SHORT COMMUNICATIONS 411

When potassium ferrocyanide is titrated with zinc ions, K2Zn3[Fe(CN)6]2 is formed only slowly and hence only one end-point is observed corresponding to a combining ratio of 2 : 1. In highly concentrated solutions of K4Fe(CN)6 the formation of K2Zna[Fe(CN)6]2 is rapid and hence a 1.5 : 1 ratio is obtained.

Chemical Laboratories, Department of Chemistry, Aligarh Muslim University, Aligarh, U.P. (India)

M. Qureshi J. P. Rawat

T. Kaur

1 I. M. KOLTHOFF AND P. J. ELVING, Treatise on Analytical Chemistry, Part II, Vol. 3, Interscience, New York, 1961, pp. 125 35.

2 I. M. KOLTHOFF AND V. A. STENGER, Volumetric Analysis, Vol. 2, Interscience, New York, 2nd edn., 1947, pp. 301-6.

3 R. BELCHER AND C. L. WILSON~ New Methods in Analytical Chemistry, Chapman and bIall, London, 1956, pp. 275-76.

4 A. L. WOODSON, B. H. JOHNSON AND S. R. COOPER, Anal. Chem., 24 (1952) 1198. 5 I. V. TANANAEV AND A. P. KOROL'KOV, Izv. Akad. Nauk SSSR, Neorgan. Materialy, 1 (1965) 100.

Received September 19th, 1969; in revised form January 7th, 1970

J. Electroanal. Chem., 26 (1970) 409-411

Complexes of ehromium(II) chloride with some sulphur-containing amino acids

The interaction of transition metal ions with amino acids covers a vast area of research because of its biological importance, and the coordinating ability of amino and carboxylic groups 1-3. However, very few papers deal with the interaction of amino acids with transition metals having abnormal oxidation states. Recently a systematic study on this aspect of the problem was undertaken by the authors and the complexes of Cr 2 +, UO 2 +, VO 2 + and Ti 3 + with non-sulphur containing amino acids were studied 4'5. The present communication deals with the composition of various sulphur-containing amino acid complexes, based upon pH-metric and potentiometric measurements.

Experimental The amino acids, DL-methionine, taurine and cysteine (B.D.H. Biologically

pure products) were used and 0.01 M solutions were prepared in doubly distilled air- free water. Chromous chloride was prepared by the method of Bathis and Bailer 6. The aqueou s solution ofchromous chloride was prepared as described previously, and stored in an air-tight storage vessel under an atmosphere of oxygen-free nitrogen (pH 3.5). The solution was standardized potentiometrically by titration with standard copper sulphate. Carbonate-free KOH was used to prepare an aqueous solution of KOH and the solution was kept in a Pyrex bottle fitted with a guard tube containing K OH for protection against atmospheric carbon dioxide. The solution was standard-

J. Electroanal. Chem., 26 (1970) 411-414

412 SHORT COMMUNICATIONS

ized by titrating with standard oxalic acid and checked periodically before carrying out the pH-metric titrations.

The potentiometric titrations were carried out with a Tinsley potentiometer having a lamp and scale arrangement and employing platinum and calomel electrodes. The pH-metric titrations were performed with a direct reading EIL pH-meter model 23A (England). All the titrations were carried out in a specially designed cell described previously. The concentration of chromous chloride was checked before studying each system. All investigations were carried out at 25°C.

Results and discussion Chromous chloride forms 1:1 complexes with DL-methionine, taurine and

cysteine as determined from potentiometric curves (Fig. 1). The titrations were carried out using various concentrations of chromous chloride in the cell and equimolar solution of amino acids. The information regarding complex formation was obtained by noting the shifts in the pH-titration curves.

For each amino acid three sets of pH-metric titrations using 0.1 N KOH as a titrant were carried out in the order: (a) amino acid 0.01 M, (b) chromous chloride 0.005 M and (c) a mixture of chromous chloride and amino acid having a total con- centration of 0.005 M and 0.01 M, respectively, (Fig. 2).

The values of the logarithm of the formation constant, log Ks at fi = 1 (where is the average number of molecules of amino acid bound by one atom of the metal)

for the amino acids were calculated from the values of - log [Sc] obtained by the plot of fi vs. - log [Sc], where [Sc] is the concentration of free amino acid (Fig. 3). By applying the relation log Ks = - 2 log [Sc], values of the formation constants for all three amino acids were evaluatedV. The values obtained graphically and by calculation

-560

-520

-480

-440

> E

~ - 4 0 0 o

o n -360

-32(

-28( 0

A

4.0 8.0 12.0 16.0 20.0 Volume of DL-methionine/ml

Fig. 1. Potentiometric titrations: (A) 10 ml 0.666 x 10-1 M CrC1 z (in cell) vs. 0.666 x 10-1 M DL-methionine (from burette), (B) 10 ml 0.50 x 10-1 M CrC12 (in cell) vs. 0.50 x 10- , M DL-methionine (from burel~te), (C) 10 ml 0.44 x 10- ~ M CrC12 (in cell) vs. 0.44 x 10- ~ M DL-methionine (from burette).

J. Electroanal. Chem., 26 (1970)411414

SHORT COMMUNICATIONS 413

pH 12

A

lo

9

8

6 I

5

4 i i i

Volume of 0.1N KOt4/ml

1.8

1.6

1.4

1.2

_ 1 . 0 n

0.8

0.6

0.4

0.2

O.C I I I -z5 -3.5 -4'.5 '-~:5 ' ~

-~.~ -~.~ -4.s -~.5 B 40 -~.0 4.0 So c

-log [s~]

Fig. 2. pH-metric titrations : (A) 50 ml 0.01 M DL-methionine (in cell), (B) 50 ml 0.005 M CrCI 2 (in cell), (C) 25 ml 0.02 M DL-methionine+ 25 ml of 0.01 M CrC12 (in cell).

Fig. 3. Formation curves : (A) DL-methionine-chromium(II) complex, (B) taurine~hromium(II) complex, (C) cysteine~hromium(II) complex.

are given in Table 1. The values of overall stability constants obtained from the formation curves

are in good agreement with those calculated. The values of log Ks given in Table 1

TABLE 1

log (KJmol 2 l 2)

Graphically Calcd.

1 Cysteine-chromium(II) chloride complex 9.80 9.77 2 DL-Methionine-chromium(II) chloride complex 7.32 7.30 3 Taurine-chromiurn(II) chloride complex 7.02 7.06

give an indication of the possible correlation between the nature of the amino acids and overall stability constants. The value of log Ks decreases as the distance between amino and carboxylic group increases (between NH 2 and -SO2OH in the case of taurine). The present studies do not clarify the part played by sulphur atoms.

Acknowledgement It is a pleasure to thank Dr. S. M. F. Rahman, Department of Chemistry

Aligarh Muslim University, Aligarh, for his encouragement during the progress of this work. One of the authors (O.F. 1 is grateful to the Vice-Chancellor, Aligarh Muslim University, Aligarh, for financial assistance.

Department of Chemistry, Omar Farooq Aligarh Muslim University, A.U. Malik Aligarh U.P. (India) Naseer Ahmad*

* Chemistry Department, Louisiana State University, Baton Rouge, La. 70803, U.S.A.

J. Electroanal. Chem., 26 (1970) 411-414

414 SHORT COMMUNICATIONS

1 D. J. PERKIN, Biochem. J., 51 (1952) 487. 2 J. B. GILBERT, M. C. CLYDE AND J. Z. HEARON, J. Am. Chem. Soc., 77 (1955) 2599. 3 A. A. KHAN AND W. U. MALIK, J. Indian Chem. Soe., 40 (1963) 565. 4 0 . FAROOQ, A. U. MALIK AND N. AHMAD, J. Electroanal. Chem., 24 (1970) 233. 5 0 . FAROOQ, A. U. MALIK, N. AHMAD AND S. M. F. RAHMAN, J. Electroanal. Chem., 24 (1970) 464. 6 J. H. BATHIS JR. AND J. C. BAILER JR., J. Am. Chem. Soc., 58 (1936) 1474. 7 A. ALBERT, Biochem. J., 47 (1950) 531.

Received December 18th, 1969

J. Electroanal. Chem., 26 (1970) 411-414

The binding of molybdate ions with gelatin and ovalbumin

The precipitation of proteins by various cations and anions is of considerable importance. Many workers 1 -4 have found that the binding of anions may be taken as a measure of the acid binding capacity of the proteins. Compounds such as tungstic, phosphotungstic and ferrocyanic acid have also been used for the preparation of protein-free filtrates of blood and other biological materials. Kumar and Sinha 5 have reported on the reaction of the oxy-acids of V, Mo and W with gelatin below its isoelectric point but no attention was paid to the physicochemical aspects of the interactions.

The physiological importance of Mo is well established 6'7, but the reaction of the metal cation cannot be studied on account of the rapid hydrolysis which occurs in acid solutions to give highly charged polyanions s. The interaction at high pH values, where the simple molybdate ion (Mo 0 2-) exists, has been thoroughly investigated 1° with fibrillar and globular proteins. In acidic solutions the polyanions do not form soluble complexes but give suspensoids which cannot be studied by techniques such as equilibrium dialysis, pH-metry and polarography. A detailed and comprehensive study of these reactions can, however, be undertaken by measuring the transmittance of the colloidal dispersion. The results on the binding of molybdate with gelatin and albumin obtained by this method are reported.

Experimental

Apparatus. pH-measurements were carried out by means of a Beckman Model H pH-meter using wide range glass electrodes. The transmittance of the protein- ammonium molybdate mixtures, which can be taken as an index of the turbidity developed, was measured by means of the Bausch and Lomb 'Spectronic-20' in- strument at 375 nm.

Solutions and reagents. Doubly-distilled water was used throughout. An aqueous solution of ovalbumin (E. Merck), molecular weight 45,000, was prepared. This solution was centrifuged and the concentration of the clear solution determined by drying a known aliquot in an air oven at 105-110°C. The protein solution was then diluted with water and the desired pH obtained by the addition of HC1.

A solution of high grade gelatin, (Oxo Ltd. London) was prepared by dissolving a weighed amount in warm water (60°C) with continuous stirring. It was then centri-

J. Electroanal. Chem. , 26 (1970) 414-419