Abstract
Different species of L-ascorbic acid and their corresponding complex formation ability with Cu(II) and Ni(II) metal ions in aqueous medium has been studied in the pH range from 2.0-12.5. The stability constants of different complexes of Cu(II) and Ni(II) with the bidentate ligand, L-ascorbic acid were determined theoretically using MINIQUARD software. Speciation of ligand and complex of Cu(II)/Ni(II) ascorbate were experimentally investigated by the titration method in solution within this pH range. Different Cu (II) and Ni(II)-L-ascorbic acid species percentages with variation of pH were calculated within the studied pH range with the help of another computer programs SIM and SPECIES. Different species distribution diagrams and the equilibria for the formation of the species were also investigated and at higher pH, ML2 species was found to be the major species in the case of both the metal complexes. All the theoretical possible structures of Cu(II) and Ni(II) complexes with L-ascorbic acid were optimized and square pyramidal and square planer geometry have been evaluated for Cu(II) and Ni(II) respectively by Gaussian09 software. Their corresponding HOMO-LUMO energy and reactivity parameters such as chemical hardness (ç), ionization potential (I), electron affinity (A), electro negativity (χ), chemical potential («), electrophilicity index (ω) have been calculated in order to provide a better understanding of the electronic structure of complexes with the experimental results. Keywords: Cu(II)and Ni(II) complexes, ascorbic acid, molecular speciation, DFT, reactivity parameters.
Highlights
Different metal ions play an important role[1] in our physiological system, in the presence of biologically relevant molecules such as vitamins, proteins, peptides and different enzymes
The species distribution diagram of metal ions Ni(II) and Cu(II) form different species[21,22] with L-ascorbic acid are shown in (Fig. 2C and Fig. 2B).The probable metal-ligand complexes are ML2, MLH+ and ML2H, (where, M = Cu(II)/ Ni(II) ions and L = L-ascorbic acid) which are shown in Figs 1 and 2
L-ascorbic acid exists as LH2, LH– and L2– species in aqueous medium and form metal chelates with Cu(II) and Ni(II) as MLH2, MLH and ML2 in the pH range 2.0–12.5
Summary
Different metal ions play an important role[1] in our physiological system, in the presence of biologically relevant molecules such as vitamins, proteins, peptides and different enzymes. These molecules exist in different structures and conformations at different pH. Ca(II), Fe(III), Cu(II), Ni(II) and Mg(II) govern the metabolic steps in our biological system.[2,3,4,5,6] Ascorbic acid is an essential micronutrient for several biochemical roles It acts as a cofactor for various enzymes involved in several processes like hydroxylation of collagen, conversion of dopamine to nor epinephrine and in tyrosine metabolism.[7,8] Ironically, ascorbic acid is a recognized pro-oxidant in vitro. The oral route of absorption for both is via different parts of the human gastrointestinal tract where, a variation of the pH of the micro-environment exists
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