Detailed studies on complexation behaviour and mechanism of fluorescence quenching of naphthalene linked hydroxamic acid with transition metal ions by UV-visible and fluorescence spectra

A Copper-Selective Sensor and Its Inhibition of Copper-Amyloid Beta Aggregation

Copper is an essential trace metal for biological processes in humans and animals. A low level of copper detection at physiological pH using fluorescent probes is very important for in vitro applications, such as the detection of copper in water or urine, and in vivo applications, such as tracking the dynamic copper concentrations inside cells. Copper homeostasis is disrupted in neurological diseases like Alzheimer's disease, and copper forms aggregates with amyloid beta (Ab42) peptide, resulting in senile plaques in Alzheimer's brains. Therefore, a selective copper detector probe that can detect amyloid beta peptide-copper aggregates and decrease the aggregate size has potential uses in medicine. We have developed a series of Cu2+-selective low fluorescent to high fluorescent tri and tetradentate dentate ligands and conjugated them with a peptide ligand to amyloid-beta binding peptide to increase the solubility of the compounds and make the resultant compounds bind to Cu2+-amyloid aggregates. The copper selective compounds were developed using chemical scaffolds known to have high affinity and selectivity for Cu2+, and their conjugates with peptides were tested for affinity and selectivity towards Cu2+. The test results were used to inform further improvement of the next compound. The final Cu2+ chelator-peptide conjugate we developed showed high selectivity for Cu2+ and high fluorescence properties. The compound bound 1:1 to Cu2+ ion, as determined from its Job's plot. Fluorescence of the ligand could be detected at nanomolar concentrations. The effect of this ligand on controlling Cu2+-Ab42 aggregation was studied using fluorescence assays and microscopy. It was found that the Cu2+-chelator-peptide conjugate efficiently reduced aggregate size and, therefore, acted as an inhibitor of Ab42-Cu2+ aggregation. Since high micromolar concentrations of Cu2+ are present in senile plaques, and Cu2+ accelerates the formation of toxic soluble aggregates of Ab42, which are precursors of insoluble plaques, the developed hybrid molecule can potentially serve as a therapeutic for Alzheimer's disease.

Unravelling the Metal Sensing Activity of a Biologically Relevant Fluorescent Crown Ether: A Unified Experimental and Theoretical Study

1,4-dihydropyridines (DHPs) are biologically active. 1,4-DHP analogs with appropriate substituents also show characteristic fluorescence activity. Here, for the first time, we report a simple and easy synthesis of a novel fluorescent 1,4- DHP derivative of dibenzo[18]-crown-6 (2), which showed promising sensing ability towards physiologically important metal ions. The covalent linking of 1,4-DHP analog with dibenzo[18]-crown-6 instigates its fluorescence activity in (2) and makes it biologically relevant. (2) shows a noteworthy enhancement of fluorescence intensity toward Fe3+ and Ba2+ in methanol medium. DFT studies revealed that metal binding by the crown ether-O atoms leads to structural rigidity, enhancing the fluorescence intensity. Interestingly, (2) shows utility in the quantitative detection of Fe3+ ions in the biological (human blood serum) and food samples.

Effects of metals released in strong-flavor baijiu on the evolution of aroma compounds during storage

Storage is essential in improving the quality of strong-flavor baijiu (SFB). Here, we investigated the release behaviors of metals from containers into SFB and their effects on the evolution of aroma compounds during storage. Twenty-six metals were identified in SFB samples. The concentrations of Na, K, Ca, Mg, Al, and Fe obviously increased after storing in pottery jar, whereas those of Fe and Cu greatly increased after storing in stainless-steel vessel. The volatility of most esters, alcohols, ketone, furan, and aldehyde decreased, whereas that of most acids increased after adding the metal ions into fresh SFB. The fluorescence intensity of SFB decreased with increased aging time in pottery jar, whereas the fluorescence intensity of acids was quenched with adding Fe3+ and Cu2+. All these results suggested that some metals released from containers had binding affinities with acids, thereby reducing SFB organoleptic stimulation by forming metal-aroma compound complexes during storage.

Interaction of cobalt(II) and copper(II) hydroxamates with polyriboadenylic acid: An insight into RNA based drug designing

The polyadenylic acid [poly(A)] tail of mRNA plays a noteworthy role in the initiation of the translation, maturation, and stability of mRNA. It also significantly contributes to the production of alternate proteins in eukaryotic cells. Hence, it has recently been recognized as a prospective drug target. Binding affinity of bis(N-p-tolylbenzohydroxamato)Cobalt(II), [N-p-TBHA-Co(II)] (1) and bis(N-p-naphthylbenzohydroxamato)Copper(II), [N-p-NBHA-Cu(II)] (2) complexes with poly(A) have been investigated by biophysical techniques namely, absorption spectroscopy, fluorescence spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy, circular dichroism spectroscopy, viscometric measurements and through molecular docking studies. The intrinsic binding constants (K_b) of complexes were determined following the order of N-p-TBHA-Co(II)] > N-p-NBHA-Cu(II), along with hyperchromism and a bathochromic shift for both complexes. The fluorescence quenching method revealed an interaction between poly(A)-N-p-TBHA-Co(II)/poly(A)-N-p-NBHA-Cu(II). The mode of binding was also determined via the fluorescence ferrocyanide quenching method. The increase in the viscosity of poly(A) that occurred from increasing the concentration of the N-p-TBHA-Co(II)/N-p-NBHA-Cu(II) complex was scrutinized. The characteristics of the interaction site of poly(A) with N-p-TBHA-Co(II)/N-p-NBHA-Cu(II) were adenine and phosphate groups, as revealed by DRS-FTIR spectroscopy. Based on these observations, a partial intercalative mode of the binding of poly(A) has been proposed for both complexes. Circular dichroism confirmed the interaction of both the complexes with poly(A). The molecular docking results illustrated that complexes strongly interact with poly(A) via the relative binding energies of the docked structure as -259.39eV and -226.30eV for N-p-TBHA-Co(II) and N-p-NBHA-Cu(II) respectively. Moreover, the binding affinity of N-p-TBHA-Co(II) is higher in all aspects than N-p-NBHA-Cu(II) for poly(A).

Naphthyl “capped” triazole-linked calix[4]arene hosts as fluorescent chemosensors towards Fe3+ and Hg2+: an experimental and DFT computational study

The metal binding properties of four new “capped” 1,2,3-triazole linked receptors 6a–d are reported.

Fluorescence quenching study of 2,6-bis(5-(4-methylphenyl)-1-H-pyrazol-3-yl)pyridine with metal ions

A novel bispyrazole derivative 2,6-bis(5-(4-methylphenyl)-1-H-pyrazol-3-yl)pyridine was synthesized and its structure was confirmed by (1)H NMR, FTIR, MS techniques and elemental analysis. The binding interactions of BMPP with Cd(2+), Co(2+), Pb(2+) and Cu(2+) ions were investigated in MeOH-H2O solution by fluorescence quenching technique at two temperatures (25 and 35°C). Their quenching constants KSV, binding constants K, binding sites n and thermodynamic parameters (ΔH, ΔG and ΔS) were determined. The results indicated that the metal ions quenched the intrinsic fluorescence of the bispyrazole by forming the bispyrazole-metal complexes and their quenching process was a static quenching mechanism. In addition, the process of interaction was spontaneous and mainly ΔS-driven.

Incorporating a piperidinyl group in the fluorophore extends the fluorescence lifetime of click-derived cyclam-naphthalimide conjugates

Ligands incorporating a tetraazamacrocycle receptor, a ‘click’- derived triazole and a 1,8-naphthalimide fluorophore have proven utility as probes for metal ions. Three new cyclam-based molecular probes are reported, in which a piperidinyl group has been introduced at the 4-position of the naphthalimide fluorophore. These compounds have been synthesized using the copper(I)-catalyzed azide-alkyne Huisgen cycloaddition and their photophysical properties studied in detail. The alkylamino group induces the expected red-shift in absorption and emission spectra relative to the simple naphthalimide derivatives and gives rise to extended fluorescence lifetimes in aqueous buffer. The photophysical properties of these systems are shown to be highly solvent-dependent. Screening the fluorescence responses of the new conjugates to a wide variety of metal ions reveals significant and selective fluorescence quenching in the presence of copper(II), yet no fluorescence enhancement with zinc(II) as observed previously for the simple naphthalimide derivatives. Reasons for this different behaviour are proposed. Cytotoxicity testing shows that these new cyclam-triazole-dye conjugates display little or no toxicity against either DLD-1 colon carcinoma cells or MDA-MB-231 breast carcinoma cells, suggesting a potential role for these and related systems in biological sensing applications.

Iron(III) selective molecular and supramolecular fluorescent probes

Iron is one of the most important elements in metabolic processes, being indispensable for all living systems and therefore it is extensively distributed in environmental and biological materials. However, both its deficiency and excess from the normal permissible limit can induce serious disorders. Therefore, several analytical techniques have been adopted for the detection of iron. Among the various techniques used for its detection, the method based on fluorescent sensors has received considerable interest in recent years because of its ability to provide online monitoring of very low concentrations without any pre-treatment of the sample together with the advantages of spatial and temporal resolution. In this article, efforts have been made to review the various molecular and supramolecular fluorescent sensors that have been developed for the selective detection of iron(III).

Cobalt complexes with tripodal ligands: implications for the design of drug chaperones

Extensive research is currently being conducted into metal complexes that can selectively deliver cytotoxins to hypoxic regions in tumours. The development of pharmacologically suitable agents requires an understanding of appropriate ligand-metal systems for chaperoning cytotoxins. In this study, cobalt complexes with tripodal tren (tris-(2-aminoethyl)amine) and tpa (tris-(2-pyridylmethyl)amine) ligands were prepared with ancillary hydroxamic acid, β-diketone and catechol ligands and several parameters, including: pK(a), reduction potential and cytotoxicity were investigated. Fluorescence studies demonstrated that only tpa complexes with β-diketones showed any reduction by ascorbate in situ and similarly, cellular cytotoxicity results demonstrated that ligation to cobalt masked the cytotoxicity of the ancillary groups in all complexes except the tpa diketone derivative [Co(naac)tpa](ClO(4))(2) (naac = 1-methyl-3-(2-naphthyl)propane-1,3-dione). Additionally, it was shown that the hydroxamic acid complexes could be isolated in both the hydroxamate and hydroximate form and the pK(a) values (5.3-8.5) reveal that the reversible protonation/deprotonation of the complexes occurs at physiologically relevant pHs. These results have clear implications for the future design of prodrugs using cobalt moieties as chaperones, providing a basis for the design of cobalt complexes that are both more readily reduced and more readily taken up by cells in hypoxic and acidic environments.

Copper extraction by fatty hydroxamic acids derivatives synthesized based on palm kernel oil

Fatty hydroxamic acids derivatives based on palm kernel oil which are phenyl fatty hydroxamic acids (PFHAs), methyl fatty hydroxamic acids (MFHAs), isopropyl fatty hydroxamic acids (IPFHAs) and benzyl fatty hydroxamic acids (BFHAs) were applied as chelating agent for copper liquid-liquid extraction. The extraction of copper from aqueous solution by MFHAs, PFHAs, BFHAs or IPFHAs were carried out in hexane as an organic phase through the formation of copper methyl fatty hydroxamate (Cu-MFHs), copper phenyl fatty hydroxamate (Cu-PFHs), copper benzyl fatty hydroxamate (Cu-BFHs) and copper isopropyl fatty hydroxamate (Cu-IPFHs). The results showed that the fatty hydroxamic acid derivatives could extract copper at pH 6.2 effectively with high percentage of extraction (the percentages of copper extraction by MFHAs, PFHAs, IPFHs and BFHAs were found to be 99.3, 87.5, 82.3 and 90.2%, respectively). The extracted copper could be quantitatively stripped back into sulphuric acid (3M) aqueous solution. The obtained results showed that the copper recovery percentages from Cu-MFHs, Cu-PFHs, Cu-BFHs and Cu-IPFHs are 99.1, 99.4, 99.6 and 99.9 respectively. The copper extraction was not affected by the presence of a large amount of Mg (II), Ni (II), Al (III), Mn (II) and Co (II) ions in the aqueous solution.

Quenching of fluorophore-labeled DNA oligonucleotides by divalent metal ions: implications for selection, design, and applications of signaling aptamers and signaling deoxyribozymes

Recent years have seen a dramatic increase in the use of fluorescence-signaling DNA aptamers and deoxyribozymes as novel biosensing moieties. Many of these functional single-stranded DNA molecules are either engineered to function in the presence of divalent metal ion cofactors or designed as sensors for specific divalent metal ions. However, many divalent metal ions are potent fluorescence quenchers. In this study, we first set out to examine the factors that contribute to quenching of DNA-bound fluorophores by commonly used divalent metal ions, with the goal of establishing general principles that can guide future exploitation of fluorescence-signaling DNA aptamers and deoxyribozymes as biosensing probes. We then extended these studies to examine the effect of specific metals on the signaling performance of both a structure-switching signaling DNA aptamer and an RNA-cleaving and fluorescence-signaling deoxyribozyme. These studies showed extensive quenching was obtained when using divalent transition metal ions owing to direct DNA-metal ion interactions, leading to combined static and dynamic quenching. The extent of quenching was dependent on the type of metal ion and the concentration of supporting monovalent cations in the buffer, with quenching increasing with the number of unpaired electrons in the metal ion and decreasing with the concentration of monovalent ions. The extent of quenching was independent of the fluorophore, indicating that quenching cannot be alleviated simply by changing the nature of the fluorescent probe. Our results also show that the DNA sequence and the local secondary structure in the region of the fluorescent tag can dramatically influence the degree of quenching by divalent transition metal ions. In particular, the extent of quenching is predominantly determined by the fluorophore location with respect to guanine-rich and duplex regions within the strand sequence. Examination of the effect of both the type and concentration of metal ions on the performance of a fluorescence-signaling aptamer and a signaling deoxyribozyme confirms that judicious choice of divalent transition metal ions is important in maximizing signals obtained from such systems.

Use of zirconium-phosphate-carbonate chemistry to immobilize polycyclic aromatic hydrocarbons on boron-doped diamond

We report on the formation of monomolecular layers of perylene- and pyrene-alkanoic acids on boron-doped diamond (BDD) substrates. The carboxylic acid layers are bound by coordination to zirconium phosphate (ZP) functionalities on the BDD substrate surface. The resulting Zr-phosphate-carbonate (ZPC) linkages between the substrate and the adlayer are asymmetric, of the form -(OPO3(2-) Zr4+-O2C-R)+ X-. Pyrene and perylene are well-established optical probes of polarity and viscosity at interfaces. We have used electrochemical and steady-state fluorescence techniques to study the loading density and behavior of these monomolecular films, allowing comparison of BDD and indium-doped tin oxide (ITO) substrates. Electrochemical data suggest that the pyrene chromophores are positioned roughly at the same distance from the surface, regardless of the length of the anchoring alkanoic acid chain, a finding that can be explained by the pyrene lying on the substrate surface. Such a conformation is plausible given the surface coverage (5 x 10(-11) mol/cm2, ca. 0.1 monolayer) we measure for these systems.

Selective sensing of Cu(II) at ng ml(-1) level based on phosphorescence quenching of 1-bromo-2-methylnaphthalene sandwiched in sodium deoxycholate dimer

Using sodium deoxycholate as a protective medium, the selective recognition of Cu(II) at ng ml(-1) level is realized through dynamic phosphorescence quenching of 1-bromo-2-methylnaphthalene (BMN) without deoxygenation. The limit of detection is 4.32 ng ml(-1), and the relative standard deviation is 1% at 10 microM, linear up to 1 x 10(-5) M.