Synthesis and antitumor activities of novel α-aminophosphonate derivatives containing an alizarin moiety

Progress in phosphorylation of natural products

Natural medicines are a valuable resource for the development of new drugs. However, factors such as low solubility and poor bioavailability of certain constituents have hindered their efficacy and potential as pharmaceuticals. Structural modification of natural products has emerged as an important research area for drug development. Phosphorylation groups, as crucial endogenous active groups, have been extensively utilized for structural modification and development of new drugs based on natural molecules. Incorporating phosphate groups into natural molecules not only enhances their stability, bioavailability, and pharmacological properties, but also improves their biological activity by altering their charge, hydrogen bonding, and spatial structure. This review summarizes the phosphorylation mechanism, modification approaches, and biological activity enhancement of natural medicines. Notably, compounds such as polysaccharides, flavonoids, terpenoids, anthraquinones, and coumarins exhibit increased antioxidation, anticancer, antiviral, immune regulatory, Antiaging, enzyme inhibition, bacteriostasis, liver protection, and lipid-lowering effects following phosphorylation modification.

β-Carboline-α-aminophosphonate Derivative: A Promising Antitumor Agent for Breast Cancer Treatment

Breast cancer is the most common type of cancer and the leading cause of cancer mortality among women worldwide. Considering the limitations of the current treatments available, we analyzed the in vitro cytotoxic potential of ((4-Fluoro-phenyl)-{2-[(1-phenyl-9H-β-carboline-3-carbonyl)-amino]-ethylamino}-methyl)-phosphonic acid dibutyl ester (BCP-1) in breast cancer cells (MCF-7 and MDA-MB-231) and in a non-tumor breast cell line (MCF-10A). BCP-1 has an α-aminophosphonate unit linked to the β-carboline nucleus, and the literature indicates that compounds of these classes have high biological potential. In the present study, the mechanism of action of BCP-1 was investigated through methods of spectrofluorimetry, flow cytometry, and protein expression analysis. It was found that BCP-1 inhibited the proliferation of both cancer cell lines. Furthermore, it induced oxidative stress and cell cycle arrest in G2/M. Upregulation of apoptosis-related proteins such as Bax, cytochrome C, and caspases, as well as a decrease in the anti-apoptotic protein Bcl-2, indicated potential induction of apoptosis in the MDA-MB-231 cells. While in MCF-7 cells, BCP-1 activated the autophagic death pathway, which was demonstrated by an increase in autophagic vacuoles and acidic organelles, in addition to increased expression of LC3I/LC3II and reduced SQSTM1/p62 expression. Further, BCP-1 demonstrated antimetastatic potential by reducing MMP-9 expression and cell migration in both breast cancer cell lines. In conclusion, BCP-1 is a promising candidate for breast cancer chemotherapy.

Anticancer activity, DFT study, ADMET prediction, and molecular docking of novel α-sulfamidophosphonates

A series of novel α-sulfamidophosphonate derivatives (3a-3 g) were synthesized and evaluated for anticancer activity against different human cancer cell lines (PRI, K562, and JURKAT). The antitumor activity of all compounds using the MTT test remains moderate compared to the standard drug chlorambucil. Compounds 3c and 3 g were found to be more active anticancer agent against PRI and K562 cells with IC50 value 0.056-0.097 and 0.182-0.133 mM, respectively. Molecular docking study related to binding affinity and binding mode analysis showed that synthesized compounds had potential to inhibit glutamate carboxypeptidase II (GCPII). Furthermore, computational analysis was performed through Density Functional Theory (DFT) utilizing the B3LYP 6-31 G (d, p) basis set and the theoretical results were correlated with experimental data. The ADME/toxicity analyses carried out by Swiss ADME and OSIRIS software show that all synthesized molecules exhibited good pharmacokinetics, bioavailability, and had no toxicity profile.

Alizarin: Prospects and sustainability for food safety and quality monitoring applications

Active and intelligent food packaging has emerged to ensure food safety, quality, or spoilage monitoring and extend the shelf life of food. The development of intelligent packaging has accelerated significantly in recent years with a focus on monitoring changes in the quality of packaged products in real-time throughout the food supply chain. As one of the popular natural colorants, alizarin has attracted much consideration due to its excellent functional properties and quality to color change under varying pH. Alizarin is an efficient and cost-effective biomaterial with numerous biological features such as antioxidant, antibacterial, non-cytotoxic, and antitumor. This review focuses on an in-depth summary and prospects for alizarin as a natural and safe colorant that has the potential to be incorporated into intelligent packaging to track the freshness of packaged foodstuffs. The use of alizarin as an intelligent packaging agent shows huge potential for the application of food packaging and brings it one step closer to real-time monitoring of food quality throughout the supply chain. Finally, various limitations and future requirements are discussed to underscore the importance of developing alizarin-based intelligent functional food packaging systems.

Comparative analysis of an anthraquinone and chalcone derivatives-based virtual combinatorial library. A cheminformatics "proof-of-concept" study

A Laplacian scoring algorithm for gene selection and the Gini coefficient to identify the genes whose expression varied least across a large set of samples were the state-of-the-art methods used here. These methods have not been trialed for their feasibility in cheminformatics. This was a maiden attempt to investigate a complete comparative analysis of an anthraquinone and chalcone derivatives-based virtual combinatorial library. This computational "proof-of-concept" study illustrated the combinatorial approach used to explain how the structure of the selected natural products (NPs) undergoes molecular diversity analysis. A virtual combinatorial library (1.6 M) based on 20 anthraquinones and 24 chalcones was enumerated. The resulting compounds were optimized to the near drug-likeness properties, and the physicochemical descriptors were calculated for all datasets including FDA, Non-FDA, and NPs from ZINC 15. UMAP and PCA were applied to compare and represent the chemical space coverage of each dataset. Subsequently, the Laplacian score and Gini coefficient were applied to delineate feature selection and selectivity among properties, respectively. Finally, we demonstrated the diversity between the datasets by employing Murcko's and the central scaffolds systems, calculating three fingerprint descriptors and analyzing their diversity by PCA and SOM. The optimized enumeration resulted in 1,610,268 compounds with NP-Likeness, and synthetic feasibility mean scores close to FDA, Non-FDA, and NPs datasets. The overlap between the chemical space of the 1.6 M database was more prominent than with the NPs dataset. A Laplacian score prioritized NP-likeness and hydrogen bond acceptor properties (1.0 and 0.923), respectively, while the Gini coefficient showed that all properties have selective effects on datasets (0.81-0.93). Scaffold and fingerprint diversity indicated that the descending order for the tested datasets was FDA, Non-FDA, NPs and 1.6 M. Virtual combinatorial libraries based on NPs can be considered as a source of the combinatorial compound with NP-likeness properties. Furthermore, measuring molecular diversity is supposed to be performed by different methods to allow for comparison and better judgment.

New Borane-Protected Derivatives of α-Aminophosphonous Acid as Anti-Osteosarcoma Agents: ADME Analysis and Molecular Modeling, In Vitro Studies on Anti-Cancer Activities, and NEP Inhibition as a Possible Mechanism of Anti-Proliferative Activity

Many organophosphorus compounds (OPs), especially various α-aminophosphonates, exhibit anti-cancer activities. They act, among others, as inhibitors of the proteases implicated in cancerogenesis. Thesetypes of inhibitors weredescribed, e.g., for neutral endopeptidase (NEP) expressed in different cancer cells, including osteosarcoma (OS). The aim of the present study isto evaluate new borane-protected derivatives of phosphonous acid (compounds 1–7) in terms of their drug-likeness properties, anti-osteosarcoma activities in vitro (against HOS and Saos-2 cells), and use as potential NEP inhibitors. The results revealed that all tested compounds exhibited the physicochemical and ADME properties typical for small-molecule drugs. However, compound 4 did not show capability of blood–brain barrier penetration (Lipiński and Veber rules;SwissAdme tool). Moreover, the α-aminophosphonite-boranes (compounds 4–7) exhibited stronger anti-proliferative activity against OS cells than the other phosphonous acid-borane derivatives (compounds 1–3),especially regarding HOS cells (MTT assay). The most promising compounds 4 and 6 induced apoptosis through the activation of caspase 3 and/or cell cycle arrest at the G₂ phase (flow cytometry). Compound 4 inhibited the migration and invasiveness of highly aggressive HOS cells (wound/transwell and BME-coated transwell assays, respectively). Additionally, compound 4 and, to a lesser extent, compound 6 inhibited NEP activity (fluorometric assay). This activity of compound 4 was involved in its anti-proliferative potential (BrdU assay). The present study shows that compound 4 can be considered a potential anti-osteosarcoma agent and a scaffold for the development of new NEP inhibitors.

Discovery of Novel α-Aminophosphonates with Hydrazone as Potential Antiviral Agents Combined With Active Fragment and Molecular Docking

A series of novel α-aminophosphonate derivatives containing hydrazone were designed and synthesized based on active fragments. Bioassay results demonstrated that title compounds possessed good activities against tobacco mosaic virus. Among them, compounds 6a, 6g, 6i, and 6j were equivalent to the commercial antiviral agents like dufulin. On structure optimization-based molecular docking, compound 6k was synthesized and displayed excellent activity with values of 65.1% curative activity, 74.3% protective activity, and 94.3% inactivation activity, which were significantly superior to the commercial antiviral agents dufulin and ningnanmycin. Therefore, this study indicated that new lead compounds could be developed by adopting a joint strategy with active fragments and molecular docking.

Synthesis and Anti-Pancreatic Cancer Activity Studies of Novel 3-Amino-2-hydroxybenzofused 2-Phospha-γ-lactones

A series of 3-amino-2-hydroxybenzofused 2-phosphalactones (4a–l) has been synthesized from the Kabachnik–Fields reaction via a facile route from a one-pot three-component reaction of diphenylphosphite with various 2-hydroxybenzaldehyes and heterocyclic amines in a new way of expansion. The in vitro anti-cell proliferation studies by MTT assay have revealed them as potential Panc-1, Miapaca-2, and BxPC-3 pancreatic cell growth inhibitors, and the same is supported by molecular docking, QSAR, and ADMET studies. The MTT assay of their SAHA derivatives against the same cell lines evidenced them as potential HDAC inhibitors and identified 4a, 4b, and 4k substituted with 1,3-thiazol, 1,3,4-thiadiazol, and 5-sulfanyl-1,3,4-thiadiazol moieties on phenyl and diethylamino phenyl rings as potential ones. Additionally, the flow cytometric analyses of 4a, 4b, and 4k against BxPC-3 cells revealed compound 4k as a lead compound that arrests the S phase cell cycle growth at low micromolar concentrations. The ADMET properties have ascertained their inherent pharmacokinetic potentiality, and the wholesome results prompted us to report it as the first study on anti-pancreatic cancer activity of cyclic α-aminophosphonates. Ultimately, this study serves as a good contribution to update the existing knowledge on the anticancer organophosphorus heterocyclic compounds and elevates the scope for generation of new anticancer drugs. Further, the studies like QSAR, drug properties, toxicity risks, and bioactivity scores predicted for them have ascertained the synthesized compounds as newer and potential drug candidates. Hence, this study had augmented the array of α-aminophosphonates by adding a new collection of 3-amino-2-hydroxybenzofused 2-phosphalactones, a class of cyclic α-aminophosphonates, to it, which proved them as potential anti-pancreatic cancer agents.

Novel α-Aminophosphonates of imatinib Intermediate: Synthesis, anticancer Activity, human Abl tyrosine kinase Inhibition, ADME and toxicity prediction

An efficient method for the synthesis of a new class of α-aminophosphonates of imatinib derivative has been developed in one-pot Kabachnik-Fields reaction of N-(5-amino-2-methyl phenyl)-4-(3-pyridyl)-2-pyrimidine amine with various aldehydes and diethyl phosphite under microwave irradiation and neat conditions using NiO nanoparticles as an reusable and heterogeneous catalyst, with 96% yield at 450 W within 15 min. All the compounds were evaluated for their in vitro cytotoxicity with various cancer cell lines by MTT assay method. Compounds with halo (4f, -4Br, IC₅₀ = 1.068 ± 0.88 µM to 2.033 ± 0.97 µM), nitro substitution (4 h, -3NO₂, IC₅₀ = 1.380 ± 0.94 µM to 2.213 ± 0.64 µM), (4 g, -4NO2, IC₅₀ = 1.402 ± 0.79 µM to 2.335 ± 0.73 µM) and (4i, 4-Cl, 3-NO₂, IC₅₀ = 1.437 ± 0.92 µM to 2.558 ± 0.76 µM) were showed better anticancer activity when compared with standard drugs Doxorubicin and Imatinib using MTT assay method. Further in silico target hunting reveals the anticancer activity of the designed compounds by inhibiting human ABL tyrosine kinase and all the designed compounds have shown significant drug-like characteristics.

Indium promoted C(sp3)–P bond formation by the Domino A3-coupling method – a combined experimental and computational study

An efficient catalytic process for the synthesis of α-aminophosphonates is developed by a one-pot three-component reaction in the presence of In complexes at room temperature. DFT based mechanistic studies of the catalytic reactions are reported.

Synthesis of α-Aminophosphonates via Phosphonylation of an Aryne-Imine Adduct

Multicomponent phosphonylation is accomplished upon the reaction of an imine with an aryne generated in situ in the presence of a dialkyl phosphite. This transition-metal-free protocol shows a broad substrate scope, providing a variety of α-aminophosphonates in moderate to good yields. A plausible mechanism for the reaction is proposed based on a deuterium exchange experiment.

Design, synthesis and biological evaluation of novel α-aminophosphonate oxadiazoles via optimized iron triflate catalyzed reaction as apoptotic inducers

α-aminophosphonate oxadiazoles (5a-m) were prepared in high yields by reacting of 1,3,4-oxadiazole acetohydrazide (3) with appropriate aldehydes and diethyl phosphite under Kabachnik-Fields conditions using Iron triflate as a catalyst. The reaction conditions were optimized using D-optimal experimental design. Possible reaction mechanisms were considered, and structures of the new products were based upon compatible elementary and spectroscopic evidence. In vitro antitumor activities of these compounds were evaluated against human cancer cell lines of colon (HCT116), breast (MCF7) and liver (HepG2) and compared with anticancer drug, Doxorubicin, employing standard MTT assay. Compounds 5i and 5l demonstrated good antiproliferative activities against HCT116 tumor cells comparable to doxorubicin with low cytotoxicity towards normal fetal colon cell (FHC). Additionally, their capacity to activate apoptosis cascade was studied in HCT116 cell line by investigating the activation of proteolytic caspases cascade, the levels of Cytochrome C, Bax and Bcl-2. Active caspase-3 level was enhanced by 6-8-folds in HCT116 cell line when stimulated with compounds 5i and 5l compared to the control. The level of Caspases 8 & 9 was also increased signifying that intrinsic and extrinsic pathways are both activated. They also induced Bax and down regulated Bcl-2 protein level in addition to over-expressing Cytochrome C level in HCT116 cell line. Also, HCT116 cell cycle was mainly arrested at the Pre-G1 and G2/M phases when treated with compounds 5i and 5l.

Phosphorus-Based α-Amino Acid Mimetic for Enhanced Flame-Retardant Properties in an Epoxy Resin

This work demonstrates the introduction of a phosphonate moiety into a commonly used curing agent, 4,4′-diaminodiphenylmethane (DDM), via an α-aminophosphonate. This compound (DDMP) can be prepared and isolated in analytical purity in under 1h and in good yield (71%). Thermoset polymer (epoxy-derived) samples were prepared using a room-temperature standard cure (SC) and a post-cured (PC) protocol to encourage incorporation of the α-aminophosphonate into the polymer network, with improved flammability properties observed for the latter. Thermogravimetric analysis under a nitrogen atmosphere showed increased char yield at 600°C, and similar observations were made when analysis was conducted in air. Significant reductions in flammability are observed at very low phosphorus content (P%=0.16–0.49%), demonstrated by higher char yields (25.5 from 14.0% in air), decreased burn time from ignition (60 to 24s), and decreased mass loss after ignition (87.6 to 58.5%). Limiting Oxygen Index for the neat polymer (P%=0%, 20.3±0.8%) increased with increasing α-aminophosphonate additive (P%=0.16%, 20.8±0.6%; P%=0.32%, 21.4±0.4%; P%=0.49%, 22.6±0.8%).

Design, synthesis and pharmacological evaluation of new 3-(1H-benzimidazol-2-yl)quinolin-2(1H)-one derivatives as potential antitumor agents

A series of new 3-(1H-benzimidazol-2-yl)quinolin-2(1H)-one derivatives (5a₁-5d₆) were designed and synthesized as antitumor agents. In vitro antitumor assay results showed that some compounds exhibited moderate to high inhibitory activity against HepG2, SK-OV-3, NCI-H460 and BEL-7404 tumor cell lines, and most compounds exhibited much lower cytotoxicity against the HL-7702 normal cell line compared to 5-FU and cisplatin. In vivo antitumor assay results demonstrated that 5a₃ exhibited effective inhibition on tumor growth in the NCI-H460 xenograft mouse model and that 5d₃ displayed excellent antiproliferative activity in the BEL-7402 xenograft model. These results suggested that both 5a₃ and 5d₃ could be used as anticancer drug candidates. Mechanistic studies suggested that compounds 5a₃ and 5d₃ exerted their antitumor activity by up-regulation of Bax, intracellular Ca²⁺ release, ROS generation, downregulation of Bcl-2, activation of caspase-9 and caspase-3 and subsequent cleavage of PARP, inhibition of CDK activity and activation of the p53 protein.

Design, synthesis and pharmacological evaluation of novel 2-chloro-3-(1H-benzo[d]imidazol-2-yl)quinoline derivatives as antitumor agents: in vitro and in vivo antitumor activity, cell cycle arrest and apoptotic response

A series of novel 2-chloro-3-(1H-benzo[d]imidazol-2-yl)quinoline derivatives were designed and synthesized as antitumor agents under the combination principle. The antitumor activity and mechanisms were then evaluated.

Design, synthesis and pharmacological evaluation of new 2-oxo-quinoline derivatives containing α-aminophosphonates as potential antitumor agents

A series of novel 2-oxo-quinoline derivatives containing α-aminophosphonates were designed and synthesized as antitumor agents. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay results demonstrated that some compounds exhibited moderate to high inhibitory activity against HepG2, SK-OV-3 and NCI-H460 tumor cell lines, and most compounds showed much lower cytotoxicity against HL-7702 normal cells than 5-FU and cisplatin. The action mechanism of representative compound 5b was investigated by fluorescence staining assay, flow cytometric analysis and western blot (WB) assay, which indicated that this compound induced apoptosis and G₂/M phase arrest accompanied by an increase in the production of intracellular Ca²⁺ and reactive oxygen species (ROS) and affecting associated enzymes and genes.

Evaluation of antioxidant, enzyme inhibition, and cytotoxic activity of three anthraquinones (alizarin, purpurin, and quinizarin)

OBJECTIVE

The aim of this work was to investigate the cytotoxic, antioxidative, and enzyme inhibition effects of alizarin, quinizarin, and purpurin, which are anthraquinones (AQ).

METHODS

Cytotoxic effects were evaluated with cell inhibition rate by 3-(4,5-dimethylthiazol- 2-yl)-2,5-diphenyltetrazolium bromide assay. Different chemical assays, including free radical scavenging activity (1,1-diphenyl-2-picrylhydrazyl and 2,2-azino-bis(3-ethylbenzothiazloine-6-sulfonic acid)), phosphomolybdenum and reducing power (ferric reducing antioxidant power and cupric ion reducing activity), were used to evaluate the antioxidant properties. Moreover, enzyme inhibitory activities were analyzed against acetylcholinesterase, butrylcholinesterase, tyrosinase, α-amylase, and α-glucosidase.

RESULTS

These components have antioxidant and enzyme inhibition activity. Especially, purpurin showed the strongest antioxidant and good enzyme inhibitory effects. According to our cytotoxicity results, alizarin, purpurin, and quinizarin induced dose- and time-dependent cell proliferation. Furthermore, when we applied AQs with mitomycin C (MC) on L929 cell line, we demonstrated that cell proliferation in MC-AQ groups compared with MC group was increased. The most effective component was alizarin at 100 µM concentration. These AQs showed positive effects on L929 cell lines with high half-maximal inhibitory concentration values.

CONCLUSION

Our results demonstrate that AQs may be used as antioxidative compounds in food and medicinal applications.

One-Pot Three-Component Synthesis of Novel Diethyl((2-oxo-1,2-dihydroquinolin-3-yl)(arylamino)methyl)phosphonate as Potential Anticancer Agents

With the aim of discovering new anticancer agents, we have designed and synthesized novel α-aminophosphonate derivatives containing a 2-oxoquinoline structure using a convenient one-pot three-component method. The newly synthesized compounds were evaluated for antitumor activities against the A549 (human lung adenocarcinoma cell), HeLa (human cervical carcinoma cell), MCF-7 (human breast cancer cell), and U2OS (human osteosarcoma cell) cancer cell lines in vitro, employing a standard 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide (MTT) assay. The results of pharmacological screening indicated that many compounds exhibited moderate to high levels of antitumor activities against the tested cancer cell lines and that most compounds showed more potent inhibitory activities comparable to 5-fluorouracil (5-FU) which was used as a positive control. The mechanism of representative compound 4u (diethyl((2-oxo-1,2-dihydroquinolin-3-yl)(phenyl-amino)methyl)phosphonate) indicated that the compound mainly arrested HeLa cells in S and G2 stages and was accompanied by apoptosis in HeLa cells. This action was confirmed by acridine orange/ethidium bromide staining, Hoechst 33342 staining, and flow cytometry.

Synthesis, antiproliferative and apoptosis-inducing effects of novel asiatic acid derivatives containing α-aminophosphonates

Novel asiatic acid derivatives containing α-aminophosphonates was designed and synthesized as antitumor agents. Compound 3d blocked the T24 cell cycle at G₁/S phase by the p53-dependent pathway and induced apoptosis through mitochondrial pathway.

Synthesis and pharmacological evaluation of dehydroabietic acid thiourea derivatives containing bisphosphonate moiety as an inducer of apoptosis

A series of DHAA thiourea derivatives containing bisphosphonate moiety were designed and synthesized as potent antitumor agents. Structures of target molecules were confirmed using HR-MS, (1)H NMR and (13)C NMR and they exhibited potent anti-tumor activities against the SK-OV-3, BEL-7404, A549, HCT-116 and NCI-H460 tumor cell lines in vitro. Especially, compound 6e (IC50 = 1.79 ± 0.43 μM) exhibited the best anticancer activity against SK-OV-3 cell line. Its role as an inducer of apoptosis was investigated in this cell line by Annexin-V/PI binding assay and by following its capability for ROS generation, depolarization of mitochondrial transmembrane potential, activation of caspases and expression of pro- and anti-apoptotic proteins. Elevated level of ROS generation, activation of caspase-3, caspase-8, caspase-9, and Fas, higher expression of Bax, lower expression of Bcl-2, and increased level of Bax/Bcl-2 ratio identified 6e as a promising inducer of apoptosis that follows both of the mitochondria dependent pathway and the death receptor-mediated pathway. In addition, the cell cycle analysis indicated that compound 6e caused cell cycle arrest at G1 phase, induced apoptosis and led to cell death by increasing the proportion of sub-G1 cells. Furthermore, molecular docking studies showed that 6e could bind to the ATP pocket sites.

Novel Coumarin-Containing Aminophosphonatesas Antitumor Agent: Synthesis, Cytotoxicity, DNA-Binding and Apoptosis Evaluation

A series of novel coumarin-containing α-aminophosphonates were synthesized and evaluated for their antitumor activities against Human colorectal (HCT-116), human nasopharyngeal carcinoma (human KB) and human lung adenocarcinoma (MGC-803) cell lines in vitro. Compared with 7-hydroxy-4-methylcoumarin (4-MU), most of the derivatives showed an improved antitumor activity. Compound 8j (diethyl 1-(3-(4-methyl-2-oxo-2H-chromen-7-yloxy) propanamido)-1-phenylethyl-Phosphonate), with IC₅₀ value of 8.68 μM against HCT-116 cell lines, was about 12 fold than that of unsubstituted parent compound. The mechanism investigation proved that 8c, 8d, 8f and 8j were achieved through the induction of cell apoptosis by G1 cell-cycle arrest. In addition, the further mechanisms of compound 8j-induced apoptosis in HCT-116 cells demonstrated that compound 8j induced the activations of caspase-9 and caspase-3 for causing cell apoptosis, and altered anti- and pro-apoptotic proteins. DNA-binding experiments suggested that some derivatives bind to DNA through intercalation. The results seem to imply the presence of an important synergistic effect between coumarin and aminophosphonate, which could contribute to the strong chelating properties of aminophosphonate moiety.