester group is involved in very
weak bifurcated hydrogen bonds with two hydroxyl groups. Within the complexes of MON3 with Li+ and K+ cations, this C@O
ester group is not hydrogen bonded, whereas in the structure of MON3 with Na+ it can also coordinate this cation. Such a struc-
ture is, however, not dominant in acetonitrile solution. It is demonstrated that the formation of a pseudo-crown ring structure
formed by MON3 prefers the complexation of Na+ cations.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Monensin A; Benzyl ester; Complexes; Monovalent cations; 1H NMR; 13C NMR; HETCOR; COSY; FT-IR; ESI mass spectrometry; PM5
calculations; Hydrogen bonds
1. Introduction
protein transport, and exhibits antibiotic, antimalarial,
and other important biological activities [15–24]. The
reason why monensin exhibits various pharmacological
and biological activities is its ability to disrupt the
Na+/K+ ion balance across the cell membranes, which
finally leads to the cell death.
Recently, we have studied the nature of complexes
between monensin A methyl ester and monovalent as
well as divalent cations, by various spectroscopic, mass
spectrometry and semiempirical methods. This ester
has been shown to have especially high affinity to
Na+ and Ca2+ cations [25,26]. As a continuation of
these studies we report here ESI MS, 1H and 13C
NMR, FT-IR as well as PM5 studies of the complexation
Monensin A is one of the most important polyether
antibiotic ionophores employed as a veterinary drug. It
is used as a growth-promoting agent and as a coccidio-
stat in beef cattle, sheep, chickens, and turkeys [1–5].
Monensin readily forms lipid-soluble pseudo-macrocyclic
complexes with mono- and divalent cations [6–14] and
regulates many cellular functions, including apoptosis.
It causes the collapse of sodium and potassium gradi-
ents at the plasma membrane, blocks intracellular
*
Corresponding author. Tel.: +48 61 829 1330; fax: +48 61 865 8008.
E-mail address: bbrzez@main.amu.edu.pl (B. Brzezinski).
0022-2860/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.03.014

100
A. Huczyn´ski et al. / Journal of Molecular Structure 797 (2006) 99–110
ability of benzyl ester of Monensin A with monovalent
cations such as the Li+, Na+ and K+ cations.
The structures of the Monensin A benzyl ester and its
complexes with monovalent cations are discussed in
detail.
2.2. Synthesis of MON3 complexes with monovalent cations
The 0.07 mol Lꢀ1 solutions of 1:1 complexes of MON3
with monovalent cations (Li+, Na+ and K+) were obtained
by adding equimolar amounts of MClO4 salt (M = Li, Na,
K) dissolved in acetonitrile to MON3 dissolved in acetoni-
trile. The solvent was evaporated under reduced pressure to
dryness and the oily residue was dissolved in an appropriate
volume of dry CH3CN or CD3CN.
2. Experimental
Monensin A sodium salt was purchased from Sigma
(90–95%). The perchlorates LiClO4, NaClO4 and KClO4
were commercial products of Sigma and were used with-
out any further purification. Because the salts were
hydrates, it was necessary to dehydrate them at several
(6–10 times) evaporation steps from a 1:5 mixture of ace-
tonitrile and absolute ethanol. The dehydration of the
perchlorates was followed by recording of their FT-IR
spectra in acetonitrile.
2.3. Mass spectrometry
The electrospray ionisation (ESI) mass spectra were
recorded on a Waters/Micromass (Manchester, UK) ZQ
mass spectrometer equipped with a Harvard Apparatus
syringe pump. All samples were prepared in acetonitrile.
The measurements were performed for the two types of
CD3CN and CH3CN spectral-grade solvents were
˚
stored over 3 A molecular sieves for several days. All
Table 1
manipulations with the substances were performed in a
carefully dried and CO2-free glove box.
The main peaks in the ESI mass spectra of the complexes of MON3 with
cations at various cone voltages
Cation
Li+
Cone voltage (V)
Main peaks m/z
2.1. Synthesis of monensin benzyl ester (MON3)
10
30
768
768
50
70
90
110
130
768
768
Monensin A sodium salt was dissolved in dichloro-
methane and stirred vigorously with a layer of aqueous
sulphuric acid (pH 1.5). The organic layer containing
MONA was washed with distilled water, and dichloro-
methane was evaporated under reduced pressure to
dryness.
768, 750
768, 750, 732, 485, 467, 445, 427
768, 750, 732, 485,467, 445, 427
Na+
10
30
784
784
A mixture of benzyl bromide (250 mg, 1.45 mmol),
MONA (500 mg, 0.75 mmol), and 1,8-diazacicyclo[5.4.0]
undec-7-en (DBU) (175 mg, 1.15 mmol) and 40 ml tolu-
ene was heated at 90 ꢁC for 5 h. After cooling, the pre-
cipitate DBU-hydrobromide (DBU Æ HBr) was filtered
and washed hexane. The filtrate and the washing were
combined and were evaporated under reduced pressure.
The residue was purified by chromatography on silica
gel (Fluka type 60) to give MON3 (510 mg, 89% yield)
as a colourless oil showing a tendency to form a glass
state. Elementary analysis: Calculated: C, 67.87%; H,
9.01%. Found: C, 67.85%; H, 9.04%.
50
70
90
110
130
784
784
784
784, 766
784, 766, 748, 507, 501, 489, 483, 479,
461, 443
K+
10
30
50
70
90
800
800
800
800
800, 782, 764
800, 782, 764
800, 782, 764
110
130
IV
OH
28
Me
27
Me
29
33
Me
Me
II
23
7
III
32
10 11
30-31
15
22
21
24
25
6
5
8
9
18
19
Me 14
H
O
OMe
Et
35
OH
XI
20
16
17
12 13
1
3
R
O
VIII
O
IX
2
4
O
VII
O
VI
O
I
O
V
26
H
H
H
H
OH
X
Me
Me
34
36
'
'
4
3
MONA R = H
MON3 R =
'
'
'
5
1
2
Scheme 1. The structures and atom numbering of MONA and MON3.

A. Huczyn´ski et al. / Journal of Molecular Structure 797 (2006) 99–110
101
samples being the solutions of MON3 (5 · 10ꢀ5 mol Lꢀ1
)
Li+, Na+ and K+ (5 · 10ꢀ5/3 mol Lꢀ1) taken together.
The samples were infused into the ESI source using a
Harvard pump at a flow rate of 20 ll minꢀ1. The ESI
with: (a) each of the cations Li+, Na+ and K+
(2.5 · 10ꢀ4 mol Lꢀ1) taken separately and (b) the cations
Fig. 1. ESI mass spectra of a 1:1 mixture of MON3 with Na+ at various cone voltages (cv).

102
A. Huczyn´ski et al. / Journal of Molecular Structure 797 (2006) 99–110
+
O
O
O
O
M
O
I
M=Li+ m/z=732
H
O
M=Na+ m/z=748
M=K+
m/z=764
O
+
-H2O
O
O
Ph
O
O
H
or
-H2O
+
O
O
M
O
O
O
H
O
H
O
O
O
M
H
M=Li+ m/z=750
M=Na+ m/z=766
M=K+ m/z=782
O
O
J
M=Li+ m/z=732
M=Na+ m/z=748
M=K+ m/z=764
O
O
O
Ph
O
-H2O
O
O
Ph
+
+
+
O
O
O
O
H
O
O
-H2O
-H2O
O
O
O
O
O
O
M
O
H
O
H
M
O
M
O
O
O
O
H
O
H
O
A
B
C
M=Li+ m/z=732
M=Na+ m/z=748
M=K+ m/z=764
O
O
O
M=Li+ m/z=768
M=Na+ m/z=784
M=K+ m/z=800
M=Li+ m/z=750
M=Na+ m/z=766
M=K+ m/z=782
O
O
Ph
O
O
Ph
O
O
Ph
+
+
O
O
O
O
-H2O
O
O
O
O
M
O
H
O
M
O
H
M=Li+ m/z=485
M=Na+ m/z=501
M=Li+ m/z=467
M=Na+ m/z=483
E
D
+
+
O
O
O
O
-H2O
O
O
O
O
M
O
H
O
M
O
H
M=Li+ m/z=427
M=Na+ m/z=443
M=Li+ m/z=445
M=Na+ m/z=461
G
F
Scheme 2. The proposed fragmentation pathways of MON3 complexes with monovalent cations.
Fig. 2. ESI mass spectrum of a 1:1:1:3 mixture of cation perchlorates LiClO4, NaClO4 and KClO4 and MON3 (cv = 30).

A. Huczyn´ski et al. / Journal of Molecular Structure 797 (2006) 99–110
103
source potentials were: capillary 3 kV, lens 0.5 kV, extrac-
and dessolvation gas at flow rates of 100 and 300 L hꢀ1
,
tor 4 V. The standard ESI mass spectra were recorded at
the cone voltages 10, 30, 50, 70, 90, 110 and 130 V. The
source temperature was 120 ꢁC and the dessolvation tem-
perature was 300 ꢁC. Nitrogen was used as the nebulizing
respectively. Mass spectra were acquired in the positive
ion detection mode with unit mass resolution at step size
of 1 m/z unit. The mass range for ESI experiments was
from m/z = 300 to m/z = 1000.
Table 2
1H NMR chemical shifts (ppm) of MON3 and its complexes in CD3CN
No. atom
Chemical shift (ppm)
Differences (D) between chemical shifts
(ppm)
MON3
MON3:Li+
MON3:Na+
MON3:K+
D1
D2
D3
1
–
–
–
–
–
–
–
2
3
4
5
6
7
8A
8B
9
2.68
3.55
1.98
4.04
1.74
3.66
1.62
1.99
–
2.67
3.60
1.99
4.16
1.69
3.96
1.56
2.05
–
2.73
3.57
2.01
3.95
1.67
3.87
1.62
2.01
–
2.70
3.56
1.92
3.98
1.68
3.76
1.64
1.99
–
ꢀ0.01
0.05
0.01
0.12
ꢀ0.05
0.30
ꢀ0.06
0.06
–
0.05
0.02
0.02
0.03
ꢀ0.09
ꢀ0.07
0.21
0.00
0.02
–
0.01
ꢀ0.06
ꢀ0.06
ꢀ0.06
0.10
0.02
0.00
–
10A
10B
11A
11B
12
1.87
1.93
1.72
1.92
–
1.85
2.05
1.85
1.89
–
1.79
2.00
1.82
1.90
–
1.76
1.86
1.80
1.86
–
ꢀ0.02
0.12
0.13
ꢀ0.03
–
ꢀ0.08
0.07
0.10
ꢀ0.02
–
ꢀ0.11
ꢀ0.07
0.08
ꢀ0.06
–
13
3.64
1.57
1.76
1.58
2.08
–
3.67
1.42
1.89
1.52
2.16
–
3.62
1.46
1.89
1.45
2.25
–
3.54
1.46
1.76
1.48
2.18
–
0.03
ꢀ0.15
0.13
ꢀ0.06
0.08
–
ꢀ0.02
ꢀ0.11
0.13
ꢀ0.13
0.17
–
ꢀ0.10
ꢀ0.11
0.00
ꢀ0.10
0.10
–
14A
14B
15A
15B
16
17
18
19A
19B
20
21
22
23A
23B
24
3.86
2.27
1.48
2.15
4.19
3.59
1.33
1.24
1.41
1.69
–
4.37
2.43
1.55
2.33
4.40
3.81
1.39
1.35
1.52
1.90
–
3.95
2.31
1.63
2.15
4.41
3.72
1.49
1.32
1.46
1.68
–
3.93
2.33
1.50
2.19
4.34
3.65
1.33
1.31
1.46
1.68
–
0.51
0.16
0.07
0.18
0.21
0.22
0.06
0.11
0.11
0.21
–
0.09
0.04
0.15
0.00
0.22
0.13
0.16
0.08
0.05
ꢀ0.01
–
0.07
0.06
0.02
0.04
0.15
0.06
0.00
0.07
0.05
ꢀ0.01
–
25
26A
26B
27
28
29
30A
30B
31
32
33
3.33
3.33
0.81
0.86
0.94
1.55
1.55
0.90
1.30
0.86
0.96
3.23
1.15
5.13
–
3.43
3.45
0.83
0.82
0.95
1.54
1.70
0.96
1.54
0.88
0.96
3.21
1.17
5.14
–
3.46
3.65
0.80
0.83
0.90
1.48
1.72
0.90
1.45
0.86
0.98
3.23
1.18
5.15
–
3.39
3.56
0.80
0.85
0.93
1.50
1.65
0.91
1.41
0.86
0.95
3.23
1.14
5.15
–
0.10
0.12
0.02
ꢀ0.04
0.01
ꢀ0.01
0.15
0.06
0.24
0.02
0.00
ꢀ0.02
0.02
0.01
–
0.13
0.32
ꢀ0.01
ꢀ0.03
ꢀ0.04
ꢀ0.07
0.17
0.00
0.15
0.00
0.02
0.00
0.03
0.02
–
0.06
0.23
ꢀ0.01
ꢀ0.01
ꢀ0.01
ꢀ0.05
0.10
0.01
0.11
0.00
ꢀ0.01
0.00
ꢀ0.01
0.02
–
34
35
36
10
20
30, 40, 50
7.39
2.79
4.15
3.81
7.39
2.95
5.81
4.57
7.39
4.03
3.80
4.36
7.39
3.16
3.62
4.06
0.00
0.16
1.66
0.76
0.00
1.24
ꢀ0.35
0.55
0.00
0.37
ꢀ0.53
0.25
O
XIH
OIV
OXH
H
D1 = dMON3:Li+ ꢀ dMON3, D2 = dMON3:Na+ ꢀ dMON3, D3 = dMON3:K+ ꢀ dMON3
.

104
A. Huczyn´ski et al. / Journal of Molecular Structure 797 (2006) 99–110
2.4. Spectroscopic measurements
The NMR spectra of MON3 and its 1:1 complexes
(0.07 mol Lꢀ1) with LiClO4, NaClO4 and KClO4 were
recorded in CD3CN solutions using a Varian Gemini 300
MHz spectrometer. All spectra were locked to deuterium
resonance of CD3CN.
The FT-IR spectra of MON3 and its 1:1 complexes
(0.07 mol Lꢀ1) with LiClO4, NaClO4, and KClO4 were
recorded in the mid-infrared region in acetonitrile solu-
tions using a Bruker IFS 113v spectrometer.
1
The H NMR measurements in CD3CN were carried
A cell with Si windows and wedge-shaped layers was
used to avoid interferences (mean layer thickness
170 lm). The spectra were taken with an IFS 113v FT-IR
spectrophotometer (Bruker, Karlsruhe) equipped with a
out at the operating frequency 300.075 MHz; flip angle,
pw = 45ꢁ; spectral width, sw = 4500 Hz; acquisition time,
at = 2.0 s; relaxation delay, d1 = 1.0 s; T = 293.0 K and
using TMS as the internal standard. No window function
or zero filling was used. Digital resolution was 0.2 Hz per
point. The error of chemical shift value was 0.01 ppm.
13C NMR spectra were recorded at the operating
frequency 75.454 MHz; pw = 60ꢁ; sw = 19,000 Hz; at =
DTGS detector; resolution 2 cmꢀ1
, NSS = 125. The
Happ–Genzel apodization function was used.
All manipulations with the substances were performed
in a carefully dried and CO2-free glove box.
Table 3
13C NMR chemical shifts (ppm) of MON3 and its complexes in CD3CN
No. atom
Chemical shift (ppm)
Differences (D) between chemical shifts
(ppm)
MON3
MON3:Li+
MON3:Na+
MON3:K+
D1
0.24
D2
0.88
D3
0.28
1
2
3
4
5
6
7
8
9
175.91
41.54
82.24
37.87
68.60
37.07
71.96
35.11
108.56
39.78
32.60
87.00
84.48
28.45
32.06
88.15
86.49
35.96
34.85
77.99
77.42
34.02
37.67
34.71
97.94
67.46
16.46
17.83
16.15
30.14
8.29
176.15
41.62
82.53
37.45
69.17
37.45
70.06
34.17
107.99
39.61
32.07
87.03
82.51
27.53
32.07
87.92
84.91
34.03
33.71
79.44
74.58
33.25
36.37
34.53
99.63
66.40
15.97
17.39
16.28
30.13
8.41
176.79
41.74
81.91
37.76
68.76
37.16
70.74
34.59
108.32
39.84
33.35
86.63
82.21
27.24
30.70
86.72
85.44
35.01
33.77
76.89
75.60
32.17
35.75
36.22
98.79
67.07
16.12
16.66
14.40
30.16
8.11
176.19
41.68
82.71
38.16
68.16
37.62
70.96
34.45
108.05
39.88
33.92
86.43
83.58
28.21
32.00
86.82
85.31
35.56
33.35
77.98
76.11
33.13
36.88
35.56
98.67
67.02
16.53
17.45
15.96
30.39
8.47
0.08
0.29
0.20
ꢀ0.33
ꢀ0.11
0.16
0.14
0.47
0.29
ꢀ0.42
0.57
ꢀ0.44
0.38
0.09
0.55
ꢀ1.90
ꢀ0.94
ꢀ0.57
ꢀ0.17
ꢀ0.53
0.03
ꢀ1.97
ꢀ0.92
0.01
ꢀ0.23
ꢀ1.58
ꢀ1.93
ꢀ1.14
1.45
ꢀ2.84
ꢀ0.77
ꢀ1.30
ꢀ0.18
1.69
ꢀ1.22
ꢀ0.52
ꢀ0.24
0.06
ꢀ1.00
ꢀ0.66
ꢀ0.51
0.10
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
10
0.75
1.32
ꢀ0.37
ꢀ2.27
ꢀ1.21
ꢀ1.36
ꢀ1.43
ꢀ1.05
ꢀ0.95
ꢀ1.08
ꢀ1.10
ꢀ1.82
ꢀ1.85
ꢀ1.92
1.51
ꢀ0.57
ꢀ0.90
ꢀ0.24
ꢀ0.06
ꢀ1.33
ꢀ1.18
ꢀ0.40
ꢀ1.50
ꢀ0.01
ꢀ1.31
ꢀ0.89
ꢀ0.79
0.85
0.85
0.73
ꢀ1.06
ꢀ0.49
ꢀ0.44
0.13
ꢀ0.39
ꢀ0.34
ꢀ1.17
ꢀ1.75
0.02
ꢀ0.44
0.07
ꢀ0.38
ꢀ0.19
0.25
0.18
1.87
ꢀ0.01
0.12
ꢀ0.18
26.34
11.28
12.50
58.46
12.25
66.96
137.41
128.95
129.40
129.02
28.50
10.85
13.13
58.71
11.28
67.09
137.31
129.04
129.44
129.06
28.28
10.92
13.20
58.81
12.15
67.23
137.21
129.05
129.44
129.09
28.21
11.02
12.78
58.75
11.68
67.06
137.30
129.01
129.42
129.07
2.16
1.94
ꢀ0.43
ꢀ0.36
0.70
0.35
ꢀ0.26
0.63
0.28
0.25
0.29
ꢀ0.97
ꢀ0.10
ꢀ0.57
0.13
0.27
0.10
20
ꢀ0.10
0.09
0.04
ꢀ0.20
0.10
0.04
ꢀ0.11
0.06
0.02
30
40
50
0.04
0.07
0.05
D1 = dMON3:Li+ ꢀ dMON3, D2 = dMON3:Na+ ꢀ dMON3, D3 = dMON3:K+ ꢀ dMON3
.

A. Huczyn´ski et al. / Journal of Molecular Structure 797 (2006) 99–110
105
1.8 s; d1 = 1.0 s; T = 293.0 K and TMS as the internal stan-
dard. Line broadening parameters were 0.5 or 1 Hz. The
error of chemical shift value was 0.01 ppm.
shown in Fig. 1. According to the data from Table 1,
MON3 forms only 1:1 complexes with the cations studied
which are very stable up to cv = 70 V. With increasing
cone voltage the fragmentation of the respective complexes
occurs. The proposed pathways of the fragmentation are
shown in Scheme 2. As follows from the m/z peaks in
Table 1 and Scheme 2, the first step of the fragmentation
is always the loss of one water molecule as a result of
the abstraction of the OIVH or OXH hydroxyl groups
yielding the B or H cations, respectively. Further fragmen-
tation of cation B, in the case of the complexes of MON3
with Li+ and Na+ cations, is realized in two directions with
the formation of other types of complexes (C, D, E, F and
finally G). Cation C is formed from cation B by the abstrac-
tion of the second water molecule. The formation of the D
and E fragmentary complexes occurs by the abstraction of
the part of the molecule including ester group. Cations F
and G can be formed from cations D and E by the loss of
one C3H4 group. Additionally cation G can be formed by
the abstraction of one water molecule from cation F. For
the complex of MON3 with K+ cation no abstraction of
the part of the molecule including ester group was detected.
The ESI spectrum of the 1:1:1:3 mixture of Li+, Na+,
K+ cations and MON3 measured at cv = 30 V is shown
in Fig. 2. This figure shows only three characteristic signals
at m/z = 768, m/z = 784 and m/z = 800 assigned to the 1:1
complexes of MON3 with Li+, Na+ and K+ cations,
respectively. The intensity of the MON3–Na+ complex sig-
nal is dominant indicating clearly that MON3 preferential-
ly forms complexes with Na+ cations. The intensity of the
signals of the MON3–Li+ and MON3–K+ complexes is
1
The H and 13C NMR signals were assigned indepen-
dently for each species using one- or two-dimensional
(COSY, HETCOR) spectra.
2.5. PM5 calculations
PM5 semiempirical calculations were performed using
the WinMopac 2003 program [27]. In all cases full geome-
try optimisation of MON3 and its complexes was carried
out without any symmetry constraints [28,29].
2.6. Elementary analysis
The elementary analysis was carried out on Perkin-El-
mer CHN 240.
3. Results and discussion
The structure of MON3 together with the atom number-
ing is shown in Scheme 1.
3.1. Electrospray mass spectrometry measurements
The main m/z peaks observed in the ESI mass spectra of
MON3 with the Li+, Na+ and K+ cations used separately
at various cone voltage values are summarized in Table 1.
Exemplary ESI mass spectra of the 1:1 complex of MON3
with Na+ cation measured at various cone voltages are
OXH
OIVH
OXIH
d
c
OXH
OXIH
OIVH
OXH
b
a
OIVH
OXIH
OIVH
OXH
OXIH
5.8
5.6
5.4
5.2
5.0
4.8
4.6
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
2.8
Fig. 3. 1H NMR in region of OH proton signals of: (a) MON3, (b) MON3–Li+, (c) MON3–Na+, and (d) MON3–K+.

106
A. Huczyn´ski et al. / Journal of Molecular Structure 797 (2006) 99–110
significantly lower and comparable demonstrating that
MON3 shows only low affinity to Li+ and K+ cations.
A comparison of the 13C NMR chemical shifts in the
spectra of complexes with those observed in the spectrum
of MON3 suggests not only that the oxygen atoms take part
in the cation coordination but also indicates more complex
conformation changes evoked by the complexation process.
For instance the D value of the C20 atom for the complex of
1
3.2. H and 13C NMR measurements
The 1H and 13C NMR data of MON3 in CD3CN and its
1:1 complexes with Li+, Na+ and K+ cations all in CD3CN
are collected in Tables 2 and 3, respectively. Unfortunately,
in contrast to these complexes, the respective MON3 com-
plexes with RbClO4 and CsClO4 are almost insoluble in
acetonitrile. The 1H and 13C signals were assigned indepen-
dently using one- and two-dimensional (COSY, HETCOR)
spectra as well as by the addition of CD3OD to the probe.
In the 1H NMR spectra of MON3 and its 1:1 complexes
with Li+, Na+ and K+ cations (Table 2), the signals of pro-
tons of OH groups are separated. The chemical shift of
these signals is strongly dependent on the kind of the cation
complexed and this fact is very well illustrated in Fig. 3. In
the spectrum of MON3 the OH proton signals observed at
2.79, 3.81, and 4.15 ppm are assigned to OXIH, OXH and
OIVH groups, respectively. In the spectra of complexes of
MON3 with the Li+, Na+ and K+ cations these signals
are shifted in different directions depending on the kind
of the cation. This demonstrates that depending on the cat-
ion, the respective hydroxyl groups form different intramo-
lecular hydrogen bonds characteristic of different structures
of the respective complexes. The conclusion about the dif-
ferent structures can also be deduced from the signal of the
protons of the methylene benzyl group (10) which arise as a
doublet of doublets due to the spin–spin geminal couplings.
The appearance of such couplings indicates the location of
both protons in different fields and formation of typical AB
spin–spin coupling structure.
a
100
50
0
4000 3500 3000 2500 2000 1500 1000
500
100
50
0
b
3324
3368
3506
3443
3500
3456
3800
3700
3600
3500
3400
3300
3200
c
100
50
0
1705
1733
1725
Fig. 3 shows that the most significantly shifted signal of
the proton from hydroxyl groups is found in the spectrum
of MON3 with Li+ cation at about 5.81 ppm. This signal
is assigned to the OIVH proton, which is involved in the
strongest intramolecular hydrogen bond in comparison with
all other hydroxyl groups. It is interesting to note that in the
spectra of MON3 complexes with Na+ and K+ cations, the
signals of the OIVH protons are shifted strongly towards
lower ppm values demonstrating the weakness of the partic-
ipation of the OIVH protons in other types of hydrogen
bonds due to the formation of different complex structures.
1800
1775
1750
1700
-1
1675
1650
Wavenumber [cm
]
Fig. 5. FT-IR spectra of: (–––) MON3, (– –) MON3–Li+, (ꢁ ꢁ ꢁ) MON3–
Na+, and (–ÆÆ–) MON3–K+ in the ranges of: (a) 4000–400 cmꢀ1; (b) m(OH)
and (c) m(C@O) stretching vibrations.
Table 4
Heat of formation (kcal molꢀ1) of MON3 and its complexes with the
cations without (A) and with (B) the engagement of carbonyl group in
coordination process calculated by PM5 method (WinMopac 2003)
Complex
HOF (kcal molꢀ1
)
DHOF (kcal molꢀ1
)
100
50
0
MON3
ꢀ603.11
ꢀ474.71
ꢀ633.89
ꢀ474.71
ꢀ553.92
ꢀ456.58
ꢀ696.84
ꢀ456.58
ꢀ687.60
ꢀ447.62
ꢀ605.39
ꢀ447.62
ꢀ496.34
–
MON3 þ LiþuncomplexedðAÞ
MON3 þ LiþcomplexedðAÞ
MON3 þ LiþuncomplexedðBÞ
MON3 þ LiþcomplexedðBÞ
MON3 þ NaþuncomplexedðAÞ
MON3 þ NaþcomplexedðAÞ
MON3 þ NaþuncomplexedðBÞ
MON3 þ NaþcomplexedðBÞ
MON3 þ KþuncomplexedðAÞ
MON3 þ KþcomplexedðAÞ
MON3 þ KþuncomplexedðBÞ
MON3 þ KþcomplexedðBÞ
ꢀ159.18
ꢀ79.21
ꢀ240.26
ꢀ231.02
ꢀ157.77
ꢀ48.72
4000
3500
3000
2500
2000
1500
-1
1000
500
Wavenumber [cm
]
Fig. 4. FT-IR spectra of: water-free MON3 (–––) and its 1:3 mixture with
water (– –).
DHOF = HOFMON3+Li+complexed ꢀ HOFMON3+Li+uncomplexed
.

A. Huczyn´ski et al. / Journal of Molecular Structure 797 (2006) 99–110
107
MON3 with Li+ cation is positive, for the complex of
MON3 with Na+ cation it is negative and for the MON3–
K+ complex it is close to zero. This result indicates that
the OVIII oxygen atom is involved in the coordination only
in the complex of MON3 with Na+ cation.
ison demonstrates that in contrast to the methyl ester of
monensin A, MON3 forms no strong bonded hydrates
and in consequence also no proton channels.
Most characteristic, in FT-IR spectrum of water-free
MON3, are the bands assigned to the m(OH) vibrations
of the OXH and OXIH groups at 3506 cmꢀ1, and of OIVH
group at ca. 3324 cmꢀ1 as well as the band assigned to the
m(C@O) vibrations at 1733 cmꢀ1. The spectra of the
MON3 complexes with Li+, Na+ and K+ cations are
similar to that of MON3 (Fig. 5). According to the
NMR data the most significant changes in these spectra
3.3. FT-IR studies
In Fig. 4 the FT-IR spectrum of water-free MON3 is
compared with the corresponding spectrum of its 1:3 mix-
ture with water, both in acetonitrile solution. This compar-
Table 5
˚
The interatomic distances (A) and partial charges for O atoms of MON3 coordinating metal cations in complex structures calculated by PM5 method
(WinMopac 2003)
˚
Complex with monovalent
cation
Monovalent cation
partial charge
Coordinating
atom
Coordinating atom
partial charge
Distance (A)
coordinating atom fi cation
MON3: Li+ (A)
MON3: Na+ (A)
MON3: Na+ (B)
MON3: K+ (A)
+0.462
+0.307
+0.311
+0.483
OI
OII
–
–
–
–
–
–
1.98
–
2.14
2.04
–
2.08
–
OIII
OIV–H
OV
OVI
OVII
OVIII
OIX
OX
ꢀ0.384
–
ꢀ0.392
ꢀ0.378
–
ꢀ0.389
–
–
OXI
–
OI
OII
–
–
–
–
–
–
2.28
–
2.29
2.23
2.25
2.30
–
OIII
OIV–H
OV
OVI
OVII
OVIII
OIX
OX
ꢀ0.405
–
ꢀ0.407
ꢀ0.399
ꢀ0.401
ꢀ0.411
–
ꢀ0.399
–
OXI
2.24
OI
OII
OIII
–
2.31
–
2.31
–
2.32
2.31
2.29
2.30
–
ꢀ0.504
–
OIV–H
ꢀ0.409
–
OV
OVI
OVII
OVIII
OIX
OX
ꢀ0.410
ꢀ0.409
ꢀ0.407
ꢀ0.409
–
ꢀ0.406
–
OXI
2.27
OI
–
OII
–
–
OIII
OIV–H
OV
OVI
OVII
OVIII
OIX
OX
–
–
ꢀ0.422
ꢀ0.423
ꢀ0.412
ꢀ0.430
–
2.79
2.80
2.64
2.85
–
ꢀ0.428
2.83
–
–
OXI
ꢀ0.419
2.73
(A) Structure of MON3 complex in which the C1@O group is not involved in coordination of metal cation; (B) structure of MON3 complex in which the
C1@O group is involved in coordination of metal cation.

108
A. Huczyn´ski et al. / Journal of Molecular Structure 797 (2006) 99–110
should be observed in the regions of the m(OH) and
m(C@O) stretching vibrations. In the spectrum of the
MON3–Li+ complex a broad band in the region 3550–
3250 cmꢀ1 is observed indicating the involvement of all
OH groups in slightly stronger intramolecular hydrogen
bonds relative to those of the structure of MON3. This
the weakening of the hydrogen bond of OIVH hydroxyl
group within the structure of this complex. This observa-
tion is also in full agreement with the H NMR data.
1
The position of the band assigned to the m(C@O) vibra-
tions at 1733 cmꢀ1, both in the spectra of MON3 and its
complexes with monovalent metal cations, is almost
unchanged (Fig. 5c). This fact demonstrates that the oxy-
gen atoms from the C@O ester groups are not engaged in
the complexation process of the cations.
The position of the band assigned to the m(C@O) vibra-
tions at 1733 cmꢀ1, in the spectra of MON3 and its
complexes with Li+ and K+ cations, is almost unchanged
(Fig. 5b). This fact demonstrates that the oxygen atom
from the C@O ester group is not engaged in the
1
observation is consistent with the H NMR data (Table 2
and Fig. 3). In the spectrum of the MON3–Na+ complex
only one broad band with a maximum at ca. 3456 cmꢀ1
is observed, which demonstrates that the hydrogen bonds
formed by the OXH and OXIH groups become slightly
stronger and that of OIVH hydroxyl group slightly weaker.
In the spectrum of the MON3–K+ complex also only one
band with maximum at 3500 cmꢀ1 is observed indicating
Table 6
˚
The length (A) and angle (ꢁ) of the hydrogen bond for MON3 complexes calculated by PM5 method (WinMopac 2003)
˚
Compound
O-atom
Hydrogen bond length (A) [hydrogen bond angle (ꢁ)]
OIVH
OXH
OXIH
MON3
OII
OIX
–
2.89[132.5]
–
2.95[133.6]
–
2.68[157.2]
MON3:Li+ (A)
OIII
OI
OIX
–
–
2.73[154.2]
–
–
2.65[143.6]
–
2.45[127.1]
MON3:Na+ (A)
MON3:Na+ (B)
MON3:K+ (A)
OIV
OIII
–
–
2.67[148.1]
–
–
2.59[158.9]
OI
OIII
–
–
2.52[133.1]
–
–
2.76[152.9]
OI
OIII
–
–
2.56[123.1]
–
–
2.55[123.8]
(A) Structure of MON3 complex in which the C1@O group is not involved in coordination of metal cation; (B) structure of MON3 complex in which the
C1@O group is involved in coordination of metal cation.
OV
OVI
OVIII OVII
OVII
OVI
Li+
OVIII
OIV
OIX
OIV
O V
OIII
OXI
OIX
OII
OX
OI
OI
OX
OXI
OII
OIII
Scheme 3. The structure of MON3 calculated by PM5 method (WinMopac
2003).
Scheme 4. The structure of MON3–Li+ complex calculated by PM5
method (WinMopac 2003).

A. Huczyn´ski et al. / Journal of Molecular Structure 797 (2006) 99–110
109
complexation process of these cations. In contrast to this
spectrum, the one of MON3 with Na+ cation shows,
besides the band at ca. 1733 cmꢀ1 a new band at
1705 cmꢀ1, assigned to the m(C@O) vibrations of the car-
bonyl groups interacting with the Na+ cations. The inten-
sity of this new band is significantly weaker than that at
1733 cmꢀ1 indicating that such a structure is not dominant.
longer than those in the A type structure. This result is
especially well consistent with the FT-IR data of this
complex.
Other oxygen atoms can be involved in the coordination
process depending on the type of cation. The coordination
distances and the number of coordinating oxygen atoms
suggest that all the cations studied can undergo fast fluctu-
ations within the structures of the complexes as it was dem-
onstrated in the complexes of crown ethers with
monovalent cations [30].
3.4. PM5 calculations
Taking into account the experimental findings, the heats
of formation (HOF) of the structures of MON3 and its
complexes with Li+, Na+ and K+ cations were calculated
and collected in Table 4. As follows from the results, the
most stable complexes in the gas phase (under the experi-
mental conditions similar to those of ESI measurements)
are formed with Na+ ꢂ Li+ ꢃ K+ cations. This result is
in good agreement with the ESI data discussed above.
The DHOF values show that the structures in which the
carbonyl group is involved in the complexation process
are energetically less favourable than those in which the
carbonyl group is not involved in the complexation pro-
cess. Only for the MON3–Na+ complexes the DHOF val-
ues are of comparable order and this result is in good
agreement with the ESI data and with the FT-IR
observations.
The interatomic distances between the oxygen atoms of
MON3 and the cations and the partial charges at these
atoms are given in Table 5. Analysis of these values shows
that some oxygen atoms such as OIV, OVI, OVII and OIX are
always involved in coordination, irrespective of the kind of
the cation, whereas the OI, OII and OIII play no role in this
coordination process within the structures of A type, which
is in agreement with the 13C NMR data. If the OII oxygen
atom from the carbonyl group participates in the coordina-
tion of the cation, B type structure in the MON3–Na+
complex, the calculated coordination distance is slightly
The lengths and angles of the hydrogen bonds in which
the OH groups are engaged are collected in Table 6. The
calculated structure of the MON3 indicates that OII oxygen
atom from the C@O ester group is engaged in the bifurcat-
ed intramolecular hydrogen bonds with two OXH and
OXIH hydroxyl groups (Scheme 3). With the formation
of MON3 complexes with cations these hydrogen bonds
OVI
OV
OVII
OIV
OXI
OVIII K+
OIX
OIII
OII
OI
OX
Scheme 6. The structure of MON3–K+ complex calculated by PM5
method (WinMopac 2003).
a
b
OVII
OVI
OV
OVII
OV
OVIII
OVI
OVIII
Na+
OIV
OIV
Na+
OXI
OIII
OXI
OIII
OIX
OIX
OII
OI
OII
OX
OI
OX
Scheme 5. The structures of two types of MON3–Na+ complexes: (a) without the engagement of the C1@O carbonyl group in coordination of the cation –
type A, (b) with the engagement of the C1@O carbonyl group in coordination of the cation – type B calculated by PM5 method (WinMopac 2003).

110
A. Huczyn´ski et al. / Journal of Molecular Structure 797 (2006) 99–110
are always broken. For this reason the 13C NMR signals of
the C1 carbon atoms are shifted toward higher ppm values
(Table 3).
In all the calculated structures the OIVHꢁ ꢁ ꢁOIX intramo-
lecular hydrogen bond in the MON3–Li+ complex was the
strongest.
The calculated structures of the MON3 and its complex-
es with the cations studied are visualized in Schemes 3–6.
The hydrogen bonds and the coordination bonds are
marked by dots. A comparison of all the calculated struc-
tures indicates that only for the MON3 complex with
Na+ cation the molecule forms a pseudo-crown ether struc-
ture and therefore, the affinity of MON3 to Na+ cation is
higher than that to the other cations.
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