Electroanalytwal Chemtstry and Interfaclal Electrochemzstrk; 53 (1974) 461-464 461 ,(~J Elsevier Sequoia S A, Lausanne - Printed m The Netherlands

SHORT COMMUNICATION

Composition and stability of osmium(1V)-amino acids complexes

OMAR FAROOQ and NASEER AHMAD Inorgamc Research Laboratory, Chem~stt y Department, Ahgarh Mushm Unwerstty, Ahgarh ( Indza)

(Receaved 28th January 1974)

The metal complexes of amino acids play very important roles in many biological systems. The interaction of transition metals with various amino acids has been extensively studied and the stabilities of the resultmg complexes have been ascertained by several electrometrtc techniques 1-8. Recently a programme of work to study the mode of binding of the transition metals of the platinum groups with certain amino acids and the determination of stabilities of their complexes has been undertaken by the authors 9-12. The present communication deals with the interaction of osmtum(IV) with some amino acids using sodium chloro-osmate m aqueous medium, pH-metrlc and potentiometric titrations were performed to calculate the successive equilibrtum constants, overall stability constants and compositions. The values of AF ° were computed from the relation, - A F ° = R T In Ks, where Ks =the overall stability constant.

Experimental Sodium chloro-osmate (Johnson and Matthey, London) was used to prepare

an aqueous solution whose strength was checked by the strychnme salt method 13. The ammo acids glycine, DL-e-alanine, /~-alanine, DL-serme, L-proline, DL- methionine, DL-threontne, DL-taurine, DL-valine (biologically pure, B.D.H., England), L-asparagine, DL-phenylalanine and L-leucine (chromatographically pure, Merck, Germany) were employed and their standard solutions m twice-distilled air-free water were used in the tltrations.

A Toshniwal titration potentiometer, model CLO6 (Indta) in conjunction with platinium and saturated calomel electrodes was used. A direct reading EIL pH-meter, model 23A (England) was employed with glass and calomel electrodes" for pH-metric titrations. All tltrations were performed m an oxygen-free atmosphere.

Results and discussion A ratio of 1:4 (metal:amino acid) was established by potentiometric titra-

tlons for all the systems reported here. The stepwise successive constants or equilibrium constants were calculated by Bjerrum's a4 and Albert's is techniques. Three solutions: 50 ml of 0.01 M amino acid, 50 ml of 0.0025 M osmate and 50 ml containing 0.01 M amino acid and 0.0025 M osmate were separately titrated against 0.1 M standard carbonate-free potassium hydroxide. Only air-free con-

462 SHORT COMMUNICATION

TABLE 1

OVERALL STABILITY CONSTANTS OF VARIOUS AMINO ACIDS-OSMATE SYSTEMS AT 28°C

Systems log ( K J m o l 2 l - 2 ) - d F° /kJ tool- 1

Calcd Graphwally

L-Asparag~ne Na2OsClo 5 17 5 20 3 10 DL<t-Alamne-Na2OsC16 6 97 6 90 4 85 fl-Alanme-Na2OsC16 6 12 6 05 4 52 DL-ct-Alanme Na2OsC16 5 58 5 60 4 3 t Glycme Na2OsC16 5 04 5 60 4 35 DL-Isoleucme-Na2OsC16 5 72 5 70 4 36 L-Leucme-Na2OsC16 5 82 5 80 4 39 DL-Methlonlnea-Na2OsC16 6 06 6 02 4 51 L-Prohne-Na2OsC16 7 02 7 00 4 89 DL-Serme-NazOsC16 5 60 ~ 50 4 18 DL-Taurlne~-Na2OsCl6 6 00 6 05 4 49 DL-Threonlne NazOsC16 6 19 6 20 3 04 DL-Vahne-Na2OsC16 7 77 7 80 5.27

a Sulphur-containing amino acids

ductivity water was used for dilution and in the tltrations. The osmium con- centration was maintained at 0.0025 M. The values of overall stability con- stants (Ks) are listed in Table 1.

In general, amino acids were found to produce complexes of high stability with metals. As the first stage of the reaction, one molecule of the amino acid combines with one atom of the metal producing a 1 : 1 complex species, MA ÷. This species, however, did not combine promptly with another molecule of amino acid to produce the 1:2 complex species, MA~-. This equilibrium is only attained when the value of h (average number of molecules of amino acid which combine with one atom of metal) reaches 80}o of the 1:1 complex. The value of ~ starts at nearly zero and reaches a maximum of approximately two. At this stage, the possibility of formation of MA~ cannot be ruled out. Flood and Loars 16 agree with this analogy. When fi = 1, the values of log K' and log K " may be written as

log K ' = log/i - log (1 - f i ) - log [Sc]

log K " = log (1 - tS) - log (2 - h) - l o g [Sc]

where [Sc] stands for the concentration of the complex-forming species and its value may be calculated by a simplified equation which holds good between pH 3 and 11 for simple amino-acid systems, namely,

log [Sc] = (pH - pK, ) + log ([HSc °] - [KOH])

where [HSc °] is defined as the concentration of the free amino acid before the addition of the metal and [KOH] is the concentration of carbonate-free potassium hydroxide present in the solution after each gradual addition, if both the amino acid and metal are supposed to be absent. At the point where h = 1 (formation curve, h vs. -- log[Sc]) the correct value for log [Sc] is obtained. The values of log Ks may

SHORT COMMUNICATION 463

be written as l o g K s = - 2 ( - l o g [ S c ] ) . The value of h is governed by the relation, h = 2[KOH]/ [HSc °] and log Ks is also equal to the logarithmic values of K' and K". A few plots of ~ vs. - l o g [Sc] are given m Fig. 1.

1.2

0 .8

OA

O.G I I 2.0 4.0 6.0

fi

1.6

I I 8 . 0

Fig 1 Plots of ~ vs -log[Sc], where [Sc] 1s the concentration of the complex-forming species (0) Glyclne-NazOsC16 system, ( . ) fl-alanme-Na2OsC16 system, (A) DL-vahne-Na2OsCl6 system, (O) L-prohne-Na2OsC16 system

The values of log Ks calculated by applying the above relations are m good agreement with those obtained graphically. These values vary from 5.17 for L- asparagine to 7.77 for DL-valine, and the order of stablhty is: DL-valine > L- proline > DL-~-alanlne > DL-threonme > fl-alanine > DL-methionlne > DL- taurine > L-leucine > DL-isoleucine > glycine > DL-serlne > DL-phenylalanine > L-asparagine. There seems to be no relationship between the values of overall stability constants and the chain length or nature of the amino acid. Keeping in view the values of log Ks for various systems reported here, the following generalizations may be made.

(a) In the case of DL-~-alanlne, fl-alanlne and DL-phenylalanine, the order of stablhty is: DL-~-alanine > fl-alanlne > DL-phenylalanine.

(b) The value of log Ks of L-leucine complex is shghtly higher than the value of the DL-isoleucine complex.

(c) In the case of the sulphur-containing amino acids, the value of log Ks decreases as the distance between the amino and carboxylic group increases (between N H 2 and SO2OH in case of DL-taurine). This does not justify the part played by the sulphur atom during complexatIon 6 9 ~0 ~7

These values are lower than those of systems stu&ed earlier 9-12 presumably due to the low avidity of osmium(IV) towards common amino acids. The actual nature of binding in these complexes is difficult to explain since the complex could not be isolated in a sufficiently pure state for analysis.

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Acknowledgemen ts The authors gratefully acknowledge the research facihties provided by Prof.

W. Rahman, Head of the Chemistry Department, A.M.U., Aligarh. Thanks too are due to CSIR India for providing a Senior Research Fellowship to one of the authors (O.F.).

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