Free metal activity and total metal concentrations as indices of micronutrient availability to barley [Hordeum vulgare (L.) ‘Klages’]

The form in which a micronutrient is found in the rhizosphere affects its availability to plants. We compared the availability to barley of the free hydrated cation form of Fe³⁺, Cu²⁺, Zn²⁺, and Mn²⁺ versus their total metal concentrations (free ion plus complexes) in chelator-buffered solutions. Free metal ion activities were estimated using the chemical equilibrium program GEOCHEM-PC with the corrected database. In experiment 1, barley was grown in nutrient solutions with different Fe³⁺ activities using chelators to control Fe levels. Chlorosis occurred at Fe³⁺ activities of 10^–18 and 10^–19 M for barley grown in HEDTA and EDTA solutions, respectively. In experiment 2, barley was grown in nutrient solutions with the same calculated Fe³⁺ activity and the same chelator, but different total Fe concentrations. Leaf, root and shoot Fe concentrations were higher from CDTA buffered solutions which had the higher total Fe concentration indicating the importance of the total Fe concentration on Fe uptake. Results from treatments using EDTA or HEDTA, with one exception, were similar to the results from the CDTA treatment. This suggests differences in critical Fe³⁺ activities found in experiment 1 were due to differences in the total Fe concentration and not errors in chelate formation constants used to estimate the critical activities. Results for Cu, Zn, and Mn were similar to Fe; despite solutions with equal free Cu²⁺, Zn²⁺ and Mn²⁺ activities, plant concentrations of these metals were generally greater when grown in the solutions with the greater total amount of Cu, Zn, or Mn. When the free Zn²⁺ activity was kept constant while the total amount of Zn was increased from 4.4 to 49 M, leaf Zn concentration increased from 77 to 146 g g^-1. In order to predict metal availability to barley and other species in chelator-buffered nutrient solutions, both free and total metal concentrations in solution must be considered. The critical Fe³⁺ activities required by barley in this study are much higher than those from tomato and soybean, 10^-28 M, which strongly supports the Strategy 2 model of Fe uptake for Poaceae. This is related to the importance of the Fe³⁺ (barley) and the Fe²⁺ (tomato and soybean) ions in Fe uptake. Fe-stressed barley is known to release phytosiderophores which compete for Fe³⁺ in the nutrient solution, while tomato and soybean reduce Fe³⁺ to Fe²⁺ at the epidermal cell membranes to allow uptake of Fe²⁺ from Fe³⁺ chelates in solution.Abbreviations CDTA trans-1,2-diaminocyclohexane-N,N,N,N-tetracetic acid - DTPA diethylenetriaminepentacetic acid - EDTA ethylenediaminetetracetic acid - EDDHA ethylenediamine-di(o-hydroxyphenylacetic acid) - HBED-N,N di(2-hydroxybenzoyl)-ethylenediamine-N,N-diacetic acid - HEDTA-N hydroxyethylenediaminetriacetic acid - MES-2 (N-morpholino)ethanesulfonic acid - NTA nitrilotriacetic acid     

Partial Protection of Carbohydrate Derivatives. Part 24.1 Synthesis of Adenylyl-(2′–5′)-Adenylyl-(2′–5′)-Adenosine Using a 3′-O-(Tetrahydropyran-2-YL) Adenosine Derivative

Abstract

The synthesis of the title compound was performed using a 3′-O-(tetrahydropyran-2-yl) adenosine derivative as the starting material, i.e., a coupling reaction of triethylammonium N ⁶-benzoyl-5′-O-dimethoxytrityl-3′-O-(tetrahydropyran-2-yl) adenosine 2′-(4-chlorophenyl)phosphate with N ⁶-benzoyl-2′,3′-di-O-benzoyladenosine, followed by a sequence of reactions, O-dedimethoxytritylation, a coupling reaction with the former triethylammonium salt, and complete deblocking of the resultant 2′, 5′-triadenylic acid derivative.     

A ‘turn-off’ fluorescence sensor for selective Hg(II) based on phenothiazine derivative

Given how crucial it is to preserve a human-safe and sustainable environment, the rapid discovery of possibly lethal heavy metals such as Hg(II) has drawn much attention in recent years. A novel sensor, known as (E)-2-((10-octyl-10H-phenothiazin-3-yl)methylene)hydrazine-1-carbothioamide (PTZHC), was synthesized as a fluorescence ‘on–off’ sensor for Hg²⁺ ions. Coordination alters organic molecule electron densities, quenching the fluorescence intensity. PTZHC was described completely with the help of FTIR and ¹H-NMR spectrum studies. The Hg²⁺ ion was successfully detected using the PTZHC sensor even when there were other metal ions present. The limit of the detection was estimated to be 2.5 × 10⁻⁸ M and the Job's plot examination implied that PTZHC was bound to Hg²⁺ with a simple 1:1 stoichiometry in s CH₃CN/H₂O (9:1, v/v) suspension. To further cast light on the bridged effect on geometric and optoelectronic characteristics, time-dependent density functional theory (TD-DFT) at the B3LYP/6-31G(d) level and DFT were both examined.     

Cd(II) and Cu(II) complexes of polydentate Schiff base ligands: synthesis, characterization, properties and biological activity

We report the synthesis of the Schiff base ligands, 4-[(4-bromo-phenylimino)-methyl]-benzene-1,2,3-triol (A₁), 4-[(3,5-di-tert-butyl-4-hydroxy-phenylimino)-methyl]-benzene-1,2,3-triol (A₂), 3-(p-tolylimino-methyl)-benzene-1,2-diol (A₃), 3-[(4-bromo-phenylimino)-methyl]-benzene-1,2-diol (A₄), and 4-[(3,5-di-tert-butyl-4-hydroxy-phenylimino)-methyl]-benzene-1,3-diol (A₅), and their Cd(II) and Cu(II) metal complexes, stability constants and potentiometric studies. The structure of the ligands and their complexes was investigated using elemental analysis, FT-IR, UV-Vis, ¹H and ¹³C NMR, mass spectra, magnetic susceptibility and conductance measurements. In the complexes, all the ligands behave as bidentate ligands, the oxygen in the ortho position and azomethine nitrogen atoms of the ligands coordinate to the metal ions. The keto-enol tautomeric forms of the Schiff base ligands A₁-A₅ have been investigated in polar and non-polar organic solvents. Antimicrobial activity of the ligands and metal complexes were tested using the disc diffusion method and the strains Bacillus megaterium and Candida tropicalis.Protonation constants of the triol and diol Schiff bases and stability constants of their Cu²⁺ and Cd²⁺ complexes were determined by potentiometric titration method in 50% DMSO-water media at 25.00 ± 0.02 °C under nitrogen atmosphere and ionic strength of 0.1 M sodium perchlorate. It has been observed that all the Schiff base ligands titrated here have two protonation constants. The variation of protonation constant of these compounds was interpreted on the basis of structural effects associated with the substituents. The divalent metal ions of Cu²⁺ and Cd²⁺ form stable 1:2 complexes with Schiff bases.The Schiff base complexes of cadmium inhibit the intense chemiluminescence reaction in dimethylsulfoxide (DMSO) solution between luminol and dioxygen in the presence of a strong base. This effect is significantly correlated with the stability constants K_CdL of the complexes and the protonation constants K_OH of the ligands; it also has a nonsignificant association with antibacterial activity.     

UV-Vis and fluorimetric Al, Zn, Cd and Pb complexation studies of two 3-hydroxyflavones and a 3-hydroxythioflavone

The complexation of Al³⁺, Zn²⁺, Cd²⁺ and Pb²⁺ by the 3-hydroxyflavones: 3-hydroxy-2-(2-methoxyphenyl)-4H-1-benzopyran-4-one (H1) and 3-hydroxy-2-(4-methoxyphenyl)-4H-1-benzopyran-4-one (H2), and by the 3-methoxythioflavone: 3-hydroxy-2-(2-methoxyphenyl)-4H-1-benzopyran-4-thione (H3) have been studied spectrophotometrically and fluorimetrically to determine the corresponding complexation constants, K_sp and K_fl, in 5:95 water:ethanol (v/v) solution for which [HClO₄] was either 10⁻² or 10⁻⁵ mol dm⁻³ and I = 0.10 mol dm⁻³ (NaClO₄) at 298.2 K. Complexation occurs dominantly through the deprotonated ligand for [Al(1)]²⁺ and [Al(2)]²⁺ for which log K_sp = 4.51 and 4.73, respectively, in 10⁻² mol dm⁻³ HClO₄ and 4.21 and 4.61 in 10⁻⁵ mol dm⁻³ HClO₄. For Pb²⁺ complexation by H1, H2 and H3 is characterized by log K_sp = 2.20, 2.57 and 3.22, respectively, in 10⁻² mol dm⁻³ HClO₄ and 4.70, 5.38 and 5.74 in 10⁻⁵ mol dm⁻³ HClO₄. Equilibrium mixtures of [Pb(H1)]²⁺ and [Pb1]⁺, [Pb(H2)]²⁺ and [Pb2]⁺, and [Pb(H3)]²⁺ and [Pb3]⁺ appear to be formed. Complexation of Zn²⁺ and Cd²⁺ by all three ligands was only detected in 10⁻⁵ mol dm⁻³ HClO₄. For Zn²⁺ complexation by H1, H2 and H3 log K_sp = 3.22, 3.74 and 4.46 and for Cd²⁺ the corresponding values are 2.39, 2.40 and 3.72 for Cd²⁺. Only [Al1]²⁺ and [Al2]²⁺ show significant fluorescence and are characterized by log K_fl = 6.30 and 7.49 in 10⁻² mol dm⁻³ HClO₄.     

Synthesis of 1,5-Anhydro-D-Mannitol Nucleosides with a Purine Base Moeety

Abstract

D-Mannitol nucleosides with a purine base moiety have been conveniently synthesized strating from 1,5-anhydro-4,6-O-benzylidene-D-glucitol. The 3-OH function of 1,5-anhydro-4,6-O-benzylidene-D-glucitol was selectively protected with t-butyldimethylsilyl group and subsequently converted to the corresponding 0-triflate derivative for the introduction of the nucleobase moietes. These nucleoside derivatives were transformed to 1,5-Anhydro-6-O-MMTr-2-(N⁶-benzoyladenin-9-yl)-2-deoxy-3-O-TBDMS-D-mannitol and 1,5-Anhydro-6-O-MMTr-2-(N²-isobutyryl-guanin-9-yl)-2-deoxy-3-O-TBDMS-D-mannitol, useful as the building blocks for oligonucleotide synthesis. Also, the synthesis of the corresponding fully deprotected anhydrohexitol nucleosides were achieved for evaluation of antiviral activity test.     

Binding of transition metal ions to albumin: Sites,affinities and rates

Background

Serum albumin is the most abundant protein in the blood and cerebrospinal fluid and plays a fundamental role in the distribution of essential transition metal ions in the human body. Human serum albumin (HSA) is an important physiological transporter of the essential metal ions Cu^2 +, and Zn^2 + in the bloodstream. Its binding of metals like Ni^2 +, Co^2 +, or Cd^2 + can occur in vivo, but is only of toxicological relevance. Moreover, HSA is one of the main targets and hence most studied binding protein for metallodrugs based on complexes with Au, Pt and V.

Scope of Review

We discuss i) the four metal-binding sites so far described on HSA, their localization and metal preference, ii) the binding of the metal ions mentioned above, i.e. their stability constants and association/dissociation rates, their coordination chemistry and their selectivity versus the four binding sites iii) the methodology applied to study issues of items i and ii and iv) oligopeptide models of the N-terminal binding site.

Major Conclusions

Albumin has four partially selective metal binding sites with well-defined metal preferences. It is an important regulator of the blood transport of physiological Cu(II) and Zn(II) and toxic Ni(II) and Cd(II). It is also an important target for metal-based drugs containing Pt(II), V(IV)O, and Au(I).

General Significance

The thorough understanding of metal binding properties of serum albumin, including the competition of various metal ions for specific binding sites is important for biomedical issues, such as new disease markers and design of metal-based drugs. This article is part of a Special Issue entitled Serum Albumin.     

A quantitative appraisal of the ambivalent metal ion binding properties of cytidine in aqueous solution and an estimation of the<Emphasis Type="Italic"> anti</Emphasis>–<Emphasis Type="Italic">syn</Emphasis> energy barrier of cytidine derivatives

The recently defined versus straight-line plots for L = pyridine-type (PyN) and ortho-aminopyridine-type (oPyN) ligands now allow the evaluation in a quantitative manner of the stability of the 1:1 complexes formed between cytidine (Cyd) and Ca²⁺, Mg²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ or Cd²⁺ (M²⁺); the corresponding stability constants, , including the acidity constant, , for the deprotonation of the (N3)H⁺ site had been determined previously under exactly the same conditions as the mentioned plots. Since the stabilities of the M(PyN)²⁺ and M(oPyN)²⁺ complexes of Ca²⁺ and Mg²⁺ are practically identical, it is concluded that complex formation occurs in an outer-sphere manner, and this is in accord with the fact that in the pK_a range 3–7 metal ion binding is independent of or . Ca(Cyd)²⁺ and Mg(Cyd)²⁺ are more stable than the corresponding (outer-sphere) M(PyN)²⁺ complexes and this means that the C2 carbonyl group of Cyd must participate, next to N3 which is most likely outer-sphere, in metal ion binding, leading thus to chelates; these have formation degrees of about 50% and 35%, respectively. Co(Cyd)²⁺ and Ni(Cyd)²⁺ show no increased stability based on the hence, the (C2)O group does not participate in metal ion binding, but the inner-sphere coordination to N3 is strongly inhibited by the (C4)NH₂ group. In the M(Cyd)²⁺ complexes of Mn²⁺, Cu²⁺, Zn²⁺ and Cd²⁺, this inhibiting effect on M²⁺ binding at N3 is partially compensated by participation of the (C2)O group in complex formation and the corresponding chelates have formation degrees between about 30% (Zn²⁺) and 83% (Cu²⁺). The different structures of the mentioned chelates are discussed in relation to available crystal structure analyses. (1) There is evidence (crystal structure studies: Cu²⁺, Zn²⁺, Cd²⁺) that four-membered rings form, i.e. there is a strong M²⁺ bond to N3 and a weak one to (C2)O. (2) By hydrogen bond formation to (C2)O of a metal ion-bound water molecule, six-membered rings, so-called semichelates, may form. (3) For Ca²⁺ and Mg²⁺, and possibly Mn²⁺, and their Cyd complexes, six-membered chelates are also likely with (C2)O being inner-sphere (crystal structure) and N3 outer-sphere. (4) Finally, for these metal ions also complexes with a sole outer-sphere interaction may occur. All these types of chelates are expected to be in equilibrium with each other in solution, but, depending on the metal ion, either the one or the other form will dominate. Clearly, the cytidine residue is an ambivalent binding site which adjusts well to the requirements of the metal ion to be bound and this observation is of relevance for single-stranded nucleic acids and their interactions with metal ions. In addition, the anti–syn energy barrier has been estimated as being in the order of 6–7.5 kJ/mol for cytidine derivatives in aqueous solution at 25 °C.Abbreviations ADP^3– adenosine 5-diphosphate - AMP^2– adenosine 5-monophosphate - ATP^4– adenosine 5-triphosphate - CDP^3– cytidine 5-diphosphate - cl closed - CMP^2– cytidine 5-monophosphate - 3-CMP^2– cytidine 3-monophosphate - CTP^4– cytidine 5-triphosphate - Cyd cytidine - DNA deoxyribonucleic acid - I ionic strength - K_a acidity constant - K_I intramolecular equilibrium constant - L general ligand - M²⁺ general divalent metal ion - NTP^4– nucleoside 5-triphosphate - op open - oPyN ortho-aminopyridine-type ligand - PyN pyridine-type ligand - t-RNA transfer ribonucleic acid - Tu tubercidin (7-deazaadenosine)In honor of Professor Liang-Nian Ji on the occasion of his 70th birthday in friendship and with best wishes.     

Low temperature optical spectroscopy of cobalt-substituted hemocyanin from Carcinus maenas

In this work we report the optical absorption spectra of three cobalt-substituted derivatives of hemocyanin (He) from Carcinus maenas, in the temperature range 300–20 K. The derivatives studied are the mononuclear (Co²⁺)-He with a single cobalt ion in the Cu_A site, the binuclear (Co²⁺)₂-He and the binuclear mixed metal (Co²⁺-Cu¹⁺)-He. At low temperature three main bands are clearly resolved; the temperature dependence of their zeroth, first and second moments sheds light on the stereodynamic properties in the surroundings of the chromophore. Within the limits of the reported analysis, in the binuclear derivatives the motions coupled to the chromophore appear to be essentially harmonic in the whole temperature range investigated; moreover the data are consistent with the presence of an exogenous ligand strongly bound to the two metal ions. For the mononuclear derivative an essentially harmonic behavior is evident only up to 200 K where the data are consistent with the presence of an exogenous ligand much less strongly bound, while at higher temperatures the behavior of the spectra indicates the onset of very large anharmonic contributions to motions, that plausibly involve the above exogenous ligand and, quite likely, the entire active site.Abbreviations He Hemocyanin - M₀ zeroth moment - M₁ first moment - M₂ second moment - (Co^2–)₂-He binuclear bicobalt hemocyanin derivative - (Co²⁺)-He mononuclear monocobalt hemocyanin derivative - (Co²⁺-Cu¹⁺)-He binuclear mixed metals hemocyanin derivative - LFT ligand field theory - CT charge transfer - EPR electronic paramagnetic resonance - XANES X-ray absorption near edge structure Correspondence to: L. Cordone     

A preparative study of some coronands and their complexation of silver(I), zinc(II), cadmium(II) and lead(II)

A study of the complexation of heavy metal ions by the coronands 3,12,20,29-tetraoxa-35,36-diazapentacyclo[29.3.1.1.^14,18.0^5,10.0^22,27]-hexatriaconta-1(35),5(10),6,8,14,16,18(36),22(27),23,25,31,33-dodecaene (1); 2,3,11,12-bis (4^′-methylbenzo)-1,4,10,13-tetrathia-7,16-dioxacyclo-octadeca-2,11-diene (2); 7,16-diaza-1,4,10,13-tetraoxa-2,3,11,12-dibenzocyclooctadeca-2,11-diene (3); 2-[19-(2-hydroxy-2-phenylethyl)-7,8,9,10, 18,19,20,21-octahydro-6H,17H-dibenzo[b,k][1,4,10,13,7,16]tetraoxadiazacyclooctadecin-8-yl]-1-phenyl-1-ethanol (4); 1,4,10,13-tetraoxa-7,16-diazacyclo-octadecane (5); and 2-[16-(2-hydroxy-2-phenylethyl)-1,4,10,13-tetraoxa-7,16-diazacyclo-octadecanyl]-1-phenyl-1-ethanol (6) is described. Coronands 1 and 3 were prepared by literature methods, improved methods were used to prepare 2, and 4 and 6 were prepared from 3 and 5 (obtained commercially), respectively. Potentiometric studies in N,N-dimethylformamide yielded (logK/dm³ mol⁻¹)=5.50, 6.49, 9.42 and 7.52 for [Ag · 1]⁺, [Ag · 2]⁺, [Ag · 5]⁺ and [Ag · 6]⁺, respectively; <2, <2, 4.30 and <2 for [Zn · 1]²⁺, [Zn · 2]²⁺, [Zn · 5]²⁺ and [Zn · 6]²⁺, respectively, <2, <2, 5.92 and >7.52 for [Cd · 1]²⁺, [Cd · 2]²⁺, [Cd · 5]²⁺, and [Cd · 6]²⁺, respectively, and 2.62, 2.38, 6.71 and >7.52 for [Pb · 1]²⁺, [Pb · 2]²⁺, [Pb · 5]²⁺, and [Pb · 6]²⁺, respectively. ESI-MS studies of the interactions of 1-6 with Ag⁺, Zn²⁺, Cd²⁺ and Pb²⁺ are also reported.     

Magnesium complexes of the antiviral 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA) and of its 1-, 3-, and 7-deaza analogues in aqueous solution

 The stability constants of the 1 : 1 complexes formed between Mg²⁺ and the anions of the N1, N3, and N7 deaza derivatives of 9-[2-(phosphonomethoxy)ethyl]adenine (PA^2–), i.e., of Mg(H;PA)⁺ and Mg(PA), were determined by potentiometric pH titration in aqueous solution (25  °C; I=0.1 M, NaNO₃) and compared with previous results [Sigel H, et al. (1992) Helv Chim Acta 75 : 2634–2656], obtained under the same conditions, for the corresponding complexes of 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA^2–) and (phosphonomethoxy)ethane (PME^2–). Based on the analysis of a microconstant scheme it is concluded that in the monoprotonated complexes, Mg(H;PA)⁺, Mg²⁺ is coordinated to a significant part at the nucleobase, H⁺ being at the phosphonate group. By making use of log K ^Mg _Mg(R-PO3) versus pK ^H _H(R-PO3) straight-line plots (also obtained previously; see above) for simple phosphonates and phosphate monoesters, it is shown that all the Mg(PA) complexes, including those with PMEA^2– and PME^2–, are more stable than expected on the basis of the basicity of the ―PO^2– ₃ group. This proves that, to some extent, five-membered chelates, Mg(PA)_cl/O, involving the ether oxygen of the ―CH₂―O―CH₂―PO^2– ₃ chain are formed; their formation degree amounts to about 30–40% in equilibrium with the isomer having only a phosphonate-Mg²⁺ coordination. In the case of Mg(1-deaza-PMEA), probably a further isomer occurs in which also N3 of the nucleobase participates. The different properties between the Mg(PA) species and the Mg(AMP) complex are discussed. Received: 26 January 1998 / Accepted: 19 May 1998     

Novel highly emissive non-proteinogenic amino acids: synthesis of 1,3,4-thiadiazolyl asparagines and evaluation as fluorimetric chemosensors for biologically relevant transition metal cations

Highly emissive heterocyclic asparagine derivatives bearing a 1,3,4-thiadiazolyl unit at the side chain, functionalised with electron donor or acceptor groups, were synthesised and evaluated as amino acid-based fluorimetric chemosensors for metal cations, such as Cu²⁺, Zn²⁺, Co²⁺ and Ni²⁺. The results suggest that there is a strong interaction through the donor heteroatoms at the side chain of the various asparagine derivatives, with high sensitivity towards Cu²⁺ in a ligand–metal complex with 1:2 stoichiometry. Association constants and detection limits for Cu²⁺ were calculated. The photophysical and metal ion sensing properties of these asparagine derivatives confirm their potential as fluorimetric chemosensors and suggest that they can be suitable for incorporation into chemosensory peptidic frameworks.     

Calculations of one-, two- and three-bond nuclear spin-spin couplings in a model peptide and correlations with experimental data

Summary We present ab initio calculations of the Fermi contact term and experimental correlations of six coupling constants, ³J_H ^N _H , ¹J_{C H} , ²J_CH , ¹J_{C N}, ²J_{C N} and ¹J_CN, in a peptide as functions of the backbone dihedral angles, and . Given estimates of experimental uncertainties, we find semiquantitative experimental correlations for ³J_H ^N _H , ¹J_{C N} and ²J_{C N}, qualitative correlations for ¹J_{C H} and ²J_CH , but no experimental correlations of practical utility for ¹J_CN, owing to its complex dependence on at least four dihedral angles. Errors in the estimation of dihedral angles from X-ray crystallographic data for proteins, which result from uncertainties in atom-to-atom distances, place substantial limitations on the quantitative reliability of coupling constant calculations fitted to such data. In the accompanying paper [Edison, A.S. et al., J. Biomol. NMR, 4, 543–551] we apply the results of the coupling constant calculations presented here to the estimation of and angles in staphylococcal nuclease from experimental coupling constants.Abbreviations AO atomic orbital - BPTI basic pancreatic trypsin inhibitor (bovine) - CI-2 chymotrypsin inhibitor 2 - E.COSY exclusive correlation spectroscopy (Griesinger et al., 1986) - ⁿJ_AB single bond (n=1), geminal (n=2), or vicinal (n=3) coupling constant between nuclei A and B - LCAO linear combination of atomic orbitals - NBO natural bond orbital - n lone pair orbitals - bonding orbitals - ^* antibonding orbitals - dihedral angle or molecular orbital wave function; r², correlation coefficient - RHF restricted Hartree-Fock; rmsd, root-mean-square deviation - 3-21G and 6-31G^* molecular orbital basis set designations (Hehre et al., 1986)     

Luminescent complexes of the zinc triad with N-substituted 8-amino-quinoline ligands: Synthesis and comparative study on the stability constants and related photophysical properties

The potentially fluorescent terdentate ligand bis-quinolin-8-yl-amine (BQAH) yields the bis-chelate complexes [M(BQAH)₂](ClO₄)₂ (M = Zn, Cd, Hg) and the mono-chelate [M(BQAH)Cl₂] (M = Zn, Cd). The aminic proton of the coordinated BQAH displays a remarkable acidity. Thus, in polar solvents (CH₃CN and methanol) the formation of the deprotonated derivatives [M(BQA)₂] and [M(BQA)Cl] is observed whose absorption and fluorescent spectra are identical with those of independently synthesized complexes (M = Zn). The affinity of the ligand BQAH with the metals of the zinc triad was studied in CH₃CN; the stability constants related to the complex [M(BQAH)(CH₃CN)]²⁺ (β₁) and [M(BQAH)₂]²⁺ (β₂) were determined and compared with those calculated in the case of the ligand 8-[(2-pyridylmethyl)amino]-quinoline (NNN(Qui)) in the same solvent. Owing to the enhanced rigidity of the ligand BQAH, a marked selectivity in coordinating the Zn²⁺ cation with respect to the larger Cd²⁺ was apparent. In the case of mercury, the equilibrium constant value was also confirmed by means of ¹H NMR technique. The low lying excited state of the BQAH and NNN(Qui) systems is ligand centered and fluo-solvato-chromism analysis reveals that in protic solvents an inter-molecular hydrogen bond between the aminic proton in the excited state and the solvent itself efficiently quenches the fluorescent signal. Coordination with metals induces a hypsochromic displacement of the absorbance maxima measured in CH₂Cl₂ with respect to those of the free ligands. On the contrary in CH₃OH the complete deprotonation of the coordinated BQAH induces a bathochromic displacement of the absorption maxima at 480 nm. In CH₃OH the fluorescent emissions of the mono- and bis-chelate deprotonated BQA⁻ complexes at ≈600 nm display a very low quantum yield and a reduced Stokes shift as compared with that of the protonated species. Such an increase can be related to the enhanced rigidity of the deprotonated ligand inducing a tight coplanarity of the aromatic rings in the first excited state. Eventually the metal coordination, while reducing the energy of the fluorescent emission of both ligands in CH₂Cl₂, does not inhibit the non radiative relaxation pathways in the BQAH system.     

Flotation separation of mercury(II) from environmental water samples using thiosemicarbazide derivatives as chelating agents and oleic acid as surfactant

Abstract

A simple and rapid procedure is developed for the quantitative flotation of mercury(II) from aqueous solutions. Thiosemicarbazide derivatives such as: 1-(amino-N-phenylmethanethio)-4-(pyridine-2-yl)thio-semicarbazide (H₂PPS), N-phenyl-2-(pyridine-2-ylcarbamothioyl) hydrazinecarboxamide (H₂PBO), 1-(amino(thioformyl)-N-phenylform)-4-(pyridine-2-yl)thiosemicarbazide (H2APO), and 1-(amino-N-(pyri-dine-3-yl)methanethio)-4-(pyridine-2-yl)thiosemicarbazide (H₂PPY) are used as organic chelating agents and oleic acid (HOL) as surfactant. The different parameters affecting the flotation process namely, metal ion, ligands and surfactant concentrations, foreign ions (which are normally present in fresh and saline waters), pH and temperature are examined. Nearly 100% of mercury ions are floated at a metal:ligand ratio of 1: 4, pH ~ 4 and at room temperature (~25°C). The procedure was successfully applied to recover Hg²⁺ ions spiked into some water samples. A flotation mechanism is suggested based on some physical and chemical studies on the solid complexes isolated from aqueous and floated layers.     

Probing of Various Physiologically Relevant Metals: Amyloid-β Peptide Interactions with a Lipid Membrane-Immobilized Protein Nanopore

One of the prevailing paradigms regarding the onset of Alzheimer’s disease endows metal ions with key roles in certain steps of the amyloid-β (Aβ) peptide aggregation cascade, through peptide conformational changes induced by metal binding. Herein, we focused on the truncated, more soluble Aβ_1–16 peptide fragment from the human Aβ_1–40, and demonstrated the utility of a sensing element based on the α-hemolysin (α-HL) protein to examine and compare at single-molecule level the interactions between such peptides and various metals. By using the same approach, we quantified Cu²⁺ and Zn²⁺ binding affinities to the Aβ_1–16 fragment, whereas the statistical analysis of blockages induced by a single Aβ_1–16 peptide on the current flow through an open α-HL pore show that the metal propensity to interacting with the peptide and entailing conformational changes obey the following order: Cu²⁺ > Zn²⁺ > Fe³⁺ > Al³⁺.     

Isolation and Molecular Characterization of Heavy Metal-Resistant Alcaligenes faecalis from Sewage Wastewater and Synthesis of Silver Nanoparticles

Environmental pollution with toxic heavy metals is increasing throughout the world alongside industrial development. Microorganisms and microbial products can be highly efficient bioaccumulators of soluble and particulate forms of metals, especially dilute external solutions. Microbe related technologies (Biotechnology) may provide an alternative or additive conventional method for metal removal or metal recovery. This study dealt with isolation, identification and characterization of heavy metal-resistant (Pb²⁺, Cd²⁺, Al³⁺, Cu²⁺, Ag²⁺ and Sn²⁺) bacteria from sewage wastewater at Taif Province, Saudi Arabia. Nine bacterial isolates were selected by using an enrichment isolation procedure based on high level of heavy metal resistance. All the isolates showed high resistance to heavy metals with Minimum Inhibitor Concentration (MIC) ranging from 800 μg/ml to 1400 μg/ml. All nine resistant isolates showed multiple tolerances to heavy metals. On the basis of morphological, biochemical and 16S rRNA characterization, the most potent isolates (Cd1-1, Ag1-1, Ag1-3 and Sn1-1) were identified as Alcaligenes faecalis. Scanning electron microscope analysis showed that the morphology of Alcaligenes faecalis Ag1-1 was unchanged after growth in medium without and with addition of Ag²⁺ indicative Ag²⁺ is not toxic to the isolate under the conditions tested. The ability of Alcaligenes faecalis Ag1-1 to synthesize sliver nanoparticles was examined. The heavy metal-resistant bacteria obtained could be useful for the bioremediation of heavy metal-contaminated environment.     

Unraveling the stability of plasma proteins upon interaction of synthesized uridine products: biophysical and molecular dynamics approach

Abstract

Most of the drugs binding to human serum albumin (HSA) are transported to various parts of the body. Here, we have studied the molecular interaction between HSA and synthesized uridine derivatives, 1-[(3R, 4S, 5?R)-2-methyl-3, 4-dihydroxy-5-(hydroxymethyl)oxolan-2-yl]pyrimidine-2,4-dion.)(C-MU); [(2R,3R,4R,5R)-5-(2,4-dioxo-3,4-dihydropyrimidin-1(2H)-yl)-3,4-dihydroxy-4-methyl-tetrahydrofuran-2-yl] methyl methyl phosphochloridate (CM-MU) and [(2R,3S,4R,5R)-5-(2,4-dioxopyrimidin-1-yl)-2-methyl-3,4-dihydroxyoxolan-2-yl] methyl dihydrogen phosphate (P-MU). Cytotoxic studies of these synthesized compounds with mouse macrophages (RAW 246.7) and HeLa cells (human cervical cancer cells) and binding mechanism of these uridine derivatives with HSA were performed. Subsequently, fluorescence quenching was observed upon titration of uridine derivatives with HSA via static mode of quenching, and the binding constants (K_2-C-MU = 4?±?0.03?×?10⁴M^?1, K_5-CM-MU = 1.95?±?0.03?×?10⁴ M^?1 and K_5-P-MU =1.56?±?0.03?×?10⁴ M^?1) were found to be in sync with the computational results. Further, molecular displacement and molecular docking data revealed that all the derivatives are binding in the subdomain IIA and IIB regions of HSA. The protein secondary structure of complexes was determined by circular dichroism, indicating partial unfolding of the protein upon addition of the uridine derivatives. Furthermore, atomic force microscopy data reveal the change in topology upon binding of 2-C-MU, 5-CM-MU and 5-P-MU with HSA, indicating change in the microenvironment around tryptophan region. Additionally, cytotoxicity studies on HeLa and Raw Cell lines suggested that these molecules have significant anti-proliferative and anti-inflammatory properties. Hence, the study may be of help for development of new drugs based on uridine derivatives which may be helpful for combating various potential diseases.

Communicated by Ramaswamy H. Sarma     

Supramolecular architectures of metal-oxalato complexes containing purine nucleobases

The reaction of purine nucleobases (adenine, 3-methyladenine and 9-methylguanine) with a metallic salt in the presence of potassium oxalate yields three compounds with formulae {[Cd(μ-ox)(H₂O)(Hade)]·H₂O}_n (1), {[Cu(μ-ox)(H₂O)(3Meade)]·H₂O}_n (2) and [Cu(ox)(H₂O)₂(9Megua)]·2.5H₂O (3). Crystal structures of compounds 1-2 consist of 1D zig-zag chains in which cis-[M(H₂O)(nucleobase)]²⁺ fragments are linked by bis-bidentate oxalato ligands. In compound 1, the nucleobase is coordinated through the minor groove N3 atom, and the resulting non-canonical 7H-adenine tautomer is stabilized by non-covalent interactions involving more basic N9 and N7 sites. In compound 2, the mutagenic 3-methyladenine is attached to the metal atoms by means of the imidazole N7 atom. The dissimilar binding pattern of the nucleobases produces significant differences in the supramolecular architectures of compounds 1 and 2 which are essentially governed by an extensive network of non-covalent interactions such as hydrogen bonded adenine-adenine base pairs, hydration of the nucleobases, carboxylato-nucleobase associations, and face-to-face π-π stacking. The model 9-methylguanine nucleobase of compound 3 exhibits its usual coordination mode through the major groove N7 atom to form two monomeric [(Cu(ox)(H₂O)₂(9Megua)] units which are held together by means of Watson-Crick like hydrogen bonds between the guanine moieties and the inorganic frameworks generating almost planar tetrameric metal-organic aggregates. The 3D packing of the complex entities affords an open structure containing voids which are filled by decameric (H₂O)₁₀ clusters. Variable-temperature magnetic susceptibility measurements of compound 2 show the occurrence of antiferromagnetic intrachain interactions in good agreement with the structural features of its 1D metal-oxalato framework.