Dinuclear Zn(2+) complexes of synthetic heptapeptides as artificial nucleases

Stable right- and left-handed peptide helices containing C(alpha)-tetrasubstituted alpha-amino acids

Short peptidomimetics with stable secondary structures in solution are of interest for applications in chemistry, biology, and medicine. One way to rigidify the backbone of a peptide is the use of cyclic C(alpha)-tetrasubstituted alpha-amino acids (TAAs) like compound 14. The structures resulting from the incorporation of this unnatural amino acid into peptides were investigated. In total, 13 different peptides with a length of up to eight residues and alternating sequences of TAA 14 and (S)- or (R)-valine were synthesized. Their structures were characterized by X-ray diffraction analysis and NMR and CD measurements showing that the all-S-backbone-configured peptides 5 and 6 (SS)(2-3) form right-handed 3(10)-helices, while the all-R-configured peptides 11-13 (RR)(2-4) form left-handed 3(10)-helices in the solid state and solution.

Synthesis of new Calpha-tetrasubstituted alpha-amino acids

C^α-Tetrasubstituted α-amino acids are important building blocks for the synthesis of peptidemimetics with stabilized secondary structure, because of their ability to rigidify the peptide backbone. Recently our group reported a new class of cyclic C^α-tetrasubstituted tetrahydrofuran α-amino acids prepared from methionine and aromatic aldehydes. We now report the extension of this methodology to aliphatic aldehydes. Although such aldehydes are prone to give aldol products under the reaction conditions used, we were able to obtain the target cyclic amino acids in low to moderate yields and in some cases with good diastereoselectivity.

Catalysis of diribonucleoside monophosphate cleavage by water soluble copper(II) complexes of calix[4]arene based nitrogen ligands

Calix[4]arenes functionalized at the 1,2-, 1,3-, and 1,2,3-positions of the upper rim with [12]ane-N(3) ligating units were synthesized, and their bi- and trimetallic zinc(II) and copper(II) complexes were investigated as catalysts in the cleavage of phosphodiesters as RNA models. The results of comparative kinetic studies using monometallic controls indicate that the subunits of all of the zinc(II) complexes and of the 1,3-distal bimetallic copper(II) complex 7-Cu(2) act as essentially independent monometallic catalysts. The lack of cooperation between metal ions in the above complexes is in marked contrast with the behavior of the 1,2-vicinal bimetallic copper(II) complex 6-Cu(2), which exhibits high catalytic efficiency and high levels of cooperation between metal ions in the cleavage of HPNP and of diribonucleoside monophosphates NpN'. A third ligated metal ion at the upper rim does not enhance the catalytic efficiency, which excludes the simultaneous cooperation in the catalysis of the three metal ions in 8-Cu(3). Rate accelerations relative to the background brought about by 6-Cu(2) and 8-Cu(3) (1.0 mM catalyst, water solution, pH 7.0, 50 degrees C) are on the order of 10(4)-fold, largely independent of the nucleobase structure, with the exception of the cleavage of diribonucleoside monophosphates in which the nucleobase N is uracil, namely UpU and UpG, for which rate enhancements rise to 10(5)-fold. The rationale for the observed selectivity is discussed in terms of deprotonation of the uracil moiety under the reaction conditions and complexation of the resulting anion with one of the copper(II) centers.

DNA cleavage promoted by 2,9-dimethyl-4,7-diazadecane-2,9-dithiol (DDD) derivatives

Three piperidine derivatives of 2,9-dimethyl-4,7-diazadecane-2,9-dithiol (DDD), NEPDDD, NEMPDDD, and NEMMPDDD, were synthesized and used as catalysts in DNA cleavage. Under physiological conditions, a series of experiments have been done. The effects of DNA cleavage with three ligands were studied under different concentrations, cleavage time, and pH values. The results strongly suggested that the plasmid DNA (pUC 19) can be cleaved efficiently by these ligands. For the cleavage reaction catalyzed by NEMPDDD, Form I DNA could convert to Form II completely, and the DNA-cleavage mechanism involved an oxidative pathway.

Solvent polarity controls the helical conformation of short peptides rich in Calpha-tetrasubstituted amino acids

The two peptides, rich in C(alpha)-tetrasubstituted amino acids, Ac-[Aib-L-(alphaMe)Val-Aib](2)-L-His-NH(2) (1) and Ac-[Aib-L-(alphaMe)Val-Aib](2)-O-tBu (2 a) are prevalently helical. They present the unique property of changing their conformation from the alpha- to the 3(10)-helix as a function of the polarity of the solvent: alpha in more polar solvents, 3(10) in less polar ones. Conclusive evidence of this reversible change of conformation is reported on the basis of the circular dichroism (CD) spectra and a detailed two-dimensional NMR analysis in two solvents (trifluoroethanol and methanol) refined with molecular dynamics calculations. The X-ray diffractometric analysis of the crystals of both peptides reveals that they assume a prevalent 3(10)-helix conformation in the solid state. This conformation is practically superimposable on that obtained from the NMR analysis of 1 in methanol. The NMR results further validate the reported CD signature of the 3(10)-helix and the use of the CD technique for its assessment.

Cleavage of double-strand DNA by zinc complexes of dicationic 2,2′-dipyridyl derivatives

Two highly charged zinc complexes, [Zn(L1)3](ClO4)8.4H2O (1) and [Zn(L2)2Br](ClO4)5.H2O (2) (L1 = 5,5'-di(1-(triethylammonio)methyl)-2,2'-dipyridyl and L2= 5,5'-di(1-(tributylammonio)methyl)-2,2'-dipyridyl) were synthesized and structurally characterized by crystallography. The zinc atom in 1 shows a distorted octahedral sphere. Variable-pH NMR studies on 1 demonstrated that the saturated six-coordinated [Zn(L1)3]8+ species can partially change into five-coordinated [Zn(L1)2(H2O)]6+ species in aqueous solution. The zinc atom in 2 shows a distorted trigonal-bipyramidal sphere. The average distance of the coordinated Br atom to the cationic N atom in 2 is ca 5.9 A, which is comparable to that of adjacent phosphodiesters in the DNA (ca. 6 A). Both complexes exhibited high nuclease activities towards cleavage of supercoiled plasmid DNA with the activity being the maximum under physiological pH. The effective DNA cleavage may be attributed to the strong electrostatic interaction of the metal moiety and two positive pendants with phosphodiester groups of nucleic acid.

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.

The first dinuclear copper(ii) and zinc(ii) complexes containing novel Bis-TACN: syntheses, structures, and DNA cleavage activities

Two novel binuclear complexes [Cu(2)(L)].(ClO(4))(2) (1) and [Zn(2)(L)].(ClO(4))(2) (2) were synthesized and crystallographically characterized {L = 1(4),5(4)-dimethyl-1(2),5(2)-dihydroxy-1(1,3),5(1,3)-dibenzene-3(1,4),7(1,4)-di-1,4,7-triazacyclononane}. The cation [Cu(2)(L)](2+) structure of 1 is similar to that of [Zn(2)(L)](2+) of 2. The central ion is bridged by the di-phenoxo of L and lies in a close to perfect square pyramidal geometry. 1 and 2 crystallize in the triclinic space group P1. The two complexes effectively promote the cleavage of plasmid DNA in the presence of activating agents at physiological pH and temperature. The pseudo-Michaelis-Menten kinetic parameters k(cat) = 1.61 h(-1), K(m) = 1.35 x 10(-5) M for complex 1 in the presence of mercaptoethanol; k(cat) = 2.48 h(-1), K(m) = 5.5 x 10(-5)M for complex 2 in the presence of hydrogen peroxide were obtained. The mechanism of plasmid DNA cleavage was studied by adding standard radical scavengers. DNA cleavage reaction by the binuclear Zn(II)/H(2)O(2) system is a hydrolytic mechanism.

Synthesis and reactivity of a C3-symmetric trinuclear zinc(ii) hydroxide catalyst efficient at phosphate diester transesterification

Inspired by trinuclear Zn(II) sites in enzymatic systems, a ligand system containing three preorganized (2-pyridyl)methyl piperazine moieties anchored onto a rigid C3-symmetric triphenoxymethane platform has been developed for preorganizing three zinc ions into an environment conducive to intramolecular interaction. Zinc(II) binding by this ligand has been analyzed by means of potentiometric measurements in 50% (v/v) CH3CN-H2O solutions. Subsequently a C3-symmetric trinuclear Zn(II) hydroxide complex of the C3-symmetric ligand was synthesized and fully characterized using NMR spectroscopy and X-ray crystallography. This complex induces a 16,900-fold rate enhancement in the catalytic cyclization of the RNA model substrate, 2-hydroxypropyl-p-nitrophenyl phosphate (HPNP, pH 6.7, 25 degrees C) over the uncatalyzed reaction with multiple catalyst turnovers. The observed differences in the pH-rate profile can be attributed to the varying concentration of various trinuclear zinc species. The trinuclear Zn(II) catalyst exhibits a higher hydrolytic activity compared to its mononuclear analogue. The reactivity and structural features of this trinuclear Zn(II) complex will be discussed.

Phosphodiester modification by zinc metalated adenine polymer with carboxyl pendants

This report describes a novel carboxyl pendant containing adenylated polymeric template, its metalation with Zn (II), and manifestation of catalytic activity for the hydrolysis of model phosphodiester, bis(p-nitrophenyl) phosphate (bNPP), and plasmid cleavage. Observation of a bell-shaped pH-K(obs) profile suggested influence of pH variation over hydrolysis rate. This metalated polymer also afforded facile relaxation of pBR322 supercoiled DNA, with an interesting reusability feature intricately associated with heterogeneous catalysis.

C(alpha)-tetrasubstituted amino acid based peptides in asymmetric catalysis

C(alpha)-tetrasubstituted alpha-amino acids constitute a powerful tool for controlling the conformation of short peptide sequences. Chiral peptides may be used in stereoselective reactions both for asymmetric induction and in kinetic resolution. By reviewing recent data from our own laboratories and by presenting new results on the stereoselective oxidation of chalcone to the corresponding oxide, this contribution shows that the control of peptide conformation is a critical issue in order to achieve successful stereoselective catalysis.

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.

DNA hydrolysis promoted by 1,7-dimethyl-1,4,7,10-tetraazacyclododecane

Several acyclic and macrocyclic polyamines were evaluated for their ability to cleave DNA. 1,7-Dimethyl-1,4,7,10-tetraazacyclododecane (DMC) could hydrolyze double-strand DNA at a concentration of 25microM. pH 7.2 was the optimal condition to cleave DNA in the presence of DMC. Supercoiled DNA hydrolytic cleavage by DMC was supported by the evidence from free radical quenching and T4 ligase ligation.

A New S4-Ligated Zinc–Peptide 1 : 2 Complex for the Hydrolytic Cleavage of DNA

A new sulfur-ligated Zn-peptide 1:2 complex, [Zn(II)(Boc-NH-Cys-Gly-Cys-OMe)2]2- (2), was prepared, characterized, and tested for its DNA-binding and -cleavage properties. Complex 2 was found to cleave DNA hydrolytically. The negative charge in 2 reduces the affinity of the complex for DNA, and enhances its binding specificity.

Ternary Nickel(II) Complexes as Hydrolytic DNA-Cleavage Agents

A series of small model complexes made from Ni(II) and the ligands ethylenediamine (en), histamine (hist), and histidylleucine (HisLeu) were prepared and studied as potential hydrolytic DNA-cleavage agents. The stability constants and species-distribution curves for these complexes were determined as a function of pH. The 1 : 1 : 1 ternary complexes [Ni(II)(en)(HisLeu)] (1) and [Ni(II)(hist)(HisLeu)] (2) were the only major species present at the physiologically relevant pH of 6-7, as further corroborated by ESI-MS analysis. The complex geometries of 1 and 2 were analyzed by UV/VIS experiments and molecular dynamics (MD) simulations. Both ternary complexes were found to intercalate with DNA, as shown by UV/VIS, thermal-denaturation, and fluorescence-titration studies with ethidium bromide (EB). The intrinsic binding constants (K(b)) for the bound complexes 1DNA and 2DNA were determined as 150 and 290, resp. Gel-electrophoresis experiments revealed that 1 and 2 cleave supercoiled (type-I) to nicked-circular (type-II) DNA at physiological pH, with rate constants of 0.64 and 0.75 h(-1), resp. A tentative mechanism for this hydrolytic cleavage is proposed.

Basicity and coordination properties of a new phenanthroline-based bis-macrocyclic receptor

The synthesis and characterisation of the new macrocyclic ligand 6-methyl-2,6,10-triaza-[11]-12,25-phenathrolinophane (L1), which contains a triamine aliphatic chain linking the 2,9 positions of 1,10-phenanthroline and of its derivative L2, composed by two L1 moieties connected by an ethylenic bridge, are reported. Their basicity and coordination properties toward Cu(II), Zn(II), Cd(II), Pb(II) and Hg(II) have been studied by means of potentiometric and spectroscopic (UV-Vis, fluorescence emission) measurements in aqueous solutions. L1 forms 1:1 metal complexes in aqueous solutions, while L2 can give both mono- and dinuclear complexes. In the mononuclear L2 complexes the metal is sandwiched between the two cyclic moieties. The metal complexes with L1 and L2 do not display fluorescence emission, due to the presence of amine groups not involved in metal coordination. These amine groups can quench the excited fluorophore through an electron transfer process. The ability of the Zn(II) complexes with L1 and L2 to cleave the phosphate ester bond in the presence has been investigated by using bis(p-nitrophenyl)phosphate (BNPP) as substrate. The dinuclear complex with L2 shows a remarkable hydrolytic activity, due to the simultaneous presence within this complex of two metals and two hydrophobic units. In fact, the two Zn(II) act cooperatively in substrate binding, probably through a bridging interaction of the phosphate ester; the interaction is further reinforced by pi-stacking pairing and hydrophobic interactions between the phenanthroline unit(s) and the p-nitrophenyl groups of BNPP.

Tetraplatinated Artificial Oligopeptides Afford High Affinity Intercalation into dsDNA

This paper reports the binding of an artificial tetrapeptide to which are tethered four Pt(II) complexes (i.e., [Pt(tpy)(py)]48+) with a 12 base pair duplex DNA oligonucleotide. Isothermal Titration Calorimetry reveals that two tetrametallic peptides stoichiometrically bind to each DNA duplex with a binding constant, KB, of 1.7 x 106 M-1, with a change in free energy of -8.5 kcal/mol. This KB represents an affinity 2 orders of magnitude greater than that of the monometallic analogue [Pt(tpy)(pic)]2+ for the same dsDNA sequence. The metalated peptides bind by intercalation into the DNA, partially unwinding the helix while stabilizing the structure, causing an increase in the dsDNA melting temperature of 25 degrees C.

Effect of Heteroatom Insertion at the Side Chain of 5-Alkyl-1H-tetrazoles on Their Properties as Catalysts for Ester Hydrolysis at Neutral pH

[reaction: see text] Herein we introduce tetrazole and its suitably designed derivatives as powerful ester-cleaving reagents. By first performing a detailed ab initio computational study, we found that, in the side chain of 5-alkyl-1H-tetrazoles, introduction of a heteroatom (e.g., N, O, or S at the alpha-position of the tetrazole ring) raises the charge on the tetrazole nucleus significantly. All calculations have been performed using restricted Hartree-Fock (RHF) and hybrid ab initio/DFT (B3LYP) methods employing 6-31G* and 6-31+G* basis sets. To estimate the nucleophilicity of these reagents, the charges on conjugate bases of various tetrazole derivatives have been calculated using natural population (NBO) analysis in gas phase and in water. Free energy of protonation (fep) of the 1H-tetrazole derivatives (1-7), free energy of solvation, deltaG(aq), and the corresponding pKa values have been calculated by self-consistent reaction field (SCRF) methods applying the polarized continuum model (PCM). Since the calculation indicates that incorporation of heteroatom leads to enhanced nucleophilicity in their deprotonated anionic tetrazole forms, a series of 5-substituted 1H-tetrazole derivatives have been synthesized. These compounds indeed catalyze the hydrolysis of p-nitrophenyl diphenyl phosphate (PNPDPP) and p-nitrophenyl hexanoate (PNPH) efficiently in cationic cetyl trimethylammonium bromide (CTABr) micelles at pH 7.0 and 25 degrees C. The pseudo-first-order rate constants (k(obs)) were determined for each catalyst against both substrates. The experimental and theoretical results show that, to achieve better k(obs) values for the cleavage of PNPDPP and PNPH under micellar conditions, charge on the N- atom (nucleophile) of conjugate base is important. Replacing the alpha-CH2 in alkyl substituent with S (3), NH (4), or O (5) enhances the accumulation of charge on N- in conjugate bases of tetrazoles and subsequently increases their intrinsic nucleophilic reactivity toward hydrolytic reactions. Significantly large rate enhancements were observed for the cleavage of PNPDPP and PNPH at pH 7.0 in the presence of catalytic system 5/CTABr over background (only CTABr). Tetrazole 4 (alpha-isomer) showed 4-5-fold superior reactivity over 6 (beta-isomer) under identical conditions. Natural charges obtained from NBO analysis (B3LYP/6-31+G*) are -0.94 and -0.852 on N- in the conjugate bases of 4 and 6, respectively. This also predicts that 4 is a better nucleophile than 6. All the newly synthesized tetrazole derivatives in micellar media display true catalytic properties by cleaving several fold excess of substrates.

Novel Peptide-Based Copper(II) Complexes for Total Hydrolytic Cleavage of DNA

Stable Cu(II) complexes with histamine- and histidine-containing dipeptides histidylserine and histidylphenylalanine have been developed. Their interaction in solution has been investigated, and the stability of their complexes was determined. The nature of binding in these complexes has been explained with the help of potentiometric pH titrations and 1H-NMR spectroscopy. The geometry of these complexes has been established by electronic spectra. The DNA-binding and -cleavage abilities of these Cu(II) complexes have been probed by the absorption, thermal denaturation, fluorescence, and electrophoresis experiments. The results suggest that these peptide-based Cu(II) complexes effectively bind and efficiently cleave DNA under mild biological conditions. Since Cu(II) complexes are known to play an important role in phosphodiester bond cleavages, these results assume importance.

Dinuclear macrocyclic polyamine zinc(II) complexes linked with flexible spacers: Synthesis, characterization, and DNA cleavage

Dinuclear macrocyclic polyamine zinc(II) complexes, which have two cyclen groups linked by flexible spacers, have been synthesized as DNA cleavage agents. The structures of these new dinuclear complexes are consistent with the data obtained from elemental analysis, MS and 1H NMR spectroscopy. The catalytic activity of these dinuclear complexes on DNA cleavage was studied. The results showed that the dinuclear zinc(II) complexes can catalyze the cleavage of supercoiled DNA (pUC 19 plasmid DNA) (Form I) under physiological conditions to produce selectively nicked DNA (Form II).

Controlled Recovery of the Transcription of Nanoparticle-Bound DNA by Intracellular Concentrations of Glutathione

Positively charged trimethylammonium-functionalized mixed monolayer protected clusters (MMPCs) bind DNA through complementary electrostatic interactions, resulting in complete inhibition of DNA transcription of T7 RNA polymerase. DNA was released from the nanoparticle by intracellular concentrations of glutathione, resulting in efficient transcription. The restoration of RNA production was dose-dependent in terms of GSH, with considerable control of the release process possible through variation in monolayer structure. This work presents a new approach to controlled release of DNA, with potential applications in the creation of transfection vectors and gene regulation systems.

Lanthanide-mediated phosphoester hydrolysis and phosphate elimination from phosphopeptides

Lanthanide ions can mediate both phosphomonoester hydrolysis and beta-elimination of inorganic phosphate from polypeptide substrates under near-physiological conditions of pH, temperature, and salt.

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.

Zn(II) coordination to polyamine macrocycles containing dipyridine units. New insights into the activity of dinuclear Zn(II) complexes in phosphate ester hydrolysis

Zn(II) binding by the dipyridine-containing macrocycles L1-L3 has been analyzed by means of potentiometric measurements in aqueous solutions. These ligands contain one (L1, L2) or two (L3) 2,2'-dipyridine units as an integral part of a polyamine macrocyclic framework having different dimensions and numbers of nitrogen donors. Depending on the number of donors, L1-L3 can form stable mono- and/or dinuclear Zn(II) complexes in a wide pH range. Facile deprotonation of Zn(II)-coordinated water molecules gives mono- and dihydroxo-complexes from neutral to alkaline pH values. The ability of these complexes as nucleophilic agents in hydrolytic processes has been tested by using bis(p-nitrophenyl) phosphate (BNPP) as a substrate. In the dinuclear complexes the two metals play a cooperative role in BNPP cleavage. In the case of the L2 dinuclear complex [Zn(2)L2(OH)(2)](2+), the two metals act cooperatively through a hydrolytic process involving a bridging interaction of the substrate with the two Zn(II) ions and a simultaneous nucleophilic attack of a Zn-OH function at phosphorus; in the case of the dizinc complex with the largest macrocycle L3, only the monohydroxo complex [Zn(2)L3(OH)](3+) promotes BNPP hydrolysis. BNPP interacts with a single metal, while the hydroxide anion may operate a nucleophilic attack. Both complexes display high rate enhancements in BNPP cleavage with respect to previously reported dizinc complexes, due to hydrophobic and pi-stacking interactions between the nitrophenyl groups of BNPP and the dipyridine units of the complexes.

An Asymmetric Dizinc Phosphodiesterase Model with Phenolate and Carboxylate Bridges

A phosphodiesterase model with two zinc centers has been synthesized and characterized. The compound, [Zn(2)(L(-)(2H))(AcO)(H(2)O)](PF(6)).2H(2)O (Zn(2)L'), was formed using an "end-off" type compartmental ligand, 2,6-bis{[(2-pyridylmethyl)(2-hydroxyethyl)amino]methyl}-4-methylphenol (L), and zinc acetate dihydrate. The X-ray crystallographic analysis shows that Zn(2)L' contains a mu-acetato-mu-cresolato-dizinc(II) core comprised of a quasi-trigonal bipyramidal Zn and a distorted octahedral Zn, and the distance between them is 3.421 Angstroms which is close to the dizinc distance in related natural metalloenzymes. Phosphodiesterase activity of Zn(2)L' was investigated using bis(4-nitrophenyl) phosphate (BNPP) as the substrate. The pH dependence of the BNPP cleavage in aqueous buffer media shows a sigmoid-shaped pH-k(obs) profile with an inflection point around pH 7.13 which is close to the first pK(a) value of 7.20 for Zn(2)L' obtained from the potentiometric titration. The catalytic rate constant (k(cat)) is 4.60 x 10(-6) s(-1) at pH 7.20 and 50 degrees C which is ca. 10(5)-fold higher than that of the uncatalyzed reaction. The deprotonated alcoholic group appended on Zn(2)L' is responsible for the cleavage reaction. The possible mechanism for the BNPP cleavage promoted by Zn(2)L' is proposed on the basis of kinetic and spectral analysis. The dizinc complex formed in situ in anhydrous DMSO exhibits a similar ability to cleave BNPP. This study provides a less common example for the phosphodiesterase model in which the metal-bound alkoxide is the nucleophile.