Synthesis of Metal Oxide Nanoparticles of Nickel(II), Vanadium(V) and Potentiometric Determination of the Stability Constants of Their Complexes with Histidine and Glutamine at Different Temperatures in Aqueous Solutions

The interaction of nanoparticles with biomolecules has attracted attention of many chemists and biochemist. The dimensional similarities of nanoparticles with biological molecules have lead to intensive research in this area. Nanostrucutred materials possess unique characteristics, namely, size and surface effects. Amino acids which are the building blocks of proteins are fascinating compounds. Among the amino acids, L-histidine is one the strongest metal coordinating ligand and plays an important role in the binding of metal ions by proteins. Glutamine is the most abundant amino acid in the body and is involved in more metabolic processes than any other amino acid. One feasible method for studying the interaction of metal ions with ligands of biological interest is through complexation. Martell et al. have been pioneers in this field. The interaction between metal ions and amino acids is of considerable attention as models for metal-protein reactions and models in a variety of biological systems. There have been different methods to study these kinds of interactions. Potentiometric titration is one of practical methods in calculating the stability constants. Amino acids (Fig. 1) also occupy a special place in the coordination chemistry of transition metal ions. In most of these complexes, amino acid ligands are bound to a metal center in a unidentate fashion by amino nitrogen. In acidic medium, where the amino group is protonated, the formation of oxygen-bound species is observed. It Synthesis of Metal Oxide Nanoparticles of Nickel(II), Vanadium(V) and Potentiometric Determination of the Stability Constants of Their Complexes with Histidine and Glutamine at Different Temperatures in Aqueous Solutions


INTRODUCTION
The interaction of nanoparticles with biomolecules has attracted attention of many chemists and biochemist. The dimensional similarities of nanoparticles with biological molecules have lead to intensive research in this area. Nanostrucutred materials possess unique characteristics, namely, size and surface effects. Amino acids which are the building blocks of proteins are fascinating compounds. Among the amino acids, L-histidine is one the strongest metal coordinating ligand and plays an important role in the binding of metal ions by proteins. Glutamine is the most abundant amino acid in the body and is involved in more metabolic processes than any other amino acid.
One feasible method for studying the interaction of metal ions with ligands of biological interest is through complexation. Martell et al. have been pioneers in this field [1][2][3][4] . The interaction between metal ions and amino acids is of considerable attention as models for metal-protein reactions and models in a variety of biological systems 5,6 . There have been different methods to study these kinds of interactions. Potentiometric titration is one of practical methods in calculating the stability constants. Amino acids (Fig. 1) also occupy a special place in the coordination chemistry of transition metal ions 7,8 . In most of these complexes, amino acid ligands are bound to a metal center in a unidentate fashion by amino nitrogen. In acidic medium, where the amino group is protonated, the formation of oxygen-bound species is observed. It has been shown that the imidazole group of histidine is the most important binding site for metal ions. In most of the enzymes with metal ions, only the functional groups of the side chains of the peptides coordinate to metal ions 9 . Glutamine has been studied extensively over the past fifteen years and has been shown to be useful in the treatment of serious illnesses, injury, trauma, burns and treatment-related side-effects of cancer as well as in wound healing for postoperative patients. Vanadium is a trace bio-element that plays an important role in several metabolic and mitogenic processes. The biological roles of vanadium in amino acids complexation and in smaller peptides are good example of such studies 10,11 . In this research work, first vanadium(V) and nickel(II) metal oxide nanopartices were synthesized, then histidine and glutamine complexes of metal ions of vanadium(V) and nickel(II) were investigated, their interactions with nanoparticles, were examined, stability constants of corresponding complexes and the thermodynamic parameters were calculated. The pH potentiometric titrations were performed using pH meter Schott, thermostat MLW16, glass cell, magnetic stirrer, micro-buret, X-ray diffraction spectrometer, transmission electron microscopy, IR, UV-VIS and FTIR spectrophotometers.
Synthesis of Vanadium(V) oxide nanoparticles: 3 g (0.037 mol) of vanadyl acetylacetonate, VO(C 5 H 7 O 2 ) 2 , was dissolved in 300 mL of deionized water, then concentrated nitric acid was added in drop wise (about 1 mL) to adjust the pH of the solution to 2. The solution was stirred at 50 ºC for 48 h and allowed the orange precipitate to form. The precipitate transferred into a round-bottom flask, 120 mL of methanol was added and aged for 24 h at room temperature, which caused a colour change from light to dark orange. After that, it was filtered off, washed with deionized water to remove the possible absorbed ions and also to reduce agglomeration. Finally it was dried in an oven at 90 ºC for 3 h 12 . The structure of product was confirmed by IR, X-ray diffraction patterns (Fig. 2) and TEM image (Fig. 3). where, d represents the average particle size, β stands for the full with at half height of the peaks λ, represents the wavelength of X-ray and θ the diffraction angle of the peak. The sample has been irradiated with X-rays at 26.04º, 31.35º, 50.56º and 61.19º. Nanocrystals of orthorhombic structure with size of 7-11 nm was obtained Synthesis of nickel oxide nanoparticles: In molar ratio of 1:4, 5.23 g of nickel(II) nitrate and 4.32 g urea was dissolved separately in beakers containing 30 mL of deionized water. Then the first solution was added to the second, stirred with magnetic bar for 2 h to mix thoroughly. After that the magnet was removed and the solution was placed in oil bath at temperature of 110 ºC and refluxed for 90 min. During this process a green precipitate was developed. Using a sinter-glass funnel the precipitate was filtered, the filtered was dried in vacuum and then it was placed in an oven at 400 ºC for 1 h, following, which it was left intact in vacuum oven for 1 h to calcinate. The product was calcinated NiO nanoparticles 13 . The structure of desired compound was confirmed by IR, X-ray diffraction patterns (Fig. 4) and TEM image (Fig. 5). Calculation of stability constant of metal-complexes: Here, histidine and glutamine act as a ligand. The stability of a complex is due to electrostatic forces between the metal ions and the ligand. In general, an amino acid forms zwitter ion at pH 7. The NH2 group accepts a proton and the COOH group loses a proton.
Disassociation of an amino acid (H2L + ) can be shown as: The deprotonated histidine and glutamine anion Lcould act as an interacting ligating species as follows: where, Kf1 and Kf2 are the overall stability constants of the complexes formed from the suggested reactions 14,15 .
We define as the average number of ligands bound per metal ion concentration.
It means that the inverse of [HL] at n = 0.5 gives K f1 and the inverse of [HL] at n = 1.5 produces K f2 .
To perform these calculations more precisely, we have developed a new software program that brings into account all possible existing species concentrations. The software displays a plot of pH reading versus the concentration of added standardized sodium hydroxide.
For calculating thermodynamic parameters we need to consider the following: The Gibb's free energy, ∆G, can be calculated from the equation below: The enthalpy change, ∆H and the entropy change, ∆S, were calculated using a temperature dependence method, where the plot of log K f versus 1/T produces a line with slope equals to -∆H/R and the intercept equals to ∆S/R.

Determination of stability constant of metal-complexes:
In a home-made glass reactor as a titrand, a 30.00 mL mixture solution, which was 1.000 × 10 -3 M with respect to the metal ion, 5.000 × 10 -3 M with respect to the amino acid and 0.0169 M with respect to the HClO 4 , was prepared. A flow of nitrogen gas was provided through solution. Next, potentiometric titration was conducted using standardized sodium hydroxide as a titrant and pH glass electrode as the working electrode. Reference electrode was a saturated calomel electrode. Our software program produces a scheme in which the pH at the intersection of the potentiometric titration curve with these two points will give K f1 and K f2 respectively (Fig. 6 ). In each potentiometric titration about 6.00 mL of the titrant was added. The experiment was repeated five times.

RESULTS AND DISCUSSION
By controlling temperature and in oxygen-free medium, potentiometric titrations were performed. The titration curve for each complex has three platforms i.e. three inflecion points. The first one is due to neutralization of extra HClO 4 in solution with titrant. In the second, the -COOH is titrated. And the third is due to -NH 3 + . It was found out that the stability constant values of glutamine complexes are larger than histidine. This is due to more steric hindrance of histidine with regard to glutamine. In all of the complexes, K f1 > K f2 is owing to electrostatic repulsions and steric hindrance among substituted ligands. These are important factors in reducing stepwise formation constants of the complexes 16 . Tables 1 and 2 listed log K f1 and log K f2 values for complexes of histidine and glutamine with vanadium and nickel bulk metal ions and with their nanoparticles respectively. With increasing temperature from 25 to 40 ºC, the stability constant values of respective complexes decreases, because the complex reactions are exothermic. Also, the stability constant of complexes of nanoparticles versus bulk metal ions have greater values. In nanoparticles there is an increase for ratio of surface area to volume which dominates the behaviour atoms in the surface in comparison with inner atoms. This phenomenon affects the properties of particles and causes the reactions to proceed faster. On the other hand ionic potential (q/r) which is an important parameter in formation of bound is raised, leading to an increase in the rate of reactions [17][18][19] .
KfV(V) -His < Kf V(V)-Gln KfNi(II)-His < KfNi(II)-Gln The enthalpy change values of complexes of Ni(II) with histidine and glutamine are smaller than that of V(V) as shown in Table-3, while the reverse trend has been observed for the  mentioned complexes with nanoparticles (Table-4). Increased change in degree of disorder is indicative of the spontaneity of the reaction. The Gibbs free energies of all complexes are negative indicating the spontaneity of the complex reactions.
In our experiments, all of enthalpy changes and entropy changes are positive, but ∆S values are greater than ∆H values, so ∆Gs have negative values (Table-5).  In UV-visible spectra of histidine complex with bulk vanadium metal ions (Fig. 9) it is observed that the spectra of the ligand have been concealed in complex spectra, a new peak appeared indicating the formation of the complex.
The TEM image of formed complex of glutamine with NiO nanoparticles has been shown in Fig. 10.

Conclusion
Potentiometric titration was performed to determine the stability constant of histidine and glutamine complexes with V2O5 and NiO nanoparticles and corresponding bulk metal ions. The results illustrated that stability constants values are greater for histidine complexes than glutamine in both cases of nanoparticles and bulk metal ions. Stability constants values for nanoparticles were not as high as we expected. There is a possibility that some kind of adsorption is involved. More research is in progress in our lab to understand the existing mechanism in the complex formation process for metal oxide nanoparticles. Moreover thermodynamics parameters for respective complexes have been calculated which are in agreement with related data in the literature.