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Complextion of thymidine with some first transition metal ion in aqueous medium.

The polarographic study of complexation of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) with thymidine were made in aqueous media at ionic strength 0.1M LiCl[O.sub.4] at 293, 298 and 303K. Crow's mean diffusion coefficient method were applied for the calculation of stability constantants, the log[K.sub.1] and log[K.sub.2] values being, Mn(II) 2.70, 1.15; Co(II) 2.77, 1.15; Ni(II) 2.95, 1.04; Cu(II) 3.09, 0.61 and Zn(II) 2.76, 1.13 at 298K respectively. The stability constant values follow Irving-William order. Thermodynamic parameters were calculated in the range 293-303K and found to have negative values.

Thymidine, a nucleoside of thymine, is a constituent of DNA. The biological roles of nucleic acids are dependent on metal ions. The nucleic acids contain a large no of oxygen and nitrogen donor sites [1, 2] of varying basicity i.e., varying donor properties and hence varying ability to interact with various metal ions. Studies of such interactions help to understand the much more complicated nucleic acid-metal system and hence enables us to understand their biological role [3-6]. Literature studies on thymidine complexes is scarce due to the lack of both precise structural and solution chemical data. This is in part due to the weak stability of the complexes formed, which makes it difficult both to interpret solution chemical data and to obtain suitable single crystal.

The present study aims primarily at a description of the chemical knowledge accumulated regarding complexes of thymidine with some transition metals in aqueous solution. Thymidine like uridine acts as a bidentate ligand involving N(3) and O(4) in metal binding [7] and rules out the possibility to bind through the deoxyribose unit [8]. The stability constants values for thymidine complexes were found to be somewhat greater than those of uridine complexes. This is due to the presence of electron-donating methyl group at the c-5 position of the pyrimidine ring in thymidine, which makes it slightly more basic than uridine [7].

The present paper comprises polarographic study of complexation of thymidine with metal ions Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) ions in aqueous media at three temperatures. The stability constants of the complexes have been determined by Crow's mean diffusion coefficient method [9-11] assuming 1:1 complextation.

Experimental

An ELICO (CL-25D) polarograph in conjunction with ELICO chart recorder (LR101P), ELICO pH-meter (LI-120), a Julabo F-40 HC (West-Germany) circulating cryostat and sergent capillary with characteristics, m=1.722[mgs.sup.-1], t=5.3s (open circuit) for 70 cm Hg head in aqueous LiCl[O.sub.4] (0.1 mol [dm.sup.-3]) at zero potential (SCE) and H-shaped double walled kalousek cell with built-in SCE in the second limb of the cell, were used.

All the reagents used were of extra pure (E. Merck) or A.R. grade. Thymidine was obtained from SRL India. Metal perchlorates were prepared from the metal oxides, hydroxides or carbonates by adding required amount of perchloric acid and then crystallizing it.

The composition of the test solution were (a) metal ion (2ml, 5mmol [dm.sup.-3]), LiCl[O.sub.4] (2.5 ml, 1.0mol [dm.sup.-3]), gelatin (0.5 ml, 0.2%) (b) metal ion (2 ml, 5mmol [dm.sup.-3]), LiCl[O.sub.4] (2.5 ml, 1.0mol [dm.sup.-3]), gelatin (0.5 ml, 0.2%) and increasing volume of ligand solutions, the final concentration of which varied from 0.08 mmol [dm.sup.-3] to 20 mmol [dm.sup.-3]. The total volumes of the solution were kept 25 ml by adding double distilled water after adjusting pH. The pH's of the solution have been kept below the precipitation pH's of the respective metal ions and have been adjusted by adding aq. LiOH and HCl[O.sub.4]. All the test solution were kept overnight for equilibration before recording polarograms. Before recording the polarogram IOLAR grade nitrogen gas (Indian Oxygen Ltd.) was passed through the solution for fifteen minutes. The pH's of the solution were checked before and after recording the polarograms. The polarograms were recorded at three temperatures 293, 298 and 303K ([+ or -] 0.1 K)

Results and Discussion

Polarographic characteristic data for all the metal ions at 298K are shown in Table-1. Electroreduction of aqueous metal ions as well as complexed metal ions for all the metal ions studied, gave a well-defined single diffusion controlled waves. In all the cases decrease in diffusion current occurred with increasing ligand concentration. Negative shifting of half-wave potential value have been seen for Co(II) and Cu(II) ions on complexation. For all other metals almost negligible shifts in half-wave potential were observed. Electroreduction process for Mn(II) (slope 36-24mV and Zn(II) (slope ~ 47mV) were quasi-reversible, whereas for Co(II) (slope 64mV-88mV), Cu(II) (slope 69mV-84mV) and Ni(II) (slope 74mV-41mV) were irreversible in nature (slope values are given for 298K)

For each metal ion, decrease in diffusion current Aid was plotted against-log[L] where L is ligand concentration. The curve so obtained is called pseudo-formation curve. Integration of the areas under the curves for different ligand concentrations give Fronaeous [9, 10] function logFo'[L] values at those concentrations. The log[F.sub.o]'[L] values are then plotted against-log[L], the limiting slope of the curve thus obtained yielded ([N.sub.max]/k) values. Assuming 1:1 complexation for each metal ions studied i.e., taking [N.sub.max] equal its one, the proportionality constant k were calculated. Multiplying log[F.sub.o]'[L] by k yielded log[F.sub.o][L], from which other Leden functions, [F.sub.J][L] were obtained. By the plot of [F.sub.J][L]Vs.[L] values, formation constants were found out [11-14]. The thermodynamic parameters are there calculated.

The stability constant values at three temperatures and thermodynamic parameters [increment of G], [increment of H] and [increment of S] for complex formation for all the metals are shown in Table-2.

Thermodynamic quantities have been derived from overall stability constants: Stability constant and Themodynamic parameters for M(II)-Thymidine complexes in aqueous media at ionic strength 0.1m LiCl[O.sub.4]

Comparison of stability constant values for Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) with thymidine for 1:1 complexes are Cu(II)>Ni(II)< Co(II)[approximately equal to]Zn(II)>Mn(II), which is in agreement with Irving-Williams [15] natural order. The values at three temperatures for all the metal ions are found to decrease with increasing temperature verifying the conclusion of Pitzer [16] that higher temperature are not favorable for complex formation.

The values of [increment of G], [increment of H] and [increment of S] are all found to be negative. The negative values of free energy [increment of G] suggest that the complex formation is spontaneous. Negative enthalpy change indicates that complexation is an exothermic reaction. Negative value of entropy change indicates that the complexation process is entropy stabilized.

References

[1] L. Chen and B.M. Carven, Acta. Cryst., B 51, 1081 (1995)

[2] B. Pullman and A. Pullman, Quantum Biochemistry, Interscience, New York (1963)

[3] A. Sigel, H. Sigel, Eds., "Metal Ions in Biological System", Dekker, N. York, Vol-42, 1-534 (2004)

[4] J.E. Wedekind, D.B. Mckay, Biochemistry, 42, 9554-9563 (2003)

[5] R.K.O. Sigel, A. Vaidya, A.M. Pyle, Nat. Struct. Biol., 7, 1111-1116 (2000)

[6] F.J. Pesch, H. Prent and B. Lippert, Inorg. Chim. Acta., 169, 195 (1990)

[7] P.R. Reddy, C.N. Keerthi and T.K. Adharani, Proc. Ind. Acad. Sci. (Chem.Sci.), 101, 99(1989)

[8] M. Goodgame and K. W. Johns, J. Chem. Soc. Dalton Trans., 1294 (1978)

[9] S. Fronaeous, Acta. Chem. Scand., 4, 72 (1950)

[10] L. Meites, Polarographic Techniques, Interscience, New York, (1969)

[11] D.R. Crow, Electrochim Acta., 28, 1799 (1983)

[12] D.R. Crow, Talanta, 29, 733 (1982)

[13] D.R. Crow. "Polarography of metal complexes", Academic, N.York, (1969)

[14] I. Leden. Z. Phys. Chem. (A), 188, 160 (1941)

[15] H. Irving and R. Williams, Nature, 162, 746 (1948)

[16] K.S. Pitzer, J. Am. Chem. Soc. 59, 2965 (1937)

Dr. Mukul Kumar Singh (1) *, Namita Bhardwaj (2) and Seema Shrivastava (1)

(1) Department of Chemistry, Govt. ERR Science PG College, Bilaspur (C.G.), India

(2) Department of Chemistry, Dr. C V Raman Univ. Kota, Bilaspur (C.G.), India
Table 1

S.N   [L]x[10.sup.4]   [i.sub.d](Mn)   [E.sub.1/2] (Mn)
      mol[dm.sup.-3]     [micro]A             V

1           00             0.645            1.305
2          0.8               -                -
3           4              0.635            1.300
4           8              0.630            1.290
5           16             0.625            1.300
6           40             0.615            1.305
7           80             0.610            1.300
8          200             0.590            1.305

S.N   [i.sub.d](Co)   -[E.sub.1/2] (Co)   [i.sub.d](Ni)
        [micro]A              V             [micro]A

1         0.745             0.93              2.075
2           -                 -               2.025
3         0.735             0.93              2.000
4         0.730             0.93              1.990
5         0.725             0.93              1.950
6         0.710             0.93              1.900
7         0.705             0.93              1.850
8         0.695             0.94              1.650

S.N   -[E.sub.1/2](Ni)   [i.sub.d](Cu)
             V             [micro]A

1           0.96              1.5
2           0.97             1.49
3           0.97             1.48
4           0.98             1.47
5           0.98             1.46
6           0.98             1.45
7           1.00             1.44
8           1.01             1.33

S.N   [E.sub.1/2](Cu)    id(Zn)    -[E.sub.1/2] (Zn)
             V          [micro]A           V

1          +0.03          1.7            1.01
2          +0.02           -               -
3          +0.03          1.68           1.01
4          +0.03          1.67           1.01
5          +0.03          1.65           1.01
6          +0.03          1.64           1.01
7          +0.03          1.62           1.01
8          +0.02          1.60           1.01

Complexation of thymidine with Mn(II), Co(II), Ni(II), Cu(II)
and Zn(II) in aqueous media at ionic strength 0.1 MLiCl[O.sub.4]
MCl[O.sub.4]:Mn(II) and Co(II): 0.2 0.2 mmol [dm.sup.-3]; Ni(II),
Cu(II), Zn(II): 0.4 0.3 mmol [dm.sup.-3] LiCl[O.sub.4]: 0.1 mol
[dm.sup.-3] 0.4 pH: Mn(II) 8.0, Co(II)6.5, Ni(II) 6.0, Cu(II) 5.0,
0.5 Zn(II) 7.0 Temperature 298K

Table 2

M(II)     Temperature   LogK1   LogK2     -[DELTA]G

               k                        kJ[mol.sup.-3]

Mn(II)        293       2.80    1.02        21.43
              298       2.70    1.15        21.97
              303       2.60    1.23        22.20

Co(II)        293       2.87    1.09        22.23
              298       2.77    1.15        22.37
              303       2.67    1.11        21.93

Ni(II)        293       3.03    1.03        22.78
              298       2.95    1.04        22.77
              303       2.85    0.93        21.93

Cu(II)        293       3.18    0.82        27.83
              298       3.09    0.61        27.56
              303       2.93    1.00        23.21

Zn(II)        293       2.85    0.98        21.54
              298       2.76    1.13        22.20
              303       2.65    1.04        21.41

M(II)     Temperature     -[DELTA]H        [DELTA]s

               k        kJ[mol.sup.-1]   J[mol.sup.-1]
                                          [k.sup.-1]

Mn(II)        293           31.72           -69.57
              298
              303

Co(II)        293           37.34           -50.90
              298
              303

Ni(II)        293           152.22          -435.40
              298
              303

Cu(II)        293           45.95           -66.21
              298
              303

Zn(II)        293           60.31           -129.55
              298
              303
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Author:Singh, Mukul Kumar; Bhardwaj, Namita; Shrivastava, Seema
Publication:International Journal of Applied Chemistry
Article Type:Report
Geographic Code:9INDI
Date:Jan 1, 2012
Words:1964
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