DISCUSSION:
The qualitative analysis of proton-ligand,
metal-ligand (binary systems) and metal-ligand A-ligand B (ternary systems)
equilibria were done by examination of titration curves (Fig. 1.1 and 1.2).
Fig 1.1
Fig 1.2
Fig. 1. Representative pH vs. 'a' Curves
Where, Curve 1 represents Ligand 'A' (Tyr) Titration
Curve
Curve 2 represents Ligand 'B'
(Ala/Phe/Asp/Glu) Titration Curve
Curve 3 represents Metal-Ligand 'A' (1:1)
Titration Curve
Curve 4 represents Metal-Ligand 'B' (1:1)
Titration Curve
Curve 5 represents Mixed-Ligand (1:1:1)
Titration Curve
Curve 'T' represents Theoretical Composite
Curve
This examination reveals that nickel ion coordinates
simultaneously with both the ligands tyrosine and alanine/ phenylalanine.
Whereas, stepwise equilibria is observed in Ni(II)-tyrosine-aspartic
acid/glutamic acid systems. Aspartic acid/glutamic acid behaves as primary
ligand in respective systems.
The values of formation constants for
Ni(II)-tyrosine-alanine system are found to be greater as compared to those
obtained for Ni(II)-tyrosine-phenylalanine system (Table 2.1 and 2.2).
Speciation curves for these systems (Fig. 2.1 and 2.2)
show that the concentrations of NiAH and NiB increase in solution up to pH ≈ 7.5. Percentage concentrations of
NiABH and NiA increase above pH ≈ 7.5. Simultaneously, nonprotonated
ternary complex NiAB also come in existence and its concentration increases
appreciably, which is also supported by an inflection at ‘a’ ≈ 3.0 (pH ≈
9.8).
Fig 2.1
Fig 2.2
Where, Curve 1: [M]; 2: [MB]; 3 : [MAH]; 4 :
[MA]; 5 : [MABH];6 : [MAB]
Fig 2.3
Fig 2.4
Fig 2 Representative Speciation Curves
Where, Curve 1: [M]; 2: [MBH]; 3 : [MB]; 4 : [MABH2];
5: [MABH]; 6 : [MAB]; 7 : [MAH]; 8 : [MA]
Percentage concentration of nonprotonated NiAB complex
is found to be low in case of Ni(II)-tyrosine-phenylalanine system as compared
to Ni(II)-tyrosine-alanine system. But, this order is reversed as far as
percentage of NiA is concerned. This is attributed to the repulsion between
bulky side groups of two ligands in the former system as well as hydrophobic
nature of phenyl ring of phenylalanine, whose free ligand concentration
increases at higher pH.
In case of Ni(II)-tyrosine-aspartic acid/glutamic acid
systems, the values of formation constants are found to be greater for the
complexes involving aspartic acid due to the presence of additional – CH2 – group in glutamic acid
(Table 2.3 and 2.4).
Further, it is clearly seen in the speciation curve
(Fig. 2.3 and 2.4) that at lower pH (upto pH ≈ 5.0) NiBH type monoprotonated complex is
formed as major species above which concentrations of NiBH and free metal
decrease and those of NiABH2 and NiB increase upto pH ≈ 6.5
and pH ≈ 7.5 respectively. In NiABH2 complex metal is attached
with tyrosinate and HB (aspartate and glutamate) forms of ligand i.e.,
out of two dissociable protons one proton is attached to each ligand.
Afterwards, concentrations of NiABH2 and
NiB decrease and that of NiABH increases upto pH ≈ 8.5 showing inflection at ‘a’ ≈ 3.0. The
proton in NiABH complex is attached to the hydroxyl group of tyrosine. This is
evidenced by higher dissociation constant of phenolic proton of tyrosine and
formation constant of Ni(II)-tyrosine nonprotonated complex at higher pH as
compared to Ni(II)-aspartic acid/glutamic acid nonprotonated complex.
Above pH ≈ 8.5 concentration of NiABH decreases and that of NiAB
increases, which is clearly depicted in the percentage distribution curves.
Thermodynamic parameters ΔGo, ΔHo,
ΔSo for various systems (table 2.1-2.4) supports the formation
of mixed ligand species in solution with appreciably high thermodynamic
stability. These studies may be helpful in understanding in vitro
behavior of nickel and selected ligands.
ACKNOWLEDGEMENT:
Dr. Neerja Upadhyaya (Dwivedi) is thankful to Jiwaji
University, Gwalior, M.P, India for
the award of research scholarship for carrying this work. Thanks are also due
to Dr. Neelima Kulkarni, Prof., Dept. of Chemistry, M.S. University, Baroda,
Gujarat, India for extending help.
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