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Studies on the composition and stability of uranyl, vanadyl and titanous complexes with some aminoacids
464
ELECTROANALYTICALCHEMISTRYAND INTERFACIALELECTROCHEMISTRY Elsevier SequoiaS.A., Lausanne- Printed in The Netherlands
SHORT COMMUNICATIONS
Studies on the composition and stability of uranyl, vanadyl and titanous complexes with some aminoacids A number of papers on the metal complexes of amino acids I -4 have appeared in the literature in recent years. These studies are mainly concerned with the determination of the stability constant of a complex by various electrometric methods, viz. potentiometry, pH-metry, polarography, etc. References to studies on the complexes of Cu, Zn, Fe, Ni, Co, Mn, Cd and other transition metals are available but no attempt has yet been made to investigate possible complex formation between VO 2+, UO22+ and Ti 3+ ions with amino acids. The present communication deals with the behaviour of these ions towards some amino acids. Formation of 1 : 1 complexes has been indicated by conductometric titrations, and the stability constants of the complexes have been computed from the results of pH-metric titrations. Experimental Amino acids such as glycine,/%alanine, DL-c~-alanine, L-asparagine, DL-serine, L-leucine, DL-valine and L-proline (B.D.H. biologically pure) were used for the experiments, and their solutions (0.01 M) were prepared in doubly-distilled water. Uranyl sulphate (B.D.H. AnalaR), and vanadyl sulphate (B.D.H.) were employed and solutions of these salts were analysed gravimetrically as the metal oxides 5'6. An aqueous solution of titanium(III) chloride was prepared by dissolving crystals 7 of TiC13 • 6 H20 in air-free doubly-distilled water, and the solution standardized 8. Fresh solutions were always prepared before use and kept covered with a layer of kerosene oil or toluene throughout the investigations to avoid oxidation. 'Carbonate-free KOH solution was used for pH-metric titrations. It was stored in a Pyrex bottle fitted with a KOH tube for protection against atmospheric C02. The solution was standardized by titrating with standard oxalic acid solution, and the strength was checked periodically before carrying out pH-metric titrations. The conductometric titrations were performed using a Philips conductivity bridge model PR 9500/90 and a dip type conductivity cell (cell constant 1.48). The pH-metric titrations were carried out with a direct reading EIL pH-meter model 23A using glass and calomel electrodes. All the titrations were carried out in a specially designed cell, with provision for adding metal salt solutions from a burette, to a stirred oxygen-free system. Results and discussion The composition of the vanadyl, uranyl and titanous complexes with various amino acids was determined conductometrically. The conductometric titrations were reversible. In all cases a ratio of 1 : 1 (metal : amino acid) was established. Typical curves are given in Fig. 1. The pH-metric titrations were performed in triplicate for each amino acid. The titrations were carried out in the order: (a) amino acid (0.01 M), J. Electroanal. Chem., 24 (1970)464-467
465
SHORT COMMUNICATIONS o4
L
L
L
{.. 3
7 2 o
113 6
12 -
1
13
11 ~5 JE E
10 -
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11
9
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4
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4
2
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2
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1 0
0
1
2
4
3
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Volume of rnetoL s Q l t / m [
Fig. 1. Reverse conductometric titrations: (1) M/15 TiC13 added to 40 ml of M/300 DL-serine, (2) M/12 UOzSO 4 added to 40 ml of M/240 DL-c~-alanine, (3) M/15 VOSO¢ added to 40 ml of M/300 DL-valine.
1.8[C
A
B
1.4 n
1,2 1,0 0.8 0,6 0,4
!
-3.0
!
-4.0 -3.0
-5.0 -4.0 -tog
!
-6.0 5.0 [Sc]
-6.0
Curve A, C Curve B
Fig. 2. Formation curves: (A) Dk-valine-VOSO¢ complex, (B) DL-~-alanine-UO2SO¢ complex, (C) Dkserine-TiC13 complex.
J. Electroanal. Chem., 24 (1970) 464467
466
SHORT COMMUNICATIONS
u
b
,u L
Z
b
lO
lO 9 9 8
~
a 7 7 6 6 5 5 4 4 3
For c u r v e s
3
,
2
3
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~
~
~
5
u af n A,B,C
For curves A', B', C' For c u r v e s A, 13,C
Volume of 0.1N K O H / m l Fig. 3. pH-metric titrations: (A, B, C) DL-serine-TiC13, (A', B', C') DL-c~-alanine-UO2SO4, (A", B", C") DL-valine-VOSO4. TABLE 1 VALUES OF LOG (KJmol 21-2) FOR VANADYL~URANYL AND TITANOUSAMINO ACIDS COMPLEXES OBTAINED BY TWO METHODS
Amino acids
L-Asparagine fl-Alanine DL-c~-Alanine Glycine L-Leucine L-Proline DL-Serine DL-Valine
Vanadyl sulphate
Uranyl sulphate
Titanous chloride
Theoretical
Graphical
Theoretical
Graphical
Theoretical
Graphical
6.95 9.77 8.75 x x 9.08 10.33 7.54 8.65
6.90 9.80 8.70 x x 9.10 10.30 7.50 8.65
6.88 9.92 9.00 8.62 8.61 10.46 ' 6.86 8.59
6.85 9.90 9.00 8.65 8.60 10.45 6.90 8.60
7.25 9.72 8.53 8.52 8.55 10.08 7.60 8.12
7.20 9.70 8.50 8.50 8.50 10.05 7.60 8.20
(b) metal salt (0.005 M) and (c) metal salt and amino acid (total concentration 0.005 M and 0.01 M, respectively), employing 0.1 N K O H as titrant. The pH-titration curves show appreciable shifts, indicating the formation of complexes with the amino acid (Fig. 3). The complex formation constant K s was evaluated by the method modified by Albert 4. The values of log Ks at ~ = 1; where ~ is the average number of molecules of J. Electroanal. Chem., 24 (1970) 464-467
SHORT COMMUNICATIONS
467
amino acid bound by one atom of the metal ion, were calculated from the values of - l o g [Sc] obtained by plotting ~ vs. -log [Sc] (Fig. 2). [Sc] is the concentration of the free amino acid. The values of log K s for various amino acids evaluated from the formation curves, and those calculated are given in Table 1. The values of the overall stability constants log K s obtained from the formation curves (Fig. 2), are in good agreement with those calculated. Uranyl sulphate, vanadyl sulphate and titanous chloride form 1 : 1 complexes with various amino acids (L-asparagine, fi-alanine, DL-c~-alanine, glycine, L-leucine, L-proline, L-serine and DL-valine). In the vanadyl and titanous complexes the values of log Ks vary from L-asparagine (6.95, 7.25) to L-proline (10.33, 10.08), whereas in uranyl complexes the variation is from DL-serine (6.86) to L-proline (10.46). There does not seem to be any correlation between the nature of the aminoacids and the K s values although, with a few exceptions, the latter decreases with increase in chain length of carbon atoms. The real nature of bonding in these complexes is a matter of speculation as no complex could be isolated in a sufficiently pure form.
Acknowledyement Thanks are due to the Vice Chancellor, A.M.U. Aligarh for granting a Research Fellowschip to one of us (O.F.).
Department of Chemistry, Aligarh Muslim University, Aligarh (India)
Omar Farooq A. U. Malik N. Ahmad* S. M. F. Rahman
D. J. PERKIN,Biochem. J., 51 (1952) 487. J. B. GILBERT, O. CLYDE AND J. Z. HEARON, J. Am. Chem. Soc., 77 (1955) 2599. A. A. KHAN AND W. U. MALIK, J. Indian Chem. Sot., 40 (1963) 565. A. ALBERT, Biochem. J., 47 (1950) 531. A. C. CUMMING AND S. A. KAY, Quantitative Chemical Analysis, Oliver and Boyd, London, IX edn., 1945, p. 367. 6 N. H. FURMAN (Edl), Standard Methods o f Chemical Analysis, VoL 1, D. van Nostrand, New York, VI edn., 1962, p. 1189. 7 J. W. MELLOR, Inorganic and Theoretical Chemistry, Vol. VII, Longmans, London, 1963, p. 74. 8 N. H. FURMAN (Ed.), Standard Methods of Chemical Analysis, Vol. 1, D. van Nostrand, New York, VI edn., 1962, p. 984. 1 2 3 4 5
Received July 3rd, 1969
* Present address : Department of Chemistry, Louisiana State University, Baton Rouge, La. 70803, U.S.A.
J. Electroanal. Chem., 24 (1970) 464-467
ELECTROANALYTICALCHEMISTRYAND INTERFACIALELECTROCHEMISTRY Elsevier SequoiaS.A., Lausanne- Printed in The Netherlands
SHORT COMMUNICATIONS
Studies on the composition and stability of uranyl, vanadyl and titanous complexes with some aminoacids A number of papers on the metal complexes of amino acids I -4 have appeared in the literature in recent years. These studies are mainly concerned with the determination of the stability constant of a complex by various electrometric methods, viz. potentiometry, pH-metry, polarography, etc. References to studies on the complexes of Cu, Zn, Fe, Ni, Co, Mn, Cd and other transition metals are available but no attempt has yet been made to investigate possible complex formation between VO 2+, UO22+ and Ti 3+ ions with amino acids. The present communication deals with the behaviour of these ions towards some amino acids. Formation of 1 : 1 complexes has been indicated by conductometric titrations, and the stability constants of the complexes have been computed from the results of pH-metric titrations. Experimental Amino acids such as glycine,/%alanine, DL-c~-alanine, L-asparagine, DL-serine, L-leucine, DL-valine and L-proline (B.D.H. biologically pure) were used for the experiments, and their solutions (0.01 M) were prepared in doubly-distilled water. Uranyl sulphate (B.D.H. AnalaR), and vanadyl sulphate (B.D.H.) were employed and solutions of these salts were analysed gravimetrically as the metal oxides 5'6. An aqueous solution of titanium(III) chloride was prepared by dissolving crystals 7 of TiC13 • 6 H20 in air-free doubly-distilled water, and the solution standardized 8. Fresh solutions were always prepared before use and kept covered with a layer of kerosene oil or toluene throughout the investigations to avoid oxidation. 'Carbonate-free KOH solution was used for pH-metric titrations. It was stored in a Pyrex bottle fitted with a KOH tube for protection against atmospheric C02. The solution was standardized by titrating with standard oxalic acid solution, and the strength was checked periodically before carrying out pH-metric titrations. The conductometric titrations were performed using a Philips conductivity bridge model PR 9500/90 and a dip type conductivity cell (cell constant 1.48). The pH-metric titrations were carried out with a direct reading EIL pH-meter model 23A using glass and calomel electrodes. All the titrations were carried out in a specially designed cell, with provision for adding metal salt solutions from a burette, to a stirred oxygen-free system. Results and discussion The composition of the vanadyl, uranyl and titanous complexes with various amino acids was determined conductometrically. The conductometric titrations were reversible. In all cases a ratio of 1 : 1 (metal : amino acid) was established. Typical curves are given in Fig. 1. The pH-metric titrations were performed in triplicate for each amino acid. The titrations were carried out in the order: (a) amino acid (0.01 M), J. Electroanal. Chem., 24 (1970)464-467
465
SHORT COMMUNICATIONS o4
L
L
L
{.. 3
7 2 o
113 6
12 -
1
13
11 ~5 JE E
10 -
04
8
11
9
7 E
s3
6 e
5
o
2
1
4
5
3-
4
2
3
1-
2
C-
1 0
0
1
2
4
3
5
Volume of rnetoL s Q l t / m [
Fig. 1. Reverse conductometric titrations: (1) M/15 TiC13 added to 40 ml of M/300 DL-serine, (2) M/12 UOzSO 4 added to 40 ml of M/240 DL-c~-alanine, (3) M/15 VOSO¢ added to 40 ml of M/300 DL-valine.
1.8[C
A
B
1.4 n
1,2 1,0 0.8 0,6 0,4
!
-3.0
!
-4.0 -3.0
-5.0 -4.0 -tog
!
-6.0 5.0 [Sc]
-6.0
Curve A, C Curve B
Fig. 2. Formation curves: (A) Dk-valine-VOSO¢ complex, (B) DL-~-alanine-UO2SO¢ complex, (C) Dkserine-TiC13 complex.
J. Electroanal. Chem., 24 (1970) 464467
466
SHORT COMMUNICATIONS
u
b
,u L
Z
b
lO
lO 9 9 8
~
a 7 7 6 6 5 5 4 4 3
For c u r v e s
3
,
2
3
4
~
~
~
5
u af n A,B,C
For curves A', B', C' For c u r v e s A, 13,C
Volume of 0.1N K O H / m l Fig. 3. pH-metric titrations: (A, B, C) DL-serine-TiC13, (A', B', C') DL-c~-alanine-UO2SO4, (A", B", C") DL-valine-VOSO4. TABLE 1 VALUES OF LOG (KJmol 21-2) FOR VANADYL~URANYL AND TITANOUSAMINO ACIDS COMPLEXES OBTAINED BY TWO METHODS
Amino acids
L-Asparagine fl-Alanine DL-c~-Alanine Glycine L-Leucine L-Proline DL-Serine DL-Valine
Vanadyl sulphate
Uranyl sulphate
Titanous chloride
Theoretical
Graphical
Theoretical
Graphical
Theoretical
Graphical
6.95 9.77 8.75 x x 9.08 10.33 7.54 8.65
6.90 9.80 8.70 x x 9.10 10.30 7.50 8.65
6.88 9.92 9.00 8.62 8.61 10.46 ' 6.86 8.59
6.85 9.90 9.00 8.65 8.60 10.45 6.90 8.60
7.25 9.72 8.53 8.52 8.55 10.08 7.60 8.12
7.20 9.70 8.50 8.50 8.50 10.05 7.60 8.20
(b) metal salt (0.005 M) and (c) metal salt and amino acid (total concentration 0.005 M and 0.01 M, respectively), employing 0.1 N K O H as titrant. The pH-titration curves show appreciable shifts, indicating the formation of complexes with the amino acid (Fig. 3). The complex formation constant K s was evaluated by the method modified by Albert 4. The values of log Ks at ~ = 1; where ~ is the average number of molecules of J. Electroanal. Chem., 24 (1970) 464-467
SHORT COMMUNICATIONS
467
amino acid bound by one atom of the metal ion, were calculated from the values of - l o g [Sc] obtained by plotting ~ vs. -log [Sc] (Fig. 2). [Sc] is the concentration of the free amino acid. The values of log K s for various amino acids evaluated from the formation curves, and those calculated are given in Table 1. The values of the overall stability constants log K s obtained from the formation curves (Fig. 2), are in good agreement with those calculated. Uranyl sulphate, vanadyl sulphate and titanous chloride form 1 : 1 complexes with various amino acids (L-asparagine, fi-alanine, DL-c~-alanine, glycine, L-leucine, L-proline, L-serine and DL-valine). In the vanadyl and titanous complexes the values of log Ks vary from L-asparagine (6.95, 7.25) to L-proline (10.33, 10.08), whereas in uranyl complexes the variation is from DL-serine (6.86) to L-proline (10.46). There does not seem to be any correlation between the nature of the aminoacids and the K s values although, with a few exceptions, the latter decreases with increase in chain length of carbon atoms. The real nature of bonding in these complexes is a matter of speculation as no complex could be isolated in a sufficiently pure form.
Acknowledyement Thanks are due to the Vice Chancellor, A.M.U. Aligarh for granting a Research Fellowschip to one of us (O.F.).
Department of Chemistry, Aligarh Muslim University, Aligarh (India)
Omar Farooq A. U. Malik N. Ahmad* S. M. F. Rahman
D. J. PERKIN,Biochem. J., 51 (1952) 487. J. B. GILBERT, O. CLYDE AND J. Z. HEARON, J. Am. Chem. Soc., 77 (1955) 2599. A. A. KHAN AND W. U. MALIK, J. Indian Chem. Sot., 40 (1963) 565. A. ALBERT, Biochem. J., 47 (1950) 531. A. C. CUMMING AND S. A. KAY, Quantitative Chemical Analysis, Oliver and Boyd, London, IX edn., 1945, p. 367. 6 N. H. FURMAN (Edl), Standard Methods o f Chemical Analysis, VoL 1, D. van Nostrand, New York, VI edn., 1962, p. 1189. 7 J. W. MELLOR, Inorganic and Theoretical Chemistry, Vol. VII, Longmans, London, 1963, p. 74. 8 N. H. FURMAN (Ed.), Standard Methods of Chemical Analysis, Vol. 1, D. van Nostrand, New York, VI edn., 1962, p. 984. 1 2 3 4 5
Received July 3rd, 1969
* Present address : Department of Chemistry, Louisiana State University, Baton Rouge, La. 70803, U.S.A.
J. Electroanal. Chem., 24 (1970) 464-467