SYNTHESIS, CHARACTERIZATION, THEORETICAL STUDY AND BIOLOGICAL EVALUATION OF SCHIFF BASE AND THEIR La(III), Ce(IV) AND UO 2 (II) COMPLEXES

. A new three metal complexes of La(III), Ce(IV) and UO 2 (II) ions have been synthesized based on a Schiff base derived from the condensation of L-histidine and anisaldehyde. All prepared compounds were characterized by different spectroscopic techniques and Density-functional theory (DFT) calculations. The complexes were proposed to have an octahedral structure based on the investigated results. The optimized shape, numbering system, and dipole moment vector of Ligand and La, Ce, and UO 2 (1:1) chelates were investigated. The Schiff base ligand and complexes exhibit moderate action against all of the bacteria tested, with P. aeruginosa , Klebsiella sp. , and E. faecalis respectively being the order of inhibition.


INTRODUCTION
Amino acids and their derivatives are very useful ligands for coordination properties, and biological significance. Also, their biological uptake due to specific mechanisms of transport [1]. The L-histidine amino acid is a very interesting bioactive amino acid with multiple physiological functions. The transition metals play good roles in biological systems when binding to L-histidine amino acid has a major physicochemical role in several proteins, as the X-ray structural determination studies of metallo proteins like carbonic anhydrase, carboxypeptidase, plastocyanin, or azurin among others have demonstrated [2]. L-Histidine is an important to play a major role in the zinc metabolism acting as the major zinc-binding moiety in serum [3]. Schiff bases [4] formed when any primary amine reacts with an aldehyde or a ketone under specific conditions. Structurally, a Schiff base (also known as imine or azomethine). Several Schiff bases are reported to possess remarkable antibacterial, antifungal and anticancer activities. In such class of compounds, the C=N moiety is important for biological activity. The number of transition metal complexes have reported by using variety of Schiff base ligands and have studied their different biological activities such as antimicrobial, anticancer, antifungal. Metals have an esteemed place in medicinal chemistry [5]. Transition metal hydroxides of the late transition metals are now relatively common, and play a special role in both the synthesis of new complexes and in important catalytic reactions and many drugs possess modified pharmacological and toxicological properties when administered in the form of metallic

Synthesis of Schiff base ligand
Sodium hydroxide (0.40 g) in 30 mL ethanol and L-histidine (1.55 g, 1 mmol) in 20 mL of ethanol were mixed in a 500 mL flask and the mixture was stirred at room temperature. Anisaldehyde (1.36 g, 1mmol) was added to the concentrated mixture. Then 1 mL of acetic acid was added to cold mixture. The obtained product was filtered, washed and dried in desiccator over calcium chloride ( Figure 1). The White product yielded 80% of (C14H15N3O3) [

Synthesis of Schiff base metal complexes
The following Schiff base complexes with La(III), Ce(IV) and UO2(II) ions were synthesized by adding 2.73 g, 0.01 mol of the Schiff base (HL) in 25 mL of the absolute methanol to 0.01 mol (4.33, 4.04 and 4.29 g, respectively) of the metal salts (La, Ce and UO2). A few drops of 10% sodium hydroxide solution were added slowly to adjust the pH value at 8 and then the mixtures were refluxed for three hours. The mixtures were filtered and rinsed with hot ethanol multiple times until the filtrates were clear. The obtained new amino acid Schiff base complexes were dried in desiccator under calcium chloride.

Antibacterial activity tests
Three pathogenic bacteria species, including one species of Gram positive bacteria E. faecalis and two species of Gram negative bacteria Klebsiella sp. and P. aeruginosa were collected in the microbiology laboratory of Benghazi Medical Centre (BMC), Libya.
Mueller Hinton Agar (MHA) was prepared by suspending 38 g in 1000 mL of distilled water. The media was sterilized by autoclaving at 121 o C for 15 min. Cool to 45-50 o C, and then pour into sterile Petri plates.

Antibacterial activity Assay
The prepared compounds were tested against three pathogenic bacteria species E. faecalis, Klebsiella sp. and P. aeruginosa by the agar well diffusion method [8,9]. Tetracycline was used as the standard antibacterial agents. The Schiff base ligand was dissolved in DMF solvent. Petri plates were poured with (MHA) medium and allowed to solidify. The bacterial suspension of each test was evenly spread over the media by sterile cotton swabs. A sterile cork borer (6 mm in diameter) was then used to punch wells (4 wells) in the agar medium. Subsequently, wells were filled with 100 μL of ligand at concentrations of 50, 100 and 150 mg/mL and allowed to diffuse at room temperature for 30 min. The plates were incubated at 37 o C for 24 h. After the incubation the plates were observed for formation of clear inhibition zone around the well indicated the presence of antibacterial activity. The absence of a clear zone around the well was taken as inactivity [10,11] and the diameter of inhibition zone was measured with a ruler. The experiment was performed in triplicate for each bacteria and tested ligand and the mean zone of inhibition was calculated for each ligand and standard antibiotic.

RESULTS AND DISCUSSION
The analytical data and physical properties of the ligand and its complexes were studied. The data are consistent with the calculated results from the empirical formula of each compound. The analytical data of the complexes confirm the existence of 1:1 [M:L] ratio. The molar conductance measurements were carried out in DMF solution (10 -3 M). The obtained data reveal the presence of non-electrolytic nature for all complexes at room temperature have low values of 12-16 Ω -1 cm 2 mol -1 that suggest their non-electrolyte nature [12,13].

Characterization of compounds
The ligand and its metal complexes' infrared spectral data were examined. The infrared spectrum of the Schiff base shows a strong band located at 1581 cm −1 assigning to the ν(HC=N) stretching vibrations. This band shifted to lower frequencies 1573-1450 cm -1 in the spectra of the complexes indicating the participation of azomethine group in chelation through nitrogen atom [14]. The bands in the range of 3441-3536 cm -1 assigned to the existence of water molecules in the mixed ligand chelates. New bands in the range of 617 and 532 cm -1 which are not present in the free ligands are assigned to v(M-O) and v(M-N) vibrations and the appearance of these bands supports the involvement of oxygen and nitrogen atoms of the azomethine and OH groups of the free ligands in chelation process [15,16]. Infrared spectra data (cm -1 ) for prepared compounds:

Electronic study
The electronic spectrum of the Schiff base (HL) exhibits absorption bands at 283 nm and 331 nm corresponding to π→π * (phenyl ring) and n→π * transition, respectively [16]. The electronic absorption spectrum of La(III) chelate shows three bands at 260 nm, 283 nm and 317 nm. The magnetic moment value of this complex is (0.00 B.M.) indicates the presence of diamagnetic character. Based on these data, an octahedral geometry was suggested [17]. The electronic spectra of the cerium(IV) and UO2(II) complexes display weak bands in the range of 260-303 nm due to f-f transition as shown in Figure 3

Antibacterial activity
Antibacterial screening of the synthesized ligand was carried out using three against E. faecalis, Klebsiella sp. and P. aeruginosa using agar well diffusion method. In order to ensure that no effect of solvent a control test was performed with DMF and found inactive in the culture medium. Tetracycline is used as positive control. The antibacterial activity of the Schiff base ligand against pathogenic bacteria species as shown in Figure 7. The results show that the Schiff base ligand has moderate activity against all the tested bacteria and order of inhibition was found to be P. aeruginosa > Klebsiella sp. > E. faecalis. Tetracycline showed highest inhibitory effect on E. faecalis, and was moderate effect on Klebsiella sp., while was lowest effect P. aeruginosa. The lipid membrane surrounding the cell favor the passage of any lipid soluble materials and it is known that lipophilicity is an important factor that controls the antibacterial activity. This increase in lipophilic nature enhances the penetration of Schiff base into the lipid membranes and seizing the growth of the organism [26][27][28][29][30].

CONCLUSION
Three novel La(III), Ce(IV) and UO2(II) complexes have been synthesized and characterized using various physicochemical techniques. It has been spectroscopically demonstrated that the Schiff base ligand derived from L-histidine and anisaldehyde coordinated to La(III), Ce(IV) and UO2(II) ions producing an octahedral geometry. DFT studies were also performed to confirm their experimental results. In addition, the antibacterial activity of the Schiff base ligand was tested against some pathogenic bacteria species and the obtained results showed a good activity.

ACKNOWLEDGEMENT
The authors would like to thank their Universities for facilities.