Eu(III) macrocyclic complexes promote cleavage of and bind to models for the 5'-cap of mRNA. Effect of pendent group and a second metal ion

Controlled Hydrolysis of Phosphate Esters: A Route to Calixarene-Supported Rare-Earth Clusters

Phosphate ester bonds are widely present in nature (e. g. DNA/RNA) and can be extremely stable against hydrolysis without the help of catalysts. Previously, we showed how the combination of phosphoryl and calix[4]arene moieties in the same organic framework (L_PO ) allows isolation of single lanthanide (Ln) metal ions as [Ln^III (L_PO )₂ ](O₃ SCF₃ )₃ . Here we report how by controlling the reaction conditions a new hydrolyzed phosphoryl-calix[4]arene ligand (H₃ L_HPO ) is formed as a result of Ln^III -mediated P-OEt bond cleavage in three out of the eight possible sites in L_PO . The chelating nature of H₃ L_HPO traps the Ln^III species in the form of [Ln^III (L_HPO )((EtO)₂ P(O)OH)]₂ dimers (Ln=La, Dy, Tb, Gd), where the Dy derivative shows slow magnetization relaxation. The strategy presented herein could be extended to access a broader library of hydrolyzed platforms (H_x L_HPO ; x=1-8) that may represent mimics of nuclease enzymes.

Luminescent Lanthanide Probes for Inorganic and Organic Phosphates

Inorganic and organic phosphates-including orthophosphate, nucleotides, and DNA-are some of the most fundamental anions in cellular biology, regulating numerous processes of both medical and environmental significance. The characteristic long lifetimes of emitting lanthanides, including the brighter europium(III) and terbium(III), make them ideally suited for the development of molecular probes for the detection of phosphates directly in complex aqueous media. Moreover, given their high oxophilicity and the exquisite sensitivity of their quantum yields to their hydration number, those luminescent lanthanides are perfect for the detection of phosphates. Herein we discuss the principles that have guided the recent developments of molecular probes selective for inorganic or organic phosphates and how these lanthanide complexes facilitate the study of numerous biological processes.

Mechanistic Insight of Sensing Hydrogen Phosphate in Aqueous Medium by Using Lanthanide(III)-Based Luminescent Probes

The development of synthetic lanthanide luminescent probes for selective sensing or binding anions in aqueous medium requires an understanding of how these anions interact with synthetic lanthanide probes. Synthetic lanthanide probes designed to differentiate anions in aqueous medium could underpin exciting new sensing tools for biomedical research and drug discovery. In this direction, we present three mononuclear lanthanide-based complexes, EuLCl₃ (1), SmLCl₃ (2), and TbLCl₃ (3), incorporating a hexadentate aminomethylpiperidine-based nitrogen-rich heterocyclic ligand L for sensing anion and establishing mechanistic insight on their binding activities in aqueous medium. All these complexes are meticulously studied for their preferential selectivities towards different anions such as HPO₄²⁻, SO₄²⁻, CH₃COO⁻, I⁻, Br⁻, Cl⁻, F⁻, NO₃⁻, CO₃²⁻/HCO₃⁻, and HSO₄⁻ at pH 7.4 in aqueous HEPES (2-[4-(2-hydroxyethyl)piperazin-1-yl]ethanesulfonic acid) buffer. Among the anions scanned, HPO₄²⁻ showed an excellent luminescence change with all three complexes. Job’s plot and ESI-MS support the 1:2 association between the receptors and HPO₄²⁻. Systematic spectrophotometric titrations of 1–3 against HPO₄²⁻ demonstrates that the emission intensities of 1 and 2 were enhanced slightly upon the addition of HPO₄²⁻ in the range 0.01–1 equiv and 0.01–2 equiv., respectively. Among the three complexes, complex 3 showed a steady quenching of luminescence throughout the titration of hydrogen phosphate. The lower and higher detection limits of HPO₄²⁻ by complexes 1 and 2 were determined as 0.1–4 mM and 0.4–3.2 mM, respectively, while complex 3 covered 0.2–100 μM. This concludes that all complexes demonstrated a high degree of sensitivity and selectivity towards HPO₄²⁻.

Binding Studies of a New Water-Soluble Iron(III) Schiff Base Complex to DNA Using Multispectroscopic Methods

A novel iron(III) complex [Fe(SF)](ClO₄)₃.2H₂O; in which SF = N,N₀-bis{5-[(triphenylphosphonium chloride)-methyl] salicylidene}-o-phenylenediamine) has been synthesized and characterized using different physicochemical methods. The binding of this complex with calf thymus (CT) DNA was investigated by circular dichroism, absorption studies, emission spectroscopy, voltammetric studies, and viscosity measurements. The results showed that this complex can bind to DNA via external and groove binding modes.

Enantiomeric self-recognition in homo- and heterodinuclear macrocyclic lanthanide(III) complexes

The controlled formation of lanthanide(III) dinuclear μ-hydroxo-bridged [Ln(2)L(2)(μ-OH)(2)X(2)](n+) complexes (where X = H(2)O, NO(3)(-), or Cl(-)) of the enantiopure chiral macrocycle L is reported. The (1)H and (13)C NMR resonances of these complexes have been assigned on the basis of COSY, NOESY, TOCSY, and HMQC spectra. The observed NOE connectivities confirm that the dimeric solid-state structure is retained in solution. The enantiomeric nature of the obtained chiral complexes and binding of hydroxide anions are reflected in their CD spectra. The formation of the dimeric complexes is accompanied by a complete enantiomeric self-recognition of the chiral macrocyclic units. The reaction of NaOH with a mixture of two different mononuclear lanthanide(III) complexes, [Ln(1)L](3+) and [Ln(2)L](3+), results in formation of the heterodinuclear [Ln(1)Ln(2)L(2)(μ-OH)(2)X(2)](n+) complexes as well as the corresponding homodinuclear complexes. The formation of the heterodinuclear complex is directly confirmed by the NOESY spectra of [EuLuL(2)(μ-OH)(2)(H(2)O)(2)](4+), which reveal close contacts between the macrocyclic unit containing the Eu(III) ion and the macrocyclic unit containing the Lu(III) ion. While the relative amounts of homo- and heterodinuclear complexes are statistical for the two lanthanide(III) ions of similar radii, a clear preference for the formation of heterodinuclear species is observed when the two mononuclear complexes contain lanthanide(III) ions of markedly different sizes, e.g., La(III) and Yb(III). The formation of heterodinuclear complexes is accompanied by the self-sorting of the chiral macrocyclic units based on their chirality. The reactions of NaOH with a pair of homochiral or racemic mononuclear complexes, [Ln(1)L(RRRR)](3+)/[Ln(2)L(RRRR)](3+), [Ln(1)L(SSSS)](3+)/[Ln(2)L(SSSS)](3+), or [Ln(1)L(rac)](3+)/[Ln(2)L(rac)](3+), results in mixtures of homochiral, homodinuclear and homochiral, heterodinuclear complexes. On the contrary, no heterochiral, heterodinuclear complexes [Ln(1)L(RRRR)Ln(2)L(SSSS)(μ-OH)(2)X(2)](n+) are formed in the reactions of two different mononuclear complexes of opposite chirality.

N-benzoylated 1,4,8,11-tetraazacyclotetradecane and their copper(II) and nickel(II) complexes: spectral, magnetic, electrochemical, crystal structure, catalytic and antimicrobial studies

A series of N-benzoylated cyclam ligands incorporating three different benzoyl groups 1,4,8,11-tetra-(benzoyl)-1,4,8,11-tetraazacyclotetradecane (L(1)), 1,4,8,11-tetra-(2-nitrobenzoyl)-1,4,8,11-tetraazacyclotetradecane (L(2)) and 1,4,8,11-tetra-(4-nitrobenzoyl)-1,4,8,11-tetraazacyclotetradecane (L(3)) and their nickel(II) and copper(II) complexes are described. Crystal structure of L(1) is also reported. The ligands and complexes were characterized by elemental analysis, electronic, IR, (1)H NMR and (13)C NMR spectral studies. N-benzoylation causes red shift in the lambda(max) values of the complexes. The cyclic voltammogram of the complexes of ligand L(1) show one-electron, quasi-reversible reduction wave in the region -1.00 to -1.04 V, whereas that of L(2) and L(3) show two quasi-reversible reduction peaks. Nickel complexes show one-electron quasi-reversible oxidation wave at a positive potential in the range +1.05 to +1.15 V. The ESR spectra of the mononuclear copper(II) complexes show four lines, characteristic of square-planar geometry with nuclear hyperfine spin 3/2. All copper(II) complexes show a normal room temperature magnetic moment values mu(eff) 1.70-1.73 BM which is close to the spin-only value of 1.73 BM. Kinetic studies on the oxidation of pyrocatechol to o-quinone using the copper(II) complexes as catalysts and hydrolysis of 4-nitrophenylphosphate using the copper(II) and nickel(II) complexes as catalysts were carried out. All the ligands and their complexes were also screened for antimicrobial activity against Gram-positive, Gram-negative bacteria and human pathogenic fungi.

Activation of a PARACEST agent for MRI through selective outersphere interactions with phosphate diesters

Ln(S-THP)(3+) complexes are paramagnetic chemical exchange saturation transfer (PARACEST) agents for magnetic resonance imaging (MRI; S-THP = (1S,4S,7S,10S)-1,4,7,10-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane, Ln(III) = Ce(III), Eu(III), Yb(III)). CEST spectra at 11.7 T show that the PARACEST effect of these complexes is enhanced at neutral pH in buffered solutions containing 100 mM NaCl upon the addition of 1-2 equiv of diethylphosphate (DEP). CEST images of phantoms at 4.7 T confirm that DEP enhances the properties of Yb(S-THP)(3+) as a PARACEST MRI agent in buffered solutions at neutral pH and 100 mM NaCl. Studies using (1)H NMR, direct excitation Eu(III) luminescence spectroscopy, and UV-visible spectroscopy show that DEP is an outersphere ligand. Dissociation constants for [Ln(S-THP)(OH(2))](DEP) are 1.9 mM and 2.8 mM for Ln(III) = Yb(III) at pH 7.0 and Eu(III) at pH 7.4. Related ligands including phosphorothioic acid, O,O-diethylester, ethyl methylphosphonate, O-(4-nitrophenylphosphoryl)choline, and cyclic 3,5-adenosine monophosphate do not activate PARACEST. BNPP (bis(4-nitrophenyl phosphate) activates PARACEST of Ln(S-THP)(3+) (Ln(III) = Eu(III), Yb(III)), albeit less effectively than does DEP. These data show that binding through second coordination sphere interactions is selective for phosphate diesters with two terminal oxygens and two identical ester groups. A crystal structure of [Eu(S-THP)(OH(2))]((O(2)NPhO)(2)PO(2))(2)(CF(3)SO(3)) x 2 H(2)O x iPrOH has two outersphere BNPP anions that form hydrogen bonds to the alcohol groups of the macrocycle and the bound water ligand. This structure supports (1)H NMR spectroscopy studies showing that outersphere interactions of the phosphate diester with the alcohol protons modulate the rate of alcohol proton exchange to influence the PARACEST properties of the complex. Further, DEP interacts only with the nonionized form of the complex, Ln(S-THP)(OH(2))(3+) contributing to the pH dependence of the PARACEST effect.

Spectroscopic system for direct lanthanide photoluminescence spectroscopy with nanomolar detection limits

A new spectroscopic system for direct photoluminescence of lanthanide ions (Ln(III)) through electronic transitions within the 4f(n) manifold is described. The system is based on an injection seeded frequency tripled (lambda = 355 nm) Nd:YAG pump laser coupled with a master oscillator power oscillator (MOPO). The MOPO delivers an average pulse energy of approximately 60 mJ/pulse, is continuously tunable from 425 to 690 nm (Signal) and 735 to 1800 nm (Idler) with a linewidth of <0.2 cm(-1), and has a pulse duration of 10-12 ns. Aqueous solutions containing two polyaminocarboxylate complexes, ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA), and Ln(3+) aqua ion for several lanthanides including Eu(III), Tb(III), Dy(III), and Sm(III)) are used as steady-state and time-resolved photoluminescence standards. The versatility of the instrument is demonstrated by excitation scans over a broad visible range for aqueous solutions of complexes of Eu(III), Dy(III), Sm(III), and Tb(III). The Eu(III) excitation band ((7)F(o)-->(5)D(o)) is recorded over a range of complex concentrations that are 1000-fold less than reported previously, including Eu(EDTA) (1.00 nM), Eu(DTPA) (1.00 nM), and Eu(III) aqua ion (50.0 nM). Emission spectra are recorded in the visible range for Ln(III) complexes at pH 6.5 and 1.00 mM. Excited-state lifetimes for the standards were constant as a function of concentration from 10.0 nM to 1.00 mM for Eu(EDTA) and Eu(DTPA) and from 100 nM to 1.00 mM for Eu(III) aqua ion. Photoluminescence lifetimes in H(2)O and D(2)O are recorded and used to calculate the number of bound water molecules for all complexes.

Cu2+TerPy complexes as catalysts of the cleavage of the 5'-cap structure of mRNA

Cu(2+)TerPy is a fairly good catalyst of the cleavage of dinucleoside triphosphates, but its efficiency is not sufficient for use in artificial RNA cleaving enzymes. The present work is aimed at improving the catalysis by Cu(2+)TerPy with additional catalysts. Electrophilic and general acid catalysis have been studied and bifunctional catalysts have been synthesized. The most efficient catalysis was achieved with a Cu(2+)TerPy-dimer.

Binding of Nitrate to a CuII−Cyclen Complex Bearing a Ferrocenyl Pendant: Synthesis, Solid-State X-ray Structure, and Solution-Phase Electrochemical and Spectrophotometric Studies

The reaction of Cu(NO3)2.3H2O with the ligand 1-(ferrocenemethyl)-1,4,7,10-tetraazacyclododecane (L) in acetonitrile leads to the formation of a blue complex, [Cu(L)(NO3)][NO3] (C1). The X-ray structure determination shows an unexpected binding of a nitrate anion in that the CuII center is surrounded by four N atoms of the 1,4,7,10-tetraazacyclododecane (cyclen) macrocycle and two O atoms from a chelating nitrate anion, both Cu-O distances being below the sums of the van de Waals radii. Hydrogen-bonding interactions in the crystal lattice and a weak interaction between a second nitrate O and the CuII center in C1 give rise to a highly distorted CuII geometry relative to that found in the known structure of [Cu(cyclen)(NO3)][NO3] (C5). Electrochemical studies in acetonitrile containing 0.1 M [Bu4N][NO3] as the supporting electrolyte showed that oxidation of C1 in this medium exhibits a single reversible one-electron step with a formal potential E degrees f of +85 mV vs Fc0/+ (Fc = ferrocene). This process is associated with oxidation of the ferrocenyl pendant group. Additionally, a reversible one-electron reduction reaction with an E degrees f value of -932 mV vs Fc0/+, attributed to the CuII/I redox couple, is detected. Gradual change of the supporting electrolyte from 0.1 M [Bu4N][NO3] to the poorly coordinating [Bu4N][PF6] electrolyte, at constant ionic strength, led to a positive potential shift in E degrees f values by +107 and +39 mV for the CuII/I(C1) and Fc0/+(C1) redox couples, respectively. Analysis of these electrochemical data and UV-vis spectra is consistent with the probable presence of the complexes C1, [Cu(L)(CH3CN)2]2+ (C2), [Cu(L)(CH3CN)(NO3)]+ (C3), and [Cu(L)(NO3)2] (C4) as the major species in nitrate-containing acetonitrile solutions. In weakly solvating nitromethane, the extent of nitrate complexation remains significant even at low nitrate concentrations, due to the lack of solvent competition.

Metal ion complexes of macrocyclic polyamines enhance both the phosphate hydrolysis and imidazole ring opening of RNA 5'-cap structure

The cleavage of P1-(7-methylguanosyl-5') P3-(guanosyl-5') triphosphate, a RNA 5'-cap model, by 2-hydroxyethyl- (6a-6c) and 2-aminoethyl- (7a-7c) substituted macrocycles in the presence and absence of Zn2+ and Cu2+ ions has been studied at pH 7.2 and 60 degrees. In the presence of the metal ions, hydrolysis of the phosphate group is enhanced. The mono- and dinuclear Zn2+ complexes promote solely the phosphate hydrolysis, whereas the corresponding Cu2+ complexes accelerate both the phosphate hydrolysis and the imidazole ring opening of the 7-methylguanine base. In the absence of the metal ions, the macrocycles mainly promote breakdown of the 7-methylguanine base, most probably by enhancing the nucleophilic attack of hydroxide ion on the C(8)-atom by shielding the repulsive negative charge on the phosphate moiety. The 2-hydroxyethyl and 2-aminoethyl side arms exhibit a two- to three-fold rate acceleration. Opening of the imidazole ring eventually results in cleavage of the triphosphate bridge.

Chiral multinuclear macrocyclic polyamine complexes: Synthesis, characterization and their interaction with plasmid DNA

A series of multinuclear macrocyclic polyamine metal (Zn(2+), Cu(2+), Co(2+)) complexes containing chiral dipeptide linkage were synthesized and used as artificial nuclease enzyme model. The interaction between the complexes and plasmid DNA (pUC19) was studied, and the results revealed that these complexes could act as powerful catalysts for the cleavage of plasmid DNA under physiological conditions.

Monitoring proton dissociation and solution conformation of chiral ytterbium complexes with near-IR CD

The ytterbium complex [Yb((S)-THP)](3+) ((S)-THP = (1S,4S,7S,10S-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane) is investigated in solution through NMR, near-IR absorption, and CD spectroscopy. Quantitative analysis of the paramagnetic pseudocontact NMR shift shows Lambda helicity of the ligand cage around the metal. The NIR CD spectrum recorded at acidic pH is found to be very similar to that of [Yb((R)-DOTMA)](-) ((R)-DOTMA = (1R,4R,7R,10R)-alpha,alpha',alpha'',alpha'''-tetramethyl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), which in solution assumes a twisted square antiprism (TSA) conformation. The similarity of the NIR CD spectra is discussed, and it is the first proof of the Lambda(lambda,lambda,lambda,lambda) conformation of [Yb((S)-THP)](3+). NIR CD spectra recorded in the pH range of 2-9 allow one to easily follow proton dissociation and to calculate the pK of this equilibrium in water (pK(A) = 6.4 +/- 0.1). This value agrees well with that determined for [Lu((S)-THP)](3+) using potentiometric methods. This demonstrates once again that NIR CD spectroscopy is a powerful technique for investigating the solution structure and dynamics of these complexes.

The conjugates of uracil-cyclen Zn(II) complexes: synthesis, characterization, and their interaction with plasmid DNA

As an important nucleobase in RNA, uracil was introduced into the side chain of cyclen (1,4,7,10-tetraazacyclododecane) by using phenylene dimethylene group as bridge. The target compounds 5 were obtained in high yields. Subsequent experiments demonstrated that the uracil-cyclen conjugates can bind Zn(2+) cation rapidly in water, and the catalytic activities of their Zn(II) complexes 6 in DNA cleavage were also studied. The results showed that Zn(II) complexes can catalyze the cleavage of supercoiled DNA (pUC 19 plasmid DNA) (Form I) to produce nicked DNA (Form II and Form III) with high selectivity. In water solution, complex 6b may form a unique and stable supramolecular structure, which benefits the DNA cleavage process.

Selective Protection of 1,4,8,11-Tetraazacyclotetradecane (Cyclam) in Position 1,4 with the Phosphonothioyl Group and Synthesis of a Cyclam-1,4-bis(methylphosphonic Acid). Crystal Structures of Several Cyclic Phosphonothioamides

A new cyclam-based ligand, 1,4,8,11-tetraazacyclotetradecane-1,4-bis(methylphosphonic acid) (1,4-H4te2p), was synthesized. Cyclam was protected by the reaction with PhP(S)Cl2 to form exclusively five-membered cyclic phenylphosphonothioic diamide 2 in a moderate yield. The solid-state structures of 2 and several by-products were determined. Compound 2 was isolated as two stable conformers differing in a mutual position of benzene ring and sulfur atom with respect to the cyclam ring. Compound 2 was used for the synthesis of 1,4-dibenzylcyclam. However, the deprotection of the thiophosphoryl-protected bis(methylphosphonate diester) with aqueous HCl under non-optimized conditions led to a mixture of cyclam derivatives differently substituted with methylphosphonic acid groups. The crystal structures of the target product, 1,4-H4te2p, and also 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetrakis(methylphosphonic acid) (H8tetp) were determined. A similar reaction with cyclen (1,4,7,10-tetraazacyclododecane) led only to hardly purifiable mixtures.

Macrocyclic Lanthanide Complexes as Artificial Nucleases and Ribonucleases: Effects of pH, Metal Ionic Radii, Number of Coordinated Water Molecules, Charge, and Concentrations of the Metal Complexes

We have been interested in the design, synthesis, and characterization of artificial nucleases and ribonucleases by employing macrocyclic lanthanide complexes because their high thermodynamic stability, low kinetic lability, high coordination number, and charge density (Lewis acidity) allow more design flexibility and stability. In this paper, we report the study of the use of the europium(III) complex, EuDO2A+ (DO2A is 1,7-dicarboxymethyl-1,4,7,10-tetraazacyclododecane) and other lanthanide complexes (i.e., LaDO2A+, YbDO2A+, EuK21DA+, EuEDDA+, and EuHEDTA where K21DA is 1,7-diaza-4,10,13-trioxacyclopentadecane-N,N'-diacetic acid, EDDA is ethylenediamine-N,N'-diacetic acid, and HEDTA is N-hydroxyethyl-ethylenediamine-N,N',N'-triacetic acid), as potential catalysts for the hydrolysis of the phosphodiester bond of BNPP (sodium bis(4-nitrophenyl)-phosphate). For the pH range 7.0-11.0 studied, EuDO2A+ promotes BNPP hydrolysis with the quickest rates among LaDO2A+, EuDO2A+, and YbDO2A+. This indicates that charge density is not the only factor affecting the reaction rates. Among the four complexes, EuDO2A+, EuK21DA+, EuEDDA+, and EuHEDTA, with their respective number of inner-sphere coordinated water molecules three, two, five, and three, EuEDDA+, with the greatest number of inner-sphere coordinated water molecules and a positive charge, promotes BNPP hydrolysis more efficiently at pH below 8.4, and the observed rate trend is EuEDDA+ > EuDO2A+ > EuK21DA+ > EuHEDTA. At pH > 8.4, the EuEDDA+ solution becomes misty and precipitates form. At pH 11.0, the hydrolysis rate of BNPP in the presence of EuDO2A+ is 100 times faster than that of EuHEDTA, presumably because the positively charged EuDO2A+ is more favorable for binding with the negatively charged phosphodiester compounds. The logarithmic hydrolysis constants (pKh) were determined, and are reported in the parentheses, by fitting the kinetic k(obs) data vs pH for EuDO2A+ (8.4), LaDO2A+ (8.4), YbDO2A+ (9.4), EuK21DA+ (7.8), EuEDDA+ (9.0), and EuHEDTA (10.1). The preliminary rate constant-[EuDO2A+] data at pH 9.35 were fitted to a monomer-dimer reaction model, and the dimer rate constant is 400 times greater than that of the monomer. The fact that YbDO2A+ catalyzes BNPP less effectively than EuDO2A+ is tentatively explained by the formation of an inactive dimer, [Yb(DO2A)(OH)]2, with no coordination unsaturation for BNPP substrate binding.

Molecular design of an acid?base cooperative catalyst for RNA cleavage based on a dizinc complex

The effects of donor groups of dizinc complexes, formed from a 2:1 mixture of Zn(II) and a dinucleating ligand, on adenylyl(3'-5')adenosine (ApA) cleavage have been studied. Two dinucleating ligands were used: one had two 2-pyridylmethyl and two 2-hydroxyethyl moieties on the 1,3-diamino-2-propanol linker moiety (2), and the other had two 2-pyridylmethyl and two carboxymethyl moieties on the 1,3-diamino-2-propanol linker moiety (3(2-)). The dizinc complex with2 [(Zn(2+))(2)-2] showed higher activities toward ApA cleavage than the dizinc complex using an analogous dinucleating ligand having four 2-pyridylmethyl donor moieties [(Zn(2+))(2)-1] at pH 5-8. The former showed a bell-shaped pH-rate constant profile, whereas the latter showed a sigmoidal pattern. The differences in the pH-rate constant profile are attributable to the various distributions of the monohydroxo-dizinc species, i.e. dideprotonated species, which are responsible for ApA cleavage. The monohydroxo species of (Zn(2+))(2)-2 has two acidic protons, which are not present in the corresponding monohydroxo species of (Zn(2+))(2)-1. The existence of both intracomplex acid (ROH or H(2)O) and base catalysts (RO(-) or OH(-)) in (Zn(2+))(2)-2 can explain its higher activity toward ApA cleavage than that of (Zn(2+))(2)-1. In contrast, (Zn(2+))(2)-3(2-) showed lower activity toward ApA cleavage at pH 7.0, which can be ascribed to the absence of the monohydroxo-dizinc species under these conditions.

Macrocyclic amines as catalysts of the hydrolysis of the triphosphate bridge of the mRNA 5'-cap structure

The reactions of a 5'-cap model compound P1-(7-methylguanosine) P3-guanosine 5',5'-triphosphate, m7GpppG, were studied in the presence of three different macrocyclic amines (2-4) under neutral conditions. The only products observed in the absence of the macrocycles resulted from the base-catalysed imidazole ring-opening and the acid-catalysed cleavage of the N7-methylguanosine base, whereas in the presence of these catalysts hydrolysis of the triphosphate bridge predominated. The latter reaction yielded guanosine 5'-monophosphate, guanosine 5'-diphosphate, 7-methylguanosine 5'-monophosphate and 7-methylguanosine 5'-diphosphate as the initial products, indicating that both of the phosphoric anhydride bonds were cleaved. The overall catalytic activity of all three macrocycles was comparable. The hydrolysis to guanosine 5'-diphosphate and 7-methylguanosine 5'-monophosphate was slightly more favoured than the cleavage to yield guanosine 5'-monophosphate and 7-methylguanosine diphosphate. All the macrocycles also enhanced the subsequent hydrolysis of the nucleoside diphosphates, 2 being more efficient than 3 and 4.

Geometry-dependent phosphodiester hydrolysis catalyzed by binuclear copper complexes

Two isomeric binuclear ligands PBTPA and MBTPA and their copper(II) complexes were prepared and examined for hydrolysis of a model phosphodiester substrate: bis(p-nitrophenyl)phosphate. A bell-shaped pH vs rate profile, which is in agreement with one mechanism proposed for bimetallonucleases/phosphatases, was observed for the binuclear complex of copper(II) and PBTPA. At pH 8.4, a maximum rate of 1.14 x 10(-6) s(-1)--more than 10(4)-fold over uncatalyzed reactions--was achieved. However, the analogous complex of MBTPA did not show significant rate enhancement. The binuclear complex of copper(II) and PBTPA also showed 10-fold acceleration over mononuclear complex of copper(II) and tris(2-pyridylmethyl)amine (TPA) catalyzed reaction. A phage phiX174 DNA assay showed that the complex of copper(II) and PBTPA promoted supercoiled phage phiX174 DNA relaxation under both aerobic and anaerobic conditions, in contrast to the hydrolytic inactivity of the mononuclear complex of copper(II) and TPA.

Hydrolytic reactions of diadenosine 5',5'-triphosphate

The hydrolysis of diadenosine 5',5'-triphosphate to AMP and ADP has been studied over a wide pH-range. Under acidic conditions the reaction shows a first-order dependence on the hydronium ion concentration. Below pH 3 the rate-increase begins to level off. From pH 6 to 9 the hydrolysis is slow and pH-independent. Base-catalysed hydrolysis is observed in NaOH-solutions. Under alkaline conditions an intramolecular nucleophilic attack on the phosphate producing 3',5'-cAMP is also observed, but it is slower than the intermolecular reaction. Depurination of the adenosine moieties competes with the hydrolysis both under acidic and alkaline conditions, but the mechanisms are different. The temperature-dependence of the hydrolysis of Ap(3)A and the depurination of adenosine moieties were studied under acidic conditions, and the activation parameters of the reactions were calculated. The results of the work reflect the fact that the negatively charged polyphosphate group is very resistant towards nucleophilic attack. An efficient catalysis is only observed under acidic conditions, where the phosphate group becomes protonated. General acids or bases did not catalyse the hydrolysis. Furthermore, hydroxide ion catalysed cleavage is only observed at high base concentrations and other negatively charged nucleophiles did not attack the phosphate groups of diadenosine polyphosphates.

Dinuclear Zn2+ complexes in the hydrolysis of the phosphodiester linkage in a diribonucleoside monophosphate diester

Dizinc complexes that were formed from 2:1 mixtures of Zn(NO3)2 and dinucleating ligands TPHP (1), TPmX (2) or TPpX (3) in aqueous solutions efficiently hydrolyzed diribonucleoside monophosphate diesters (NpN) under mild conditions. The dinucleating ligand affected the structure of the aquo-hydroxo-dizinc core, resulting in different characteristics in the catalytic activities towards NpN cleavage. The pH-rate profile of ApA cleavage in the presence of (Zn2+)(2)-1 was sigmoidal, whereas those of (Zn2+)(2)-2 and (Zn2+)(2)-3 were bell-shaped. The pH titration study indicated that (Zn2+)(2)-1 dissociates only one aquo proton (up to pH 12), whereas (Zn2+)(2)-2 dissociates three aquo protons (up to pH 10.7). The observed differences in the pH-rate profile are attributable to the various distributions of the monohydroxo-dizinc species, which are responsible for NpN cleavage. As compared to that using (Zn2+)(2)-1, the NpN cleavage using (Zn2+)(2)-2 showed a greater rate constant, with a higher product ratio of 3'-NMP/2'-NMP. The saturation behaviors of the rate, with regard to the concentration of NpN, were analyzed by Michaelis-Menten type kinetics. Although the binding of (Zn2+)(2)-2 to ApA was weaker than that of (Zn2+)(2)-1, (Zn2+)(2)-2 showed a greater kcat value than (Zn2+)(2)-1, resulting in higher ApA cleavage activity of the former.

The cleavage of the triphosphate bridge of a model for the 5'-cap mRNA promoted by dinuclear bicyclic complexes with metal ions

Dinuclear bicyclic complexes, which have two active centers, can significantly promote the hydrolysis of the triphosphate bridge in ApppA, a 5'-cap model compound.

Formation of two diverse classes of poly(amino-alkoxide) chelates and their mononuclear and polynuclear lanthanide(III) complexes

Factors that influence aggregation of lanthanide(III) (Ln(III)) ions to form polynuclear complexes were studied utilizing 1-aziridineethanol as a versatile source of macrocyclic and acyclic chelates. The facile ring-opening cyclo-oligomerization of 1-aziridineethanol leads to the formation of a series of polyaza cyclic oligomers (series A). In the presence of ethylenediamine, a competing N-alkylation reaction occurs to produce a new class of acyclic ligands (series B). The cyclo-oligomerization of four 1-aziridineethanol units is the most favorable process, leading to the formation of the 12-membered cyclen-type macrocycle, H(4)L(1) (1,4,7,10-tetrakis(2-hydroxyethyl)-1,4,7,10-tetraaza-cyclododecane). Ring-opening cyclo-oligomerization of 1-aziridineethanol in the presence of Ln(III) ions produces self-assembled mononuclear, tetranuclear, and pentanuclear compounds of H(4)L(1). In the presence of ethylenediamine, oligomerization of 1-aziridineethanol results in a dinuclear complex of an acyclic poly(amino-alkoxide) H(2)L(2). The coordinative unsaturation of (i) the alkoxy sites of [H(x)L(1)](x)(-)(4) (where x < 4) and (ii) Ln(III) ions in coordination numbers less than nine are critical factors in the formation of the polynuclear Ln(III) complexes. The identities of mononuclear, dinuclear, tetranuclear, and pentanuclear complexes herein discussed were established by X-ray crystallography.