Pb(II)-MA 30 % v/v DMF -water mixture
Acid
|
0
|
7.11(17)
|
12.10(18)
|
10.51(17)
|
-5
|
Rejected
|
13.82(09)
|
12.88(08)
|
-2
|
7.05(60)
|
12.90(08)
|
11.50(09)
|
+2
|
6.78(16)
|
Rejected
|
9.56(39)
|
+5
|
6.16(21)
|
Rejected
|
Rejected
| |
Alkali
|
-5
|
5.69(40)
|
Rejected
|
Rejected
|
-2
|
6.63(19)
|
Rejected
|
9.38(48)
|
+2
|
Rejected
|
13.04(06)
|
11.75(05)
|
+5
|
Rejected
|
14.00(14)
|
13.33(12)
| |
Ligand
|
-5
|
6.82(86)
|
12.62(12)
|
11.24(13)
|
-2
|
7.06(28)
|
12.33(15)
|
10.81(15)
|
+2
|
7.05(15)
|
11.85(25)
|
10.19(22)
|
+5
|
6.98(11)
|
11.21(73)
|
9.59(45)
|
Metal
|
-5
|
7.09(20)
|
12.12(18)
|
10.61(16)
|
-2
|
7.10(18)
|
12.11(18)
|
10.54(16)
|
+2
|
7.10(16)
|
12.09(17)
|
10.46(17)
|
+5
|
7.10(15)
|
12.07(17)
|
10.40(18)
|
Effect of Solvent
DMF is a polar aprotic solvent. The dielectric constant of DMF-water mixture decreases with increasing concentration of DMF and these solutions are expected to mimic physiological conditions where the concept of the equivalent solution dielectric constant for protein cavities is applicable [36]. The dielectric constants of DMF at different percentages (0.0-50.0 v/v %) of water were taken from literature [37]. The increase in organic solvent content decreases the dielectric constant of the medium. The change in log β values of Pb(II), Cd(II) and Hg(II) complexes of LPA and MA with 1/D (Fig.1-2) is almost linear which indicates that the dielectric constant or long range interactions are responsible for the stability trend in both DMF-water media. The deviation from linearity may be due to some contributions from non- electrostatic forces.
Fig.1: Variation of overall stability constant values of metal-LPA complexes with reciprocal of dielectric constant (1/D) of DMF-water mixtures (A) Pb(II); (B) Cd(II); (C) Hg(II); (■)log βML2; (▲) log βML3; (●) log βML2H.
Fig.2: Variation of overall stability constant values of metal-MA complexes with reciprocal of dielectric constant (1/D) of DMF-water mixtures (A) Pb(II); (B) Cd(II); (C) Hg(II); (■)log βML2; (▲) log βML3; (●) log βML2H.
Distribution diagrams
L-phenylalanine is a bidentate ligand that has one dissociable (carboxyl group) and one associable (amino) protons. The form of LPA that exist in the pH region of 1.6 - 11.5 is LH2+, LH and L-. Hence the possible metal-ligand species are ML2, ML2H, and ML3 which are confirmed by MINIQUAD75. The equilibria can be represented as follows:
M(II) + LH2+
|
|
MLH2+ + H+
|
(1)
|
MLH2+
|
|
ML+ + H+
|
(2)
|
M(II) + LH2+
|
|
ML+ + 2H+
|
(3)
|
M(II) + 2LH2+
|
|
ML2H22+ + 2H+
|
(4)
|
MLH2+ + LH2+
|
|
ML2H22+ + H+
|
(5)
|
ML2H22+
|
|
ML2H+ + H+
|
(6)
|
MLH2+ + LH
|
|
ML2H+ + H+
|
(7)
|
ML++ LH2+
|
|
ML2H+ + H+
|
(8)
|
ML2H+
|
|
ML2 + H+
|
(9)
|
ML++ LH
|
|
ML2+ H+
|
(10)
|
ML2+ LH
|
|
ML3- + H+
|
(11)
|
M(II) + 3LH2+
|
|
ML3H32+ + 3H+
|
(12)
|
MLH2++ 2LH2+
|
|
ML3H32+ + 2H+
|
(13)
|
ML3H32+
|
|
ML3H2++ H+
|
(14)
|
ML++ 2LH2+
|
|
ML3H2++ 2H+
|
(15)
|
ML3H2+
|
|
ML3H-+ H+
|
(16)
|
ML3H-
|
|
ML32-+ H+
|
(17)
|
M(II) + 2LH2+
|
|
ML2H+ + 3H+
|
(18)
|
M(II) + 2LH2+
|
|
ML2 + 4H+
|
(19)
|
M(II) + 3LH2+
|
|
ML3H2+ + 4H+
|
(20)
|
M(II) + 3LH2+
|
|
ML3H + 5H+
|
(21)
|
M(II) + 3LH2+
|
|
ML3- + 6H+
|
(22)
|
ML2H2
|
|
ML22- + 2H+
|
(23)
|
ML3H3
|
|
ML3H2- + 2H+
|
(24)
|
ML3H3
|
|
ML33-+ 3H+
|
(25)
|
Maleic acid is a bidentate ligand that has two dissociable (carboxyl groups) protons. The different forms of maleic acid are XH2, XH-, and X2- in the pH range < 4.0, 2.0–7.0 and > 5.0 respectively. Hence, the plausible binary metal-ligand complexes can be predicted from these data. The present investigation reveals the existence of MX2, MX2H, MX3 for Pb(II), Cd(II) and Hg(II).
The present study is confined to pH ranges 2.8-7.8 for Pb(II), 4.0-7.5 for Cd(II) and 4.2-6.5 for Hg(II). The formation of various maleic acid complex species is shown in the following equilibria.
M(II) + XH2
|
|
MXH+ + H+
|
(1)
|
MXH+
|
|
MX + H+
|
(2)
|
M(II) + XH2
|
|
MX + 2H+
|
(3)
|
M(II) + 2XH2
|
|
MX2H2 + 2H+
|
(4)
|
MX2H2
|
|
MX2H- + H+
|
(6)
|
MXH++ XH-
|
|
MX2H- + H+
|
(7)
|
MX+ XH2
|
|
MX2H- + H+
|
(8)
|
MX2H-
|
|
MX22- + H+
|
(9)
|
MX+ XH-
|
|
MX22- + H+
|
(10)
|
MX22-+XH-
|
|
MX34- + H+
|
(11)
|
M(II) + 3XH2
|
|
MX3H3- + 3H+
|
(12)
|
MXH++2XH2
|
|
MX3H3- + 2H+
|
(13)
|
MX3H3-
|
|
MX3H22-+ H+
|
(14)
|
MX+2XH2
|
|
MX3H22-+ 2H+
|
(15)
|
MX3H22-
|
|
MX3H3-+ H+
|
(16)
|
MX3H3-
|
|
MX34-+ H+
|
(17)
|
M(II) + 2XH2
|
|
MX2H- +3H+
|
(18)
|
M(II) + 2XH2
|
|
MX22- + 4H+
|
(19)
|
M(II) + 3XH2
|
|
MX3H22- + 4H+
|
(20)
|
M(II) + 3XH2
|
|
MX3H3- + 5H+
|
(21)
|
M(II) + 3XH2
|
|
MX34- + 6H+
|
(22)
|
MX2H2
|
|
MX22- + 2H+
|
(23)
|
MX3H3-
|
|
MX3H3- + 2H+
|
(24)
|
MX3H3-
|
|
MX34- + 3H+
|
(25)
|
Distribution diagrams were drawn for various complex species using the formation constants of the best-fit models as shown in Fig 3-4. These diagrams indicate that the percentage of ML species of Pb(II),Cd(II) and Hg(II) increases and then decreases with the increase of pH. Depending upon the active sites in the ligand and the nature of the metal ions, the structures were proposed for the species detected as shown in Fig 5-6.
Fig.3: Distribution diagrams of Pb- LPA in DMF-water mixtures % v/v.
(A) 10, (B) 20, (C) 30, (D) 40, and (E) 50.
Fig.4: Distribution diagrams of Cd-MA in DMF-water mixtures % v/v.
(A) 10, (B) 20, (C) 30, (D) 40, and (E) 50.
Amino nitrogen and carboxyl oxygen of L-phenylalanine participate in bonding with metal ions. This argument supports the structures of complexes proposed in Fig.5
Maleic acid acts as a bidentate ligand by using its two oxygen donor sites and the chelation results in highly stable seven membered rings (Fig.6). Octahedral structures are proposed to the complexes of all the metal ions.
Fig.5: Speculative structures of LPA complexes with Pb(II), Cd(II) and Hg(II).
where S is either solvent or water molecules.
Fig.6: Speculative structures of MA complexes with Pb(II), Cd(II) and Hg(II)
where S is either solvent or water molecules.
CONCLUSIONS
The common species formed due to interaction of LPA with the toxic metal ions ML2, ML2H, and ML3 in the DMF media and For MA complexes of Pb(II), Cd(II) and Hg(II) the species MX2, MX2H, MX3 are refined in DMF-water mixtures.
The linear variation of stability constants of LPA and MA complexes with the reciprocal of dielectric constant of DMF-water mixtures indicates the dominance of electrostatic forces over non-electrostatic forces in case of Pb(II) & Hg(II) and non-linear trend in case of Cd(II). A linear increasing trend with DMF content supports the predominance of the structure forming nature of DMF over its complexing ability in case of Pb(II), Cd(II) and Hg(II) indicates the dominance of electrostatic forces.
The order of ingredients in influencing the magnitudes of stability constants due to incorporation of errors in their concentrations is alkali > acid > ligand > metal.
At higher pH values, the high concentrations of chemical species indicate that the metals are more amenable for transportation at higher pH values.
| 