Biologically important complexes—I

,f im~rg mwl Chem., 1074. V:~/ ~6, pp 2155 2157. Pergamon Press Printed in Great Britain.

BIOLOGICALLY IMPORTANT COMPLEXES--I POLAROGRAPHIC STUDIES OF THE COMPLEXES OF CADMIUM LEAD WITH PYRIDOXINE ( V I T A M I N B 6)

AND

DAYA NAND CHATURVEDI and C. M. GUPTA Chemical Laboratories, University of Rajasthan, Jaipur 302004, India

(Received 4 December 1973) Abstract Polarographic investigations on the complexes of cadmium and lead with pyridoxine (vitamin B6) have been carried out at 20+_-1, 30_+-1 and 40_+-1°C. The effect due to variation of pH, temperature, ligand concentration and pressure on the reduction process has been studied. Complexes with metal-toligand ratios of I : 1 and 1:2 are observed with cadmium and lead, at pH 8.0 and 4.3 respectively. Stability constants of all complex species formed have been reported at different temperatures. Thermodynanric constants have also been evaluated.

INTRODUCTION T r ~ NATURE and stability of complexes formed by zinc and cadmium with several groups of biologically i m p o r t a n t c o m p o u n d s has been examined by Suffet and Purdy[l]. The complexation behaviour of various metal ions with certain biologically i m p o r t a n t compounds have been recently investigated in our laboratories[2 5]. A t h o r o u g h survey of the literature, however, revealed that apart from a single reference[6], there are hardly any published data regarding polarographic studies on the complex-forming ability of pyridoxine, hence the present investigations were undertaken. The communication describes a polarographic study of composition and stability of complexes of c a d m i u m and lead with pyridoxine in aqueous media. EXPERIMENTAL

(11 All chemicals used were of reagent grade and all solutions prepared in air-free conductivity water. Potassium nitrate was used as supporting electrolyte to maintain ionic strength constant at 0-I M, while 0.004 per cent gelatine in the final solution sufficed as a maxima suppressor. Experiments were performed with 0.5 mM cadmium and lead. Pyridoxine hydrochloride was used as a complexing agent and its concentration varied from 0-05 to 0-45 M. (2) Polarographic waves were recorded with a manual polarograph using an H-cell with a saturated calomel electrode as reference electrode. The dropping mercury electrode had the characteristics: rn = 2.35mg/sec and t = 4.0 sec. (3~ The experimental technique is the same as that described earlier[2]. THEORY

DeFord and Hume[7] defined an experimentally determinable function Fo(X) which depends on the concentration

of the ligand (X) and the various overall stability constants /3t, f12, etc.

Fo(X) = antilog {(0.4343nF/RT)AEt, z + log (I, /,)'~ = 1 q'- / ~ l ( X ) -{- /32(X) 2 -t- f l 3 ( X ) 3 " -

activity coefficient terms have been dropped because ionic strength was held constant. AE,/z is the shift in halfwave potential, and I s and I c are the diffusion current constants of the simple and complexed metal ions respectively. /3t,/32, ,6'3, etc. are the stepwise overall stability constants. A plot of Fo(X ) against IX] would give an intercept on the Fo(X ) axis so that another function Ft(X) may be defined as F,(X) = {Fo(X ) - 1}/IX] = /3t + /J2(X) + /331X)z . Similarly, a plot of F~(X} vs [X] gives an intercept [;~, hence where

F2(X) = {FI(X) - B1}/[X] = /32 + /33[X)'" and so on. Thus by plotting each F[X] function against the free ligand concentration, the stability and coordination numbers of all complex species present can be evaluated.

RESULTS AND DISCUSSION

Cd-pyridoxine system The reduction of Cd(II) in pyridoxine invariably gave a single well-defined wave. The slopes I31 + 2 mVI of plots of log i/lia - i) vs Ed.e. coupled with the constancy of the ia/h~(~ values indicated that the reversible two-electron reduction is entirely diffusioncontrolled. Effect OJ pH. The polarographic investigations of the Cd(II)-pyridoxine system were carried out over the pH range 1.5-9.9 on solutions of 0.5 m M Cd(ll) in 0.1 M pyridoxine, 0.1 M KNO3 and 0-004 per cent gelatine at 20 +_ .I°C. The upper pH limit in these measurements is restricted by precipitation.

2155

DAYANANDCHATURVEDIand C. M. GUPTA

2156

The half-wave potential remains constant within the pH range 1-5-7-5, beyond which a gradual shift in half-wave potential to more negative values with increase in pH shows the formation of the complex (Fig. 1): pH 8.0 was selected for further investigations on the complexation process.

overall stability constants as 25 and 230; 24 and 215; and 18 and 200 at 2 0 _ - 1 , 3 0 + . 1 and 4 . _ . 1 ° C respectively (Figs. 3-5).

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o.~ooc

I

I

2

I

4

I

6

I

8

I0

pH

I

Fig. I. Plot of - E I/2 vs pH for the Cd(II)-pyridoxine system. J

E~ect c?[ ligand concentration. The cathodic shift in half-wave potential coupled with a decrease in diffusion current on increasing pyridoxine concentration (0.025 M-0.45 M), indicated the formation of the complex at all the temperatures studied. Corresponding plots of - E l ~ 2 vs - l o g [pyridoxine] give a curve indicating the formation of more than one complex species (Fig. 2). Hence DeFord and Hume's [7] method was used for the calculation of successive stability constants of various complex species formed. Plots of F~(X) and F2(X ) vs ligand concentration, when extrapolated to zero [puridoxine], gave values of

I

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of Ligand

Fig. 3. Plots of F~(X) vs concentration ofligand at 20 + .I°C.

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Fig. 4. Plots of Fj(X) vs concentration of ligand at 30 + .I°C.

o.58

The change in free energy (AG), enthalpy (AH) and entropy (AS) have been calculated using the equations [8]: o.~2 '

_"

L

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I-2 --Log

Fig. 2. Plots of

~

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log flrz/flT ~ --

Cx

-El/2 vs log Cx system.

AG = - 2.303RTlog tip

0

for the Cd(II)-pyridoxine

and AS -

AH(T2 - T,) 4.576T1T2 AH - AG T

Biologically important complexes 120 --

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2157

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The half-wave potential remains unaffected belov, pH 3.6. Increasing the pH from 3-6 7.5 results m a cathodic shift in half-wav'e potential, which indicates complexation. It is inferred that chelation is favoured in the pH range 3,6,7.5 and further studies on the system were made at pH 4-3. Beyond pH 7.5. howe~er, the studies wrre restricted due to precipitation in the solution. Ell ~'ct O/ ligand concentration. A shift in half-waxc potential to more negative values with increase m ligand concentration is observed. Since the plot of - E ~ e vs. - l o g I p y r i d o x i n e ! is a curve indicating stepwise complex formation, the method of DcFord and Hume[71 was used to evahtate tile stabililx constants of the complex species so formed al constanl ionic strength (0I M). The values of oxerall stabilit\ constants are 9 and 7 3 12-5 and 59: and 17 and 48 at 20 ± .1, 30 + ,1 and 40 ± -I°C respccnvel_x. The values of change in free energy (A(;). enthalpv (AH) and entropy' (AS) are summarized helox~ :

Fig. 5. Plots of F / X ) vs concentration of ligand at 40 + -I°C.

Thermodynamic data for the Pb pyridoxine sxstem T h e values are s u m m a r i z e d b e l o w :

Thermodynamic data for the cadmium-pyridoxine system Temp. (°C)

AG (kcal)

AH (kcal)

AS (cal/deg/mole

20 + .I 3O + .1 40 ~ .I

3.16 / 3.23 f -3.29

1-19

6.73

-1-36

6-18

Pb{ I l)-pyridoxine system In each case a well-defined wave was obtained. The slopes (32 + 1 mV) of plots of Ea.,. vs logi/(ia - i) coupled with the direct proportionality of the diffusion current to the square root of the effective pressure ( m m Hg), i n d i c a t e d that t h e t w o - e l e c t r o n r e d u c t i o n is

reversible and entirely diffusion controlled. El[fect of" pH. The variation of half-wave potential with pH of the solution (0"5 m M lead, 0.1 M pyridoxine, 0"1 M K N O 3 and 0.004 per cent gelatine) is shown in Fig. 6.

AG (kcal)

AH (kcall

AS ical deg molcl

at)

2.50

30 40

" '~ f -'4. 2'40

3.75

4.>~

3"88

473

Temp. (°(h

A general decrease in stepwise stability constants {K~) is usually found as one passes from the lowcsl to the highest coordination number. Similar results ha~e been obtained here and such a trend is to be expected on the basis of steric and statistical considerations. .4ckmmledt{emenl.s The authors arc gralcftfl to Proti:ssor R, C. Mehrota, for providing laborator\ facililics One of the attthors (D.N.C,) acknowledges the support ~,I" the University ,fl" Rajaslhan for the grant of a sch~qatsh!p

REFERENCES

45OO

1. 1. H. Suffet and W. C. Purdy. ,I. clc~noomd. ('tram. I I, 302 (1966).

2. D. N. ('haturxedi and C. M, (;upta. Z cm~t/~/ ('hem. 260. 120 (1972). 3. D. N. Chaturvedi and C. M. Gupta. ,-tna/l~/ 98, 895 (1973). 4. D. N. Chalurvedi and C. M (Jupla. ,I pr,z/,[ ('htnl. Ii1 press, 1973. 5. P. C. Rawat and C M. (}upta. Imliall .1 (hem. |1, IS6 (1973). 6. P. A. Pella and W. C. Purdy, J. eh'clroamd. ('hem. 9,

w tO 03

4100

I

O" 3 7 0 0 0

2

4

6

8

pH

Fig. 6. Plot of - E ~ 2 vs pH for the Pb(lI}-pyridoxine system.

51 ( 19651. 7. D. D. DeFord and D. N. H u m e . , l -Ira. h e m 5321 (1951).

.Y,~ 73.

8. K. B. Yatsimirski and V. P. Vasil'ex. h>ml~i/it~ ( , n ~talltS ol Complev ('ompolmdv. Pergamon Press, Oxlord ( 1960'!.