Naringin [1,2] and antioxidant phenolates [3,4] have been the focus of attention in recent years due to their potential applications in biological, industrial, and medicinal processes. This work has studied the protonation and complexation equilibria of the antioxidant phenolate ligand with different metal ions [5,6]. A number of synthesized platinum(II) terpyridyl complexes of some phenolic ligands displayed a colorimetric response and fluorescence quenching in the presence of acetate, dihydrogen phosphate, and fluoride anions . The optimized molecular geometries of the ground and the lowest triplet states for these platinum(II) complexes of phenol and pyridine ligands were generated using time dependent density functional theory calculations . This computational study indicated a higher photoluminescence quantum efficiency of the studied complexes which can be easily controlled by adapting auxiliary ligands. Also, the introduction of fluorine ligand into these complexes can successfully increase the radiation transition rate, decrease the non-radiation d-d transition rate, resulting in a novel phosphorescent platinum(II)-fluorine complex appropriate for organic electronic devices .
Cyclic voltammetry, EPR spectroscopy, emission spectroscopy, and single crystallography studies of platinum(II)-catecholatocomplex indicated a significant change in the electronic spectra and luminescence properties of the diamagnetic complex . Also, its geometric structure was investigated using time dependent density functional theory calculation . The reactivities and the effect of ultra violet light irradiation of the synthesized hydroxyl phenyl platinum binuclear complex analogues toward nucleophiles  showed that the axial aryl- platinum bond was homolytically cleaved by UV-light to give the aryl radical and the platinum(III)-platinum(II) paramagnetic binuclear complex. Thermal stabilities of platinum clathrates including phenol and tetracyano complexes as guest molecules can be divided in two clathrates groups: those which release the guest molecules in the first step thermal decomposition, and those which lose the guest component only after partial destruction of the host cage . The temperature variations of loss of the guest constituent may control the interval for their usage in sorptive experiments as investigated in this study.
Single X-ray crystallography studies of the interaction between vanadium(V)-alkylidene complexes with phenol ligands  clearly suggested that the reaction of these alkylidene complexes with phenols proceeded via coordination of phenol. Subsequent reaction with the alkylidene was accompanied by phenoxy exchange on the vanadium or the alkyl moiety and resulted in high molecular weight polymer with a unimodal molecular weight distribution .The distorted square pyramidal and octahedral ternary complexes including vanadiumphenol-bipyridine or phenanthroline ligands , vanadium(V)-novel schiff base-phenol ligands , vanadium-cyclodextrins-heteropoly phenolic acid ligands  were also synthesized and well characterized recently using elemental analyses (EA), Fourier transform infra-red spectroscopy (FT-IR), ultraviolet-visible spectroscopy (UV-visible), thermal analysis (TA), and powder X-ray diffraction (XRD) techniques. The reaction chemistry study of these vanadium(V) complexes involving aryloxo ligands showed some remarkable activities for ethylene polymerization [16,17]. These vanadium(V)-alkyl complexes also showed unique reactivity toward phenolic ligands, and experimental results clearly suggested that the reaction proceeds through the coordination of phenol on the vanadium metal ion center . Cyclic and differential pulse voltammetry and X-ray crystallography studies of tetra- and binuclear heterometallic complexes of vanadium(IV/V) combinations involving a phenol-based primary ligand  showed constructed tetra-nuclear complexes with unprecedented hetero-metallic eight-membered.
In continuation of our recent research relating to the study of protonation, complexation equilibria and the determination of formation and molecular structure of biologically important binary, ternary, and mixed ligand complexes [20-32], this work investigates the synthesis and characterization of some platinum and vanadium binary and mixed ligand complexes of phenolic acids and naringin. Cytotoxic activities of the synthesized binary and ternary vanadium and platinum metal complexes of phenolic acids and naringin were tested and evaluated against HepG2 (human hepatocellular carcinoma), MCF-7 (human breast adenocarcinoma), and HCT116 (human colorectal carcinoma) tumor cell lines. Additionally, antioxidant activities of the synthesized complexes were examined using free radical scavenging assay.
2.1 Chemicals and Materials
All chemicals, solvents, and materials used in this work were of analytical reagent grade and were used without further purifications. Naringin (4′,5,7-Trihydroxyflavanone 7-rhamnoglucoside, ≥ 95% purity), ferulic acid (trans- 4-Hydroxy-3-methoxycinnamic acid, ≥99% purity), p-coumaric acid (trans-4-Hydroxycinnamic acid, ≥99% purity), caffeic acid (trans-3,4-Dihydroxycinnamic acid, ≥98% purity), vanillic acid (4-Hydroxy-3-methoxybenzoic acid, ≥97% purity), sinapic acid (trans-4-Hydroxy- 3,5-dimethoxy-cinnamic acid, ≥98% purity), vanillin (4-Hydroxy-3-methoxybenzaldehyde, ≥99% purity), pyrogallol (1,2,3-Trihydroxybenzene, ≥98%purity), catechol (1,2-Dihydroxybenzene, >99% purity), and gallic acid (3,4,5- Trihydroxybenzoic acid, ≥98% purity) were purchased from Sigma Aldrich (USA). Carbonate-free sodium hydroxide (NaOH, ≥98% purity) from Across Organics (USA) was standardized with potassium hydrogen phthalate (Sigma Aldrich, USA). Two metal salts; platinum chloride (PtCl2, ≥99.99% purity), and ammonium metavanadate (NH4VO3, ≥99.996% purity) purchased from Sigma Aldrich (USA) were weighed accurately before preparing the solutions.
2.2 Synthesis of binary and mixed ligand complexes
A series of platinum and vanadium binary and mixed ligand complexes involving naringin and phenolate ligands such as ferulic acid, vanillin, sinapic acid, caffeic acid, pyrogallol, and catechol were synthesized according to the following general procedure: A methanolic solution (20 mL) of metal salt (0.1 M PtCl2 or V(NH4)5) was added gradually to magnetically stirred methanolic solution (20 mL, 0.2 M) of naringin (NRG), and/or phenolic compound. Then, to the first reaction mixture, a methanol solution (20 mL) of naringin (0.2 M) or phenolic ligand (0.2 M) was added and stirred carefully and refluxed for 6 hours at temperature of about 185 °C, until the reaction was completed. Then, dehydration was done by putting the reaction product mixture in cupboard gases, leading to the isolation of precipitated solid complex.
Later the solid was filtered off, washed thoroughly with diethyl ether and ethanol mixture several times to remove any traces of unreacted starting materials and finally dried in a vacuum desiccator over fused CaCl2 (yield: 35-55%). For binary complexes, the preparation ratio was TL: TM = 2:1 where L = naringin (NRG) or Phen (phenolate ligands), M (Metal ion) = Pt(II) and V(V) metal ions; while for various mixed ligand complexes TNRG:TM:Tphen = 1:1:1. The order of mixing of solutions of different reagents was maintained strictly throughout the work. To the naringin solution vanadium/platinum metal ion solution was firstly added, then phenolate ligand was added to this binary solution and again kept for a few minutes to reach complete equilibrium and the synthesis was done as above.
2.3 Physical measurements of the synthesized complexes
Elemental analysis (carbon, hydrogen and nitrogen) of the mixed ligand complexes was performed on a ThermoScientific™ FLASH 2000Series CHNS/O Analyzerat Precision Instrumentation Center, College of Science, National Taiwan University, Taipei, Taiwan. Melting points of all the mixed ligand complexes were determined in open glass capillaries and were recorded on a Fisher Johns melting point apparatus. A digital Elico Conductivity Bridge meter (Model No. CM-180) was used to measure the molar conductance of the free ligands and the metal mixed ligand complexes in DMSO solution with a concentration of about 1 × 10−3 mol.dm-3 at room temperature, using a dip-type conductivity cell fitted with a platinum electrode. Magnetic susceptibility measurements of the powdered mixed ligand complexes were measured at room temperature with a Magway MSBMk1 magnetic susceptibility balance using Hg(Co(NCS)4) as the calibrant. Magnetic measurements were carried out according to the Gauy method. The calculations were evaluated by applying the following equations (1-3): (1) (2) (3)
Where x is the mass susceptibility per g of sample; c is the calibration constant; R is the balance reading for the sample and tube; R0 is the balance reading for the empty tube; M is the weight of the sample in g. The metal content of a complex was determined by a Buck Scientifics 210VGP atomic absorption spectrophotometer.
2.4 Spectral measurements of the synthesized complexes
Vibration infrared spectral (IR) studies of all synthesized mixed ligand complexes were recorded on a Shimadzu FT-IR 8000 spectrophotometer using KBr disc medium in the range 400–4000 cm−1 and the spectra were collected with a resolution of 2 cm−1 with 15 scans. The proton 1H NMR and 13C NMR spectra of the mixed ligand complexes were recorded on a JEOL GSX 400 MHz FT-NMR spectrometer employing TMS as internal reference to 0.0 ppm and DMSO-d6 as solvent with a field gradient operating at 500.13 MHz for proton observation, and the measurements were completed at a probe temperature of 25 °C. Fast atomic bombardment mass spectra (FAB-MS) of the ligands (naringin and phenolate ligands) and their metal complexes were recorded on a JEOL SX-102 FAB mass spectrometer using 3-nitrobenzoyl alcohol matrix. Scanning electron microscope (SEM) images were obtained using a Jeol Jem-1200 EX II Electron microscope at an acceleration voltage of 25 kV.
2.5 Bioactivity Evaluation of the synthesized complexes
Cytotoxic and antioxidant activities of the studied ligands and their platinum and vanadium synthesized complexes were in vitro assayed and quantitatively evaluated statistically using SPSS 17.0 statistical software package as in our recent published work based on the experimental measurements used for quantitative analysis that repeated at least three times.
3 Results and Discussions
3.1 Physical characterizations of synthesized complexes
Naringin (NRG) and phenolic bioligands (Phen = ferulic acid (FA), caffeic acid (CA), pyrogallol (PYG), catechol (CC), vanillin (VA), and sinapic acid (SA), as well as their Pt(II), and V(V) binary and mixed ligand complexes were subjected to elemental analysis. The physico-chemical analytical data of the elemental analysis (carbon, hydrogen, and oxygen percentages contents), the metal ion percentage with empirical molecular formula, colors, melting points, conductivities, and magnetic susceptibility, are summarized in Table 1.
The analytical and physical experimental data are in acceptable agreement with the calculated values as expected for the assigned empirical formula shown in Table 1.The elemental analytical data of the complexes show 1:2 molar ratio (Pt/V metal ion: NRG/Phen) for the binary complexes and 1:1:1 molar ratio (Pt/V metal ion: NRG: Phen) for the mixed ligand complex species. The platinum(II) metal binary and mixed ligand complexes involving naringin and different phenolic bioligands have melting points reported from 260 °C to 275 °C, while for binary and mixed ligand complexes of vanadium(V), naringin, and different phenolic ligands melting points are from 285 °C to more than 450 °C. The molar conductivity values for the studied Pt/V metal ion binary and mixed ligand complexes (1.0 × 10−3 mol/cm3) were found to be between 52 and 83 Ω−1cm2mol−1 suggesting a slightly electrolytic nature, and providing a method to test the degree of ionization of the complexes.
The observed decreasein the electrolytic nature of the mixed ligand complexes is due to the presence of chloride or nitrate ions inside the coordination sphere. These results were strongly supported by the elemental analysis data. The absence or presence of chloride ions inside or outside the coordination sphere of platinum(II) binary and mixed ligand complexes was detected by adding a few drops of saturated silver nitrate (AgNO3) reagent leading to the formation of white precipitate. The magnetic susceptibilities (μeff) of the platinum(II) and vanadium(V) binary and mixed ligand complexes (Table 1) at room temperature were found to be consistent with marked low-spin diamagnetism, which is in agreement with the structures proposed in Scheme 1.
3.2 FT-IR and NMR analysis of synthesized complexes
The elucidation of molecular structures of the synthesized Pt/V binary and mixed ligand complexes were confirmed by detailed spectroscopic IR and NMR techniques. The synthesized platinum(II) and vanadium (V) metal ions binary and mixed naringin/phenolic complexes were found to be stable at room temperature with different colors. They are partially soluble in D2O, soluble in DMSO and DMF solvents. The proposed coordination modes of the synthesized complexes depend on the routine spectral analysis. The essential infrared spectral absorption patterns of these mixed ligand complexes are shown in Supplementary Data (S1-S20). Careful inspection of the IR spectra of free ligands naringin (NRG), catechol (CC), pyrogallol (PYG), ferulic acid (FA), sinapic acid (SA) and their binary and mixed platinum(II)/vanadium(V) ligand complexes was made in order to facilitate the assignment of these bands in the free ligand and its metal binary and mixed ligand complexes.
The strong-to-broad bands for all synthesized complexes existed in the region ca. 2900–3300 cm−1 were assigned to the δ(OH) vibrations of the phenolic ring in all metal complexes. The stretching band of the carbonyl group, vs(C=O) in the free ligands was observed at 1718 cm−1, and located in the range 1698 and 1718 cm−1 for different Pt(II) and V(V) binary and mixed ligand complexes. The binding of carbonyl compounds towards metal ions resulted in a significant blue shift in the frequency of the carbonyl group. In most phenolic Pt/V complexes the stretching vibration band of the carbonyl group has no significant blue shift. The low intensity bands in the region of 550–400 cm−1 are assigned to Pt(II) and V(V)-oxygen bond vibrations. Strong absorption band at 1650 cm−1 is due to the stretching vibration of ν(C=O) of free ketonic of the carboxylic group.
This group shifted or disappeared in the spectra of its platinum and vanadium complexes (Supplementary Data SI-1 to SI-20). Interestingly, there are two bands which appeared at the range of 1500–1580 cm−1 which corresponds to Vas(COO−) and the other band in the range of 1485-1550 cm−1 is assigned to Vas(COO−).The direction of frequency shifts of Vas(COO−) and Vas(COO−) bands with respect to those of free ion depends on the coordination mode of the COO− group with the metal ion. Upon complexation, these strong bands were shifted and broadened with respect to the corresponding band in the free ligand. The present bands of the carboxylate COO− group are reflected by IR spectrum of the asymmetric (Vas) and symmetric (Vs) stretching vibrations. 1H NMR and 13C NMR spectra of free ligands naringin (NRG), catechol (CC), pyrogallol (PYG), ferulic acid (FA), sinapic acid (SA) and their binary and mixed platinum(II)/vanadium(V) ligand complexes could be found in Supplementary Data (SI-21-SI-37). The nuclear magnetic resonance spectral data were recorded in DMSO-d6 for all complexes, using TMS as an internal standard, are shown in Supplementary Data (SI-21-SI-37). From the proton and carbon assignments for free ligands and their mixed ligand complexes, the comparison between peaks of free ligands and their complexes clearly show that there are blue shifts in the assignments values of protons and carbons because of the mixed ligand complex formation (Scheme 1).
3.3 SEM analysis of synthesized complexes
SEM micrographs at various magnifications show the layered structure, surface morphology, and microstructure of PtNRG2, PtSA2, PtCA2, PtNRGCA, PtNRGSA, VNRG2, VSA2, VCA2, VNRGCA, and VNRGSA complexes (Figure 1). The SEM micrograph reveals the well sintered nature of the complexes with various grain sizes and shapes. The distribution of grain size is homogeneous for most complexes, and clearly show that very small grains were obtained with agglomerates for Pt(II) complex. The particle size distribution of all complexes was evaluated and the average particle sizes of these were observed.
3.4 Bioactivities Evaluation of synthesized complexes
A number of previous in vivo and in vitro studies showed that naringin possesses many pharmacological effects. It can inhibit drug-metabolizing cytochromeP450 enzymes, which may result in drug-drug interactions . Also, it can affect the intestinal absorption of certain drugs, leading to increase or decrease in circulating drug levels. In addition, it can be used as an inhibitor of vascular endothelial growth factor release, and to reduce diabetes-induced neuropathy in rats, and it showed protective effects against cognitive dysfunction and oxidative damage in rats [32,33]. In the same manner, almost all phenolate ligands have significant biological applications . In the present work, we examined the cytotoxicity of naringin (NRG), ferulic acid (FA), pyrogallol (PYG), catechol (CC), caffeic acid (CC), vanillin (VA), and sinapic acid (SA) and their binary and ternary complexes with platinum(II) and vanadium(V) metal ions against HepG2 (human hepatocellular carcinoma), MCF-7 (human breast adeno carcinoma), and HCT116 (human colorectal carcinoma) tumor cell lines. These results are summarized in Table 2 and Figure 2.
It is possible to conclude that the nature of naringin and phenolic ligands, the characteristics of the leaving groups at some metal ions, the number and coordination mode of the metal ions, and their chemical environment, determine the bioactivities of the complexes studied under physiological conditions, most probably through induction of DNA structural rearrangements. Thus, the design of new, more effective bioactive drugs should be governed by these crucial factors, since slight changes in the metal coordination are sufficient to significantly change the in vitro antiproliferative and/ or cytotoxic properties of these complexes.
Among the ligands and complex species tested against the three human cancer cell lines, i.e.HepG2 (human hepatocellular carcinoma), MCF-7 (humen breast adenocarcinoma), and HCT116 (human colorectal carcinoma) tumor cell lines, it was observed that: Catechol (CC), pyrogallol (PYG), ferulic acid (FA) caffeic acid (CA), and vanillin phenolic ligands, as well as the complexes (VCA2) and (VVA2) showed very strong cytotoxic activities against the MCF-7 (human breast adenocarcinoma) cancer cell. Sinapic acid (SA),(PtNRG2), (VFA2), (PtNRGCA), and (PtNRGFA) complexes showed significant cytotoxic activities against the MCF-7 cancer cell. The complexes (PtCC2) and (PtNRGSP) showed moderate cytotoxic activities against the MCF-7 cancer cell. Naringin (NRG) and (PtCA2), (PtSA2), (PtNRGCA), (VNRG2), (VSP2), and (VNRGSP) complexes showed weak cytotoxic activities against the MCF-7 cancer cell. The complexes (PtCC2), (PtNRGCA), (VFA2), (VCA2) and (VVA2) showed very strong cytotoxic activities against the HCT116 (human colorectal carcinoma) tumor cell line. Only ferulic acid (FA) showed significant cytotoxic activities against the HCT116 tumor cell line with Naringin (NRG), vanillin (VA), and (VSP2) complexes showing moderate cytotoxic activities. Catechol (CC), pyrogallol (PYG), caffeic acid (CA), and sinapic acid (SA), phenolic ligands and (PtNRG2), (PtSA2), (PtCA2), (PtNRGSP), (VNRG2), (VNRGFA), (VNRGCA), and (VNRGSP) complexes showed weak cytotoxic activities against the HCT116 tumor cell line. Ferulic acid (FA), and vanillin (VA) phenolic ligands, as well as the complexes (PtCC2), (PtNRG2), and (VVA2) showed very strong cytotoxic activities against the HepG2 (human hepatocellular carcinoma). (VCA2) and (VNRGFA) complexes showed significant cytotoxic activities against HepG2. The complexes (VSP2), (VNRG2), (VNRGCA) and (VNRGSP) showed moderate cytotoxic activities against HepG2. Naringin (NRG), catechol (CC), pyrogallol (PYG), sinapic acid (SA), caffeic acid (CA) phenolic ligands and (PtCA2), (PtSA2), (PtNRGCA), (VFA2), and (PtNRGSP) complexes showed weak activities against HepG2.
Based on the experimental data, metal complexes of naringin and phenolic acids showed highest cytotoxicity against studied cancer cell lines, followed by phenolic ligands and naringin. These experimental studies are in agreement with other studies showing weak inhibition of naringin and other phenolic acids [35-37].
Also, it is known that naringin (NRG) possesses numerous pharmacological and therapeutic applications such as anti-inflammatory, anti-carcinogenic, lipid-lowering, free radical scavenging and antioxidant effects [38-40]. Also, metal ions play a crucial role in various enzymes that catalyze oxidation/reduction reactions related to the antioxidant system of the organism concerned. However, different behaviors depend on the chemical environment and the nature of chelating agent. In the present work, naringin, phenolic ligands and their platinum(II) and vanadium(V) metal complexes were subjected to DPPH radical scavenging measurements. By looking at the data listed in Table 3 and Figure 3, we can conclude that bioligands pyrogallol (PYG), sinapic acid (SA), ferulic acid (FA), and binary and ternary complexes (PtNRG2), (PtCC2), (PtCA2), (PtSA2), (VFA2), (VVA2), (PtNRGCA), (PtNRGSA) displayed strong antioxidant activities. Bioligands naringin (NRG), catechol (CC), caffeic acid (CA), and vanillin (VA) and binary and ternary complexes (VNRG2), (VCA2), (VSA2), (VNRGFA2), (VNRGCA), and (VNRGSA) displayed moderate antioxidant activities.
Based on the natural biological activities of naringin and phenolic bioligands and their stability in forming binary, ternary, and mixed ligand complexes with different transition metal ions, the ligands studied in this work are proven to have potential use as chelating agents. Selection of a suitable ligand for metal complexation also depends on the biological pH at which complexation is likely to occur. It is claimed that platinum(II) complexes have been prepared, and the results as well line drawings of the products showed four-coordinate square-planar and low-spin diamagnetic species. Magnetic susceptibility data for a number of compounds reported as vanadium (V) complexes show marked low-spin diamagnetism, which is in agreement with the structures proposed in Scheme 1. This can be established by temperature-dependence of magnetic susceptibility measurements, EPR spectroscopy in combination with single crystal and powder X-ray diffraction data. In addition, from charge balance point of view one ligand coordinated to vanadium(V) must be monodeprotonated to explain the overall charge “zero” for these complexes. Naringin is known to be a main constituent of bioflavonoids found in several fruits so that one can merely take it as food additives. The present biological results suggested that naringin and its metal complexes are a useful compounds having antioxidant activity. However, it has a less cytotoxic activity in comparison with its binary and mixed ligand complexes.
IR, 1H, and 13C NMR spectrual data of the synthesized complexes were supplied from SI1 to SI37 in a separate file attached at the end of this article.
The work done in this paper was financially supported by King Abdulaziz City for Science and Technology (KACST), Saudi Arabia, through the project number MS_34_63.
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About the article
Published Online: 2017-08-10
Citation Information: Open Chemistry, Volume 15, Issue 1, Pages 189–199, ISSN (Online) 2391-5420, DOI: https://doi.org/10.1515/chem-2017-0022.
© 2017 Ahmed E. Fazary et al.. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. BY-NC-ND 3.0