Electroanalytical Chemistry and Interfacial Electrochemistry, 49 (1974) 141-144 141 i~ Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

S H O R T COMMUNICATION

Studies on the mode of interaction of iridium(IV) with amino acids using sodium chloroiridate

OMAR FAROOQ and NASEER AHMAD Inorganic Research Laboratory, Department of Chemistry, Aligarh Muslim University, Aligarh/U.P. (India)

(Received 2nd July 1973)

The equilibria between metal ions and amino acids in solutions of various concentrations have been studied by a number of workers 1-4. Due to the biological significance and the coordinating ability of amino and carboxylic groups, the interaction of the transition metal ions with amino acids covers a wide area of investigation 5-9. The behaviour of transition metals having unusual oxidation states towards certain amino acids was reported by the authors in earlier communica- tions 10-16. The present study deals with the determination of the stability constants and composition of certain amino acid-iridium(IV) systems formed using sodium chloroiridate. These studies are based upon pH-metric titration under anaerobic conditions. The values of AF ° are computed from the relation AF ° = - R T log Ks. All studies were carried out at 35°C.

Experimental Amino acids such as glycine, DL-ct-alanine, L-asparagine, DL-serine, DL-

valine, L-proline, taurine (BDH, biologically pure), fl-alanine, DL-phenylalanine, DL-methionine and DL-threonine (E. Merck) were used and their solutions were prepared in doubly distilled air-free water. Sodium chloroiridate (Johnson Matthey, England) in solution was standardised by the method developed by Barefoot et al. 17.

The pH-metric titrations were performed with a direct reading Elico pH- meter, model LI-10 (India) and potentiometric titrations with the help of a Toshniwal Titration Potentiometer, model CL06 (India). Glass and saturated calomel electrodes were utilised as reference and indicator electrodes respectively. All studies were carried out in a specially designed cell as described previously TM.

Results and discussion Potentiometric titrations revealed a ratio of 1:4 (metal :amino acid) in all

the cases. The pH-metric titrations were performed as described in earlier com- munications 7. lO, 18. One equivalent of carbonate-free KOH of 0.1 M was used as a titrant. Iridium being tetravalent in nature, its concentration was kept at 0.0025 M in all the systems, whereas the concentration of amino acid was constantly kept at 0.01 M. The values of partial stability constants (log K' and log K ' ) and overall stability constants (log Ks) are listed in Table 1.

i42

TABLE 1

PARTIAL A N D OVERALL STABILITY C O N S T A N T S

S H O R T C O M M U N I C A T I O N

Systems Average Average log (Ks~tool 2 1-2) log K' log K"

Graphically Calcd.

AFO/kcal tool - la

Glycine-NazlrC16 3.15 2.63 5.70 5.78 81.27 DL-~-Alanine-Na2IrCl 6 4.36 2.61 7.02 6.97 99.59 L-Asparagine-Na2IrC16 3.22 2.68 5.85 5.90 82.92 DL-Serine-NazlrCl 6 6.83 3.32 9.75 9.85 138.40 DL-Valine-Na2IrCl 6 3.71 2.76 6.42 6.47 95.85 fl-Alanine-Na2IrC16 6.76 3.19 9.90 9.95 139.70 DL-Phenyla lanine-Na 2IrC16 5.28 2.53 7.76 7.81 109.70 DL-Threonine-Nazl rCl 6 5.47 3.49 8.92 8.96 125.60 DL-Taurine-NazlrC16 4.72 3.05 7.80 7.77 109.10 DL- Meth ion ine-Na 2IrC16 7.30 3.11 10.36 10.41 146.20

" 1 ca1=4.184 J.

The amino acids were found to produce highly soluble complexes with metals. However, in the case of L-proline and L-leucine it was difficult to get the correct values of the partial stability constants (log K' and log K'). The values obtained deviated a lot from the simple amino acid complexes discussed by various authors. Probably, this was due to the lower solubility of these complexes, on the whole, than those of the other systems involving metal-amino acids. The behaviour of L-proline was also strange and this material needs a thorough in- vestigation.

The majority of the amino acids followed the same sequence as that observed for other amino acid complexes previously 7' 9-12,14.. 15. In general, at the first stage one atom of the metal combined with one molecule of the amino acid (having two ionising groups, NH 2 and COO -) and thus the 1 : 1 complex species, MA ÷, did not appreciably react with a second molecule to give the 1:2 complex MA~. This position was attained when the value of h (average number of molecules of the amino acid combined with one atom of metal) reached 0.80 or 80~o of the 1:1 complex present.

The values of various terms were evaluated from the relations given below:

and

log K' = log h - l o g (1-h)- - log [Sc]

log K" = log ( h - 1)- log ( 2 - h ) - log [So]

log Ks = log h - log ( 2 - h ) - 2 log [Sc]

log Ks = 2 log [Sc], when h = 1.00

log Ks -- log K '+log K"

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

(a) (2) (3) (4)

(5) (6)

where h = 2 [KOH]/[HSc°]; [HSc °] = concentration of the complex-forming species. Equations (1) and (2) give regular values of log K' and log K". The ranges

SHORT COMMUNICATION 143

2.0

' ° l 1.2

O.O •

0.4

0.0 ~ i i

- 2 . 0 - 3 , 0 - 4 . 0 - 5 . 0 - 6 , 0 - 7 . 0 - 8 . 0

-~og I:sc]

Fig. 1. Formation curves. (O) Glycine-Na2IrCl6 system; ( A ) L-asparagine-Na2IrC16 system; (if]) DL-e-alanine-Na2IrC16 system; (11) DL-serine-Na2IrCl 6 system; ( 0 ) taurine-Na2IrC16 system.

are: log K' (fi= 1.004).80) and log K" (fi = 1.26-1.80) respectively, in all the cases reported herein. The values of the overall stability constant, log Ks, obtained from the formation curves (Fig. 1) are in good agreement with those calculated employing the above relations. In the present study, the stability of these systems varies from 5.78 for glycine to 9.95 for fl-alanine. It is interesting to compare the stabilities of the complexes of DL-a-alanine, fl-alanine and DL-phenylalanine. The order is: fl-alanine(9.95) > DL-phenylalanine (7.81) > DL-a-alanine(6.97). It is obvious that the NHE group at the fl-position is more basic in nature than the NH z group at the a-position.

In the case of L-proline and L-leucine, the values of the stability constant could not be calculated correctly. The plot of h vs. - log [Sc] also indicated the absence of appreciable quantity of the 1:1 complex species, MA +, even in the first portion of the titration curve. Equations (3) and (4) hold good regardless of the nature of the system. Equation (3) holds good between pH 3 and 11.0 whereas eqn. (4) is valid only when h = 1.00.

In the case of sulphur-containing amino acids, the value of log Ks for DL-methionine is higher than that for taurine. In general, the value .of log Ks decreases as the distance between the amino and carboxylic groups increases (between NH 2 and SOIOH in the case of taurine). The present study does not clarify the part played by the sulphur/atom. The actual mode of coordination in these complexes is a matter of speculation. The observations could not be quan- titatively substantiated due to the lack of information since it was not possible to isolate a complex in a sufficiently pure state.

Acknowledgements The authors' thanks are due to Professor W. Rahman, Head of the Chemistry

Department, A.M.U. Aligarh, for providing the facility for this experimental work and CSIR, New Delhi for granting a Senior Fellowship to one of them (O.F.).

144 SHORT COMMUNICATION

REFERENCES

1 I. Lifschitz, Proc. Acad. Sci. Amsterdam, 39 (1937) 1192. 2 H. Flood and V. Loars, Tidsskr. Kjemi, 5 (1945) 83. 3 L. M. Volshtein, lzv. Akad. Nauk S.S.S.R., Otd. Khim. Nauk, (1952) 248. 4 J. B; Gilbert, C. M. Otey and J. Z. Hearon, J. Chem. Soc., 77 (1955) 2599. 5 M. Cefola, A. S. Tompa, A. V. Celiano and P. S. Gentile, lnorg. Chem., 1 (t960) 290. 6 L. E. Maley and D. P. Mellor, Nature, 165 (1950) 453. 7 A. Albert, Biochem. J., 47 (1950) 531. 8 D. J. Perkins, Biochem. J., 51 (1952) 487. 9 J. Curchod, J. Chim. Phys., 53 (1956) 125, 182, 241, 2561

10 O. Farooq, A. U. Malik and N. Ahmad, ,1. Electroanal. Chem., 24 (1970) 233. 11 O. Farooq, A. U. Malik, N. Ahmad and S. M. F. Rahman, J. Electroanal. Chem., 24 (1970) 464. 12 O. Farooq, A. U. Malik and N. Ahmad, d. Electroanal. Chem., 26 (1970) 411. 13 O. Farooq and A. U. Malik, Collect. Czech. Chem. Commun., 37 (1972) 3410. 14 O. Farooq, A. U. Malik and N. Ahmad, J. Eiectroanal. Chem., 46 (1973) 136. 15 O. Farooq, A. U. Malik and N. Ahmad, J. Electroanal. Chem., 48 (1973) 475. 16 O. Farooq, A. U. Malik and N Ahmad, unpublished work. 17 R. R. Barefoot, W. J. McDonnell and F. E. Beamish, Anal. Chem., 23 (1951) 514; in I. M. Kolthoff

and E. Sandell (Eds.), Treatise on Analytical Chemistry, Vol. 8, Wiley, New York, 1963, part 2, p. 45. 18 J. Bjerrum, Metal Ammine Formation in Aqueous Solution, Haase, Copenhagen, 1941. 19 O. Farooq, Ph.D. Thesis, Aligarh Muslim University, India, 1970.