Electroanalytical Chemistry and lnterfi~cial Electrochemistry, 57 (1974) 121 124 © Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

121

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Stability and stepwise association of rhodium(liD chloride with some amino acids

OMAR FAROOQ and NASEER AHMAD

Inorganic Research Laboratory, Chemistry Department, Alioarh Muslim University, Aligarh U.P. (India)

(Received 9th July 1974)

Owing to the biological importance of amino acids and their related compounds, appreciable attention has been paid to the stability of transition- metal complexes with amino acids 1~. Recent work in this laboratory on the mode of interaction of various amino acids with transition metals and with several platinum group metals has shown the likelihood of association of these metals with certain common amino acids including sulphur containing amino acids 5-15. The present communication deals with the results of composition and stability of certain amino acid complexes with trivalent rhodium. The studies are mainly based upon the pH-metric titration method in aqueous medium as suggested by Bjerrum 16 and modified by Albert iv. Potentiometric titrations were performed for each reported system herein. The relation, - AF ° = RT In K~ (where Ks = overall stability constant) was employed to compute the values of AF ° at 27°C. All studies were done under anaerobic conditions and in an oxygen-free atmosphere.

Experimental The amino acids, glycine, DL-c~-alanine, fl-alanine, L-leucine, L-proline,

DL-serine, DL-taurine, DL-methionine, DL-valine (B.D.H., biologically pure), L-asparagine, DL-phenylalanine, DL-isoleucine and DL-threonine (E. Merck, chromatographically pure) were used and their solutions were prepared in twice distilled air-free water. Rhodium chloride (Johnson Matthey, England) in aqueous solution was estimated gravimetrically is.

A Toshniwal titration potentiometer; model CL06 (India), in conjunction with platinum and calomel electrodes, was employed for potentiometric studies, whilst an EIL, Direct Reading pH-meter, model 23A (England) was used for pH-metric measurements. Glass and saturated calomel electrodes were used as indicator and reference electrodes, respectively. Carbonate-free KOH was used as the titrant. The solution was stored in a Pyrex bottle fitted with a guard tube containing anhydrous KOH to avoid atmospheric carbon dioxide. The strength was checked before titrating each set of sfimples.

Results and discussion A stoichiometric ratio of 1:3 (metal:amino acid) was observed by potentio-

metric titrations in all the systems reported herein, pH-metric titrations in tri-

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plicate were performed to evaluate the successive equilibrium constants, (K1 and Kz) and the overall stability constant (Ks) in the order: (a) amino acid, 0.01 M (b) rhodium chloride, 0.0033 M and (c) a mixture of rhodium chloride and amino acid having a total concentration of 0.0033 M and 0.0l M, respectively. Rhodium being trivalent, the concentration was kept to 0.0033 M in all the systems. The values of partial or successive stability constants (log K1 and log Kz) and the overall stability constants are listed in Table 1.

TABLE 1

VALUES OF SUCCESSIVE EQUILIBRIUM AND OVERALL STABILITY CONSTANTS FOR VARIOUS RHOD1UM(III) CHLORIDE-AMINO ACID SYSTEMS AT 25°C

System log K 1 log K 2 log (Ks~tool z 1-2) -AF° /kca l tool - z

DL-e-Alanine RhC13 7.19 3.09 10.28 14,1 fl-Alanine-RhCl3 6.63 3.29 9.92 13.6 L-Asparagine-RhCl3 6.86 2.87 9.73 13,4 Glycine RhC13 7.34 3.03 10.37 14.2 DL-Isoleucine RhC13 7.16 2.76 9,92 13.6 L-Leucine RhC13 7.13 2.79 10.10 13.9 DL-Methioninea-RhC13 6.69 2.69 9.38 12.9 DL-Phenylalanine RhCI 3 6.82 3.12 9.94 13.7 L-Proline-RhC13 8.24 3.00 11.24 15.4 DL-Serine-RhCl 3 6.92 3.03 9.95 13.7 DL-Taurine"-RhC13 6.61 2.68 9.29 12.7 DL-Threonine-RhC13 • 6.86 3.02 9.88 13.6 DL-Valine-RhC13 7.14 2.66 9.80 13.5

" Sulphur-containing amino acids.

In general, the majority of the amino acids exhibited the same binding sequence as that observed in various other systems. Amino acids produce complexes of high stability with metals. Monoaminomonocarboxylic acids, amino acids having two ionizing groups, combine with one atom of the metal to produce a 1 : 1 species MA + at the initial stage of the reaction. Another molecule of the amino acid probably does not coordinate with this species to produce a more complex species, MA~. An equilibrium is attained and the value of h (average number of complex forming agent, amino acid, bound by one atom of the metal) reaches 80% of the dominating species, MA + of 1:1 complex. The value of fi starts at zero and reaches a maximum of approximately two. Flood and Loafs 19 suggested that the probability of formation of MAr species may not be totally ruled out at this stage of the reaction, and the plot of h vs. - l o g [Sc], where [Sc] stands for the concentration of the complex forming species, shows some abnormality.

The concentration of various species at equilibrium is directly linked by a series of expressions to that of each of the other complexes at equilibrium.

KI = [MA +]/[M] [A]; K2 = [MA;]/[M] [A]; K 3 = [MA~]/[M] [a] . . . . . K, = [MA,]/[MA,_ 1] [a]

K1, K2, K3 ... K, are termed as partial stability, successive association or equili-

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brium constants. The algebraic sum of the logarithmic values of these constants is defined as the overall stability constant, log Ks and may be represented as log Ks = log K1 + log K 2 + log K 3 + . . . . . . +log K,. In the present study, the value of Ks may be rewritten as,

log K s = log K1 +log K 2 or log Ks = log [MAz]/[M][A-] 2

As the complex formation starts, hydrogen ions are liberated and the pH of the resulting mixture can be used to check the extent of complex formation in the solution phase. When the value of h reaches nearly 1, the values of successive association constants, K1 and K2 may be expressed as

log K1 =log h - l o g (1 -h ) - - log [Sc]; log K 2 = log (h-- 1)--log ( 2 - h ) - log [Sc].

[Sc] may be evaluated by a simplified equation in the pH-range 3 to 11, viz.,

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

here [HSc °] and [KOH] represent the initial concentration of amino acid before the addition of the metal salt and the concentration of titrant when complex forming agent and metal salt were both absent, respectively. The values of h from 0 to 2 were computed from the relation, h= 2[KOH]/[HSc°].

The values of the overall stability constants, log Ks, vary from 9.25 (L-asparagine) to 11.24 in L-proline and the general order of stabilities is: L-proline > glycine > DL-~-alanine > L-leucine > DL-serine >/DL-phenylalanine > fl-alanine = DL-isoleucine > DL-threonine > DL-valine > L-asparagine > DL- methionine >DL-taurine. Undoubtedly, sulphur-containing amino acids (DL- methionine and DL-taurine) appear to behave like a monoaminomonocarboxylic acid. Veidis and Palenik 2° established that thio-ether type sulphur in methionine, CH3SCHzCH2(NHE)COOH, contributed very little to the stability of its complexes. Similarly, taurine, a sulphonic amino acid, did not show any specific preference for rhodium. These values are somewhat higher than the values of other systems studied earlier, showing thereby the high affinity of rhodium(III) chloride for amino acid 5 12.

The plots of h vs. - l o g [Sc] (formation curves) correspond to the value of log K1 when h= 1 for a particular system. Similar behaviour was observed in the case of Au 3 +-amino acid systems 13. On the other hand in most of the previous studies it was noticed that the value of log Ks was equal to - 2 log [Sc] at h = 1. The possibility of formation of polynuClear compounds cannot be totally ruled out. The hydroxides of heavy and coinage metals associate themselves to produce polynuclear adducts in solution phase when the pH of the system approaches the region at which MOH is precipitated. Two common types of linkages are reported between metal atoms in crystalline hydroxides21: M ~ ) M partially covalent oxygen bridges and hydroxyl bridges of the type, M~D H-M. Most probably both types of linkages are involved in the formation of polynuclear complexes.

• In the present study, the values of log Ks deviated a lot from the other systems but they are similar in magnitude to the values of Au 3 +-amino acid systems reported earlier 13. Some curves of h against - l o g [Sc] are given in Fig. 1

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1.6

12

O8

04

p I I

°~.o 5.0 zo 90 -~og [s4

Fig. 1. Formation curves: fi vs. - l o g [Sc]. (O) DL-valine-RhCl3 system, (A) L-asparagine RhC13 system, ( x ) DL-c~-alanine-RhC13 system.

which support this claim. It is difficult to assign a definite general structure to these complexes since they could not be isolated in the pure state.

Acknowledgements It is a pleasure to thank Prof. W. Rahman for laboratory facilities.

Farooq is also grateful to CSIR, New Delhi, India for the award of a Senior Research Fellowship.

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