Solubility properties of the systems chromium salts-amino acids-water

Article abstract of DOI:10.1023/B:RUCO.0000043893.89877.8c

Solubility properties of the ternary systems of Cr(NO₃) ₃-His-H₂O, Cr(NO₃)₃-Met-H ₂O, and CrCl₃-His-H₂O (His - histidine, Met - methionine) have been investigated in t

Full text of DOI:10.1023/B:RUCO.0000043893.89877.8c

Russian Journal of Coordination Chemistry, Vol. 30, No. 10, 2004, pp. 698–702. From Koordinatsionnaya Khimiya, Vol. 30, No. 10, 2004, pp. 742–746.
Original English Text Copyright © 2004 by Chen San-Ping, Gao Sheng-Li, Shi Qi-Zhen.
Solubility Properties of the Systems Chromium
Salts–Amino Acids–Water*
Chen San-Ping, Gao Sheng-Li, and Shi Qi-Zhen
Shaanxi key laboratory of Physico-Inorganic Chemistry, Department of Chemistry, Northwest University,
Xi’an 710069, P.R. China
Received August 29, 2003
Abstract—Solubility properties of the ternary systems of Cr(NO3)3–His–H2O, Cr(NO3)3–Met–H2O, and
CrCl3–His–H2O (His—histidine, Met—methionine) have been investigated in the whole concentration range
by the phase equilibrium semimicromethod, and the corresponding phase diagrams have been constructed. It
was shown that the new complexes Cr(His)(NO3)3 · 3H2O, Cr(His)2(NO3)3 · 3H2O, Cr(His)3(NO3)3 · 3H2O,
Cr(His)Cl3 · H2O, Cr(His)2Cl3 · H2O, and Cr(His)3Cl3 · H2O are formed in the Cr(NO3)3/CrCl3–His–H2O sys-
tem, while Cr(Met)(NO3)3 · H2O and Cr(Met)2(NO3)3 · H2O complexes are formed in the system Cr(NO3)3–
Met–H2O. Under the guidance of the phase diagrams, the complexes were prepared and characterized by chem-
ical and elemental analysis, IR spectroscopy, and thermogravimetry data. The influences of metal cations,
anions and the structures of amino acids on the formation of complexes were discussed.
INTRODUCTION
Cr(Met)(NO3)3 · H2O (D) and Cr(Met)2(NO3)3 · H2O
(E), Cr(His)Cl3 · H2O (F), Cr(His)2Cl3 · H2O (G), and
Cr(His)3Cl3 · H2O (H), have been prepared and charac-
terized by chemical and elemental analysis, IR spec-
troscopy, and thermogravimetry (TG–DTG). The influ-
ences of metal cations, anions and the structures of
amino acids on the formation of complexes have been
discussed.
GTF, the indispensable cofactor of insulin, is a com-
plex by amino acid and niacin coordination to element-
chromium trace. Insulin does not maintain the normal
sugar metabolic unless GTF has affinity with its com-
plexity [1, 2]. It is not surprising that a good under-
standing of coordination behavior of chromium with
amino acid is basic to acquaintance with GTF. L-α-
Amino acids are structural units of protein, especially
for histidine and methionine that are essential to life
and have to be absorbed from foods as they are not syn-
thesized by an organism. So, there is considerable prac-
tical and fundamental importance to focus on the com-
plex of chromium and amino acid.
EXPERIMENTAL
Materials and equipment. CrCl3 · 6H2O, Cr(NO3)3 ·
6H2O (Beijing Shuanghuan Chemical Plant, A.R.) and
L-α-His, L-α-Met (Shanghai KandaAmmonia Factory,
B.R.) are recrystallized with the greater purity than
99.95%. The conductivity of the deionized water is
1.4 µ s cm–1. The thermostat temperature fluctuation is
0.05°C. ZD-2 type automatic potential titrator is of
Shanghai Leici Company manufacture. Elementary
analyses of the complexes were carried out with an
instrument of Vario EL III CHNOS (Germany). The IR
spectra of the compounds were obtained with the
BRUKER EQ UINOX-550 model infrared spectropho-
tometer (KBr pallets). TG and DTG data were deter-
mined by a Perkin-Elmer thermogravimetric analyzer.
All TG–DTG tests were performed under a dynamic
atmosphere of dry nitrogen at a flow rate of 60 ml min–1,
the heating rate was 10 K min–1 and sample masses
were about 1 mg.
As for the complexes of chromium with valine, leu-
cine, methionine, and phenylalanine, their preparation
and properties (IR, NMR, and magnetism) in the mix-
ture solvent of water and alcohol have been reported in
[3–9]. Cooper studied the structure, biological activity,
chromatography, IR spectra of the complexes of the
chromium and amino acid with the molar ratio of chro-
mium to amino acid of 1 : 2 and relationship of the
complexes with GTF [1]. However, the phase chemistry
related to chromium(III)–amino acid–H2O systems has
not been reported elsewhere. In this paper, the solubil-
ity of the ternary systems of Cr(NO3)3–His–H2O (I,
His—histidine), Cr(NO3)3–Met–H2O (II, Met—
methionine) and CrCl3–His–H2O (III) have been inves-
tigated and the phase diagrams of the systems have
been constructed. Based on the experimental results,
eight new solid complexes, Cr(His)(NO3)3 · 3H2O (A),
Cr(His)2(NO3)3 · 3H2O (B), Cr(His)3(NO3)3 · 3H2O (C),
Experimental methods. Cation Cr3+ was deter-
mined complexometrically with ammonium ferrous
sulfate. Its content was analyzed by the formalin
method. Anion Cl– was determined by the Fajans
*This article was submitted by the authors in English.
1070-3284/04/3010-0698 © 2004 åÄIä “Nauka /Interperiodica”

SOLUBILITY PROPERTIES OF THE SYSTEMS CHROMIUM SALTS–AMINO ACIDS–WATER
699
(a)
(b)
(c)
H2O
H2O
H2O
d
g
f
a
e
c
h
b
j
k
i
D
A
E
B
C
F
G
H
Cr(NO3)3
w(His)
His Cr(NO3)3
w(Met)
Met CrCl3
w(His)
His
Fig. 1. Solubility diagrams of the ternary systems: (a) I, (b) II, and (c) III. a, b, c, d, …j, k—the points to the congruent composition
of the phases; w—mass fraction.
method. Carbon, hydrogen, and nitrogen analyses were more, all of the complexes are congruently soluble
performed on the elemental analyzer.
compounds.
2. In the case of the Cr(NO3)3–Met–H2O system, the
Solubility properties of the systems were deter-
mined by the phase equilibrium semimicromethod
[10]. Distribution of system points was roughly esti-
mated for separate systems according to solubility of
salts and amino acids in water at 25°C. The material
samples were weighed with an accuracy of 0.0001 g,
sealed in small polyethylene tubes, and then were put in
a thermostat and stirred. Refractive index or other phys-
ical properties such as density and the chemical compo-
sition of these samples were determined at intervals.
When these data remained constant, the system was
assumed to be in the equilibrium state. The composi-
tions of liquid phase and wet-solid phase were analyzed
by the methods mentioned above. The solubility dia-
grams were obtained in accordance with the composi-
tion and refractive index of the saturated solution in the
systems.
solubility curves are composed of four branches, corre-
sponding to chromium salts, the complexes D, E, and
methionine, respectively. Like the A, B, C, F, G, and H
complexes, the D and E complexes are congruently sol-
uble compounds.
Characterization of the complexes. Under the
guidance of the phase diagrams, the A, B, C, D, E, F,
G, and H complexes were synthesized from water with
the excellent yield more than 95%, and were dried in
the desiccator containing P4O10. They are not hydro-
scopic and dissolve in water, but are insoluble in
organic solvents. The compositions of the complexes
are depicted in Table 3. The data from Table 3 indicate
that the compositions of the solid complexes are consis-
tent with those of the new phases existing in the phase
diagrams, which attests to the fact that the phase equi-
librium method is successful guidance to the prepara-
tion of the new complexes.
RESULTS AND DISCUSSION
The IR bands of main groups for the complexes and
ligands are summarized in Table 4, which was inter-
preted as follows:
Studies on phase equilibrium. The solubility prop-
erties of the ternary systems of Cr(NO3)3(CrCl3)–His–
H2O and Cr(NO3)3–Met–H2O have been investigated in
the whole concentration range by phase equilibrium
method at 25°C. Based on 27, 24, and 36 sets of phase
equilibrium data corresponding to Cr(NO3)3–His–H2O,
CrCl3–His–H2O and Cr(NO3)3–Met–H2O systems,
respectively, the phase diagrams have been constructed
and shown in the figure. The composition of the critical
points of three systems is depicted in Table 1 and the
description of equilibrium of solid phase is given in
Table 2.
1. For the complexes involving the histidine ligand,
the characteristic absorption peaks of amino and car-
boxyl groups have a great shift compared to those of the
free ligand. It indicates that nitrogen and oxygen atoms
of this ligand are coordinated to Cr3+ in a bidentate
fashion. In addition, the characteristic absorption peaks
of imidazole group in the complexes have a little shift,
which is attributable to the environment effect. This
confirms that nitrogen atom of the imidazole group
does not participate in coordination to chromium.
2. Like the complexes with the histidine ligand,
nitrogen and oxygen atoms of methionine ligand are
coordinated to Cr3+ in a bidentate fashion in these com-
The following conclusions could be drawn from the
phase diagrams:
1. For the systems Cr(NO3)3(CrCl3)–His–H2O, the plexes. Compared with that of the free ligand, the char-
solubility curves consist of five branches, correspond- acteristic absorption peak of methylsulfonyl group
ing to chromium salts, the complexes A, B, and C or F, shifting toward low wave lengths illustrates that sulfur
G, and H, as well as histidine, respectively. Further- atom of methylsulfonyl group is combined with chro-
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 30 No. 10 2004

700
CHEN SAN-PING et al.
Table 1. Results of phase equilibrium on the ternary systems I–III at 25°C
Composition
of liquid phase, mass %
Composition
of solid phase, mass %
Composition
of synthetic phase, mass %
Equilibrium
solid phase
System
*
*
*
L**
L**
L**
CrX3
CrX3
CrX3
I
51.03
52.28
39.58
25.10
12.22
Cr(NO3)3 · 7.5H2O
Cr(NO3)3 · 7.5H2O + A
A + B
11.10
28.99
38.88
41.10
4.87
57.66
41.98
31.50
21.42
11.48
33.82
44.45
50.87
55.70
41.53
29.04
19.00
11.32
31.11
42.13
48.24
B + C
C + His
His
II
51.03
49.29
36.11
23.84
Cr(NO3)3 · 7.5H2O
Cr(NO3)3 · 7.5H2O + D
D + E
14.03
30.94
35.12
3.31
57.96
45.12
28.04
9.43
33.59
49.61
52.61
39.59
25.43
12.33
31.77
40.66
E + Met
Met
III
52.02
47.03
28.90
21.92
10.77
59.50
54.32
36.00
27.35
9.00
CrCl3 · 6H2O
CrCl3 · 6H2O + F
F + G
17.98
43.18
51.00
57.00
4.78
14.95
47.01
59.82
73.79
49.82
33.02
23.00
10.20
17.01
47.00
54.30
61.00
G + H
H + His
His
* X—NO , Cl.
3
** L—His, Met.
Table 2. Comparison of the results of phase equilibrium for the ternary systems of MX3–L–H2O (M = Cr; X = NO3, Cl; L =
His, Met)
Complexes
Systems
variety
3
composition
M : L : H2O
properties
Cr(NO3)3–His–H2O
Cr(His)(NO3)3 · 3H2O
Cr(His)2(NO3)3 · 3H2O
Cr(His)3(NO3)3 · 3H2O
Cr(Met)(NO3)3 · H2O
Cr(Met)2(NO3)3 · H2O
Cr(His)Cl3 · H2O
1 : 1 : 3
1 : 2 : 3
1 : 3 : 3
1 : 1 : 2
1 : 2 : 2
1 : 1 : 1
1 : 2 : 1
1 : 3 : 1
Congruent dissolving
″
″
″
″
″
″
″
Cr(NO3)3–Met–H2O
CrCl3–His–H2O
2
3
Cr(His)2Cl3 · H2O
Cr(His)3Cl3 · H2O
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 30 No. 10 2004

SOLUBILITY PROPERTIES OF THE SYSTEMS CHROMIUM SALTS–AMINO ACIDS–WATER
701
Table 3. Elemental analyses data for the A–H complexes
Content (found/calcd), %
Complex
Cr
L*
X**
C
H
N
Cr(His)(NO3)3 · 3H2O
Cr(His)2(NO3)3 · 3H2O
Cr(His)3(NO3)3 · 3H2O
Cr(Met)(NO3)3 · H2O
Cr(Met)2(NO3)3 · H2O
Cr(His)Cl3 · H2O
11.68/11.63
8.54/8.63
34.58/34.69
51.31/51.49
61.20/61.39
35.14/35.23
52.41/52.10
46.79/46.80
63.68/63.72
72.57/72.52
16.18/16.11
23.97/23.93
28.60/28.54
14.38/14.19
20.98/20.98
21.68/21.74
29.68/29.62
34.00/33.68
3.43/3.38
4.07/4.02
4.44/4.39
3.68/3.57
4.63/4.58
3.31/3.34
4.19/4.14
4.60/4.55
18.85/18.87
20.87/20.92
22.16/22.19
13.17/13.24
12.20/12.23
12.81/12.67
17.26/17.27
19.47/19.64
6.92/6.86
12.43/12.28
9.14/9.08
15.71/15.69
10.77/10.68
8.13/8.10
32.11/32.08
21.47/21.85
16.11/16.57
Cr(His)2Cl3 · H2O
Cr(His)3Cl3 · H2O
* L = His, Met.
** X = Cl, NO .
3
Table 4. IR absorption data for main groups of free ligands and the A–H complexes (cm–1)
Compounds
L-α-His
3025
3124
3146
3034
3128
2921
2977
3147
3135
3127
2860
2100
2910
2910
2910
2273
2361
3023
3023
3023
1590
1620
1636
1636
1653
1624
1660
1625
1627
1641
1456
1510
1378
1385
1378
1446
1456
1443
1443
1441
1635
1582
1450
1508
1507
1508
1508
1449
1504
1502
1415
1410
1378
1350
1312
1384
1384
1397
1384
1363
1315
744
L-α-Met
1340
Cr(His)(NO3)3 · 3H2O
Cr(His)2(NO3)3 · 3H2O
Cr(His)3(NO3)3 · 3H2O
Cr(Met)(NO3)3 · H2O
Cr(Met)2(NO3)3 · H2O
Cr(His)Cl3 · H2O
Cr(His)2Cl3 · H2O
Cr(His)3Cl3 · H2O
3390
3390
3390
3421
3428
3423
3423
3423
823
1144
1144
1130
989
983
983
825
825
826
825
828
828
828
1140
1140
1144
1145
1143
980
978
978
mium in coordination bond. The structures of the amino those of water molecules being present in the com-
acids have influence on the coordination number of
chromium ion.
plexes.
Thermostability of the solid complexes is investi-
3. The difference between the characteristic IR
absorption of the main groups of A and B reveals that
the anions affect the coordination of Cr(NO3)3 and
CrCl3 with histidine.
gated by thermogravimetry. TG–DTG curves of the
compounds reflect that the experimental results of the
residual amount are in good agreement with the calcu-
lated results, and the intermediate and final products of
the thermal decompositions of the complexes are iden-
4. The characteristic IR absorption peaks in the
range of 3390–3423 and 823–828 cm–1 are assigned to tified via IR spectroscopy as well. It is thus assumed
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 30 No. 10 2004

702
CHEN SAN-PING et al.
Table 5. Thermoanalytical data for the A–H complexes
Complex
Step
Decomposition product Decomposition temperature, °C Residual rate (found/calcd), %
A
B
C
D
1
1
1
1
2
3
1
2
3
1
2
1
2
1
2
Cr2O3
51–331–400*
51–343–450
51–444–8000
51–204–300
300–390–460
460–544–800
51–158–200
200–230–450
460–480–600
51–416–494
494–534–798
51–448–500
500–542–800
51–450–521
521–568–800
16.87/16.99
12.83/12.62
31.54/31.42
91.26/91.49
50.39/50.71
16.34/16.19
93.94/93.71
41.79/41.58
13.34/13.28
53.58/53.60
47.80/47.77
91.55/91.52
32.46/32.54
75.85/75.92
24.48/24.67
Cr2O3
Cr2O3
Cr(Met)(NO3)3
Cr(NO3)3
Cr2O3
E
Cr(Met)2(NO3)3
Cr(NO3)3
Cr2O3
F
G
H
Cr(His)0.81Cl3
CrCl3
Cr(His)1.85Cl3
CrCl3
Cr(His)2.12Cl3
CrCl3
* The intermediate data are peak temperatures of DTG curves.
that the thermal decompositions of the complexes are
summarized in Table 5.
ACKNOWLEDGMENTS
We thank the National Science Foundation of China
for financial support.
It is interesting that the A, B, and C complexes are
directly decomposed to oxides without the intermediate
stage, and thermostability is in decreasing order of C,
B, and A. Apparently, water molecules in the com-
plexesA, B, and C cannot be regarded as crystallization
water. With respect to those of compounds H, G, and F,
the thermal decomposition processes proceeded in two
steps. When the complexes lose water, this process is
accompanied with loss of part of histidine firstly; in
these complexes, water molecules are not crystalliza-
tion water. Subsequently, the interminate products are
decomposed into CrCl3. The thermostability of the H,
G, and F complexes is in decreasing sequence for H, G,
and F. As for the D and E complexes, the thermal
decomposition processes occur in three steps. The first
step is the loss of water from the complexes, which
indicates that water in the complexes is crystallization
water. The following step is the loss of methionine, and
the chromium nitrate is produced. In the last stage, the
complexes are completely decomposed into chromium
hemitrioxide.
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