Dinuclear Zn Research Articles

Investigation of the Effect of Oxy Bridging Groups in Dinuclear Zn(II) Complexes that Catalyze the Cleavage of a Simple Phosphate Diester RNA Analogue

Two sets of dinuclear Zn(II) complexes were prepared to determine the effect of the presence of oxyanionic bridging groups between the metal centers on the catalytic activity toward the methanolysis of the RNA analogue 2-hydroxypropyl-4-nitrophenyl phosphate (HPNPP, 2). The Zn(II)2 complexes of bis(di-(2-pyridylmethyl)amino)-m-xylene (6) and 2,6-bis(di-(2-pyridylmethyl)amino)-4-methylphenol (7) were compared to assess the effect of a bridging phenoxide ligand, while the Zn(II)2 complex of 1,3-bis-N1-(1,5,9-triazacyclododecyl)-propan-2-ol (8) was prepared to determine the effect of the 2-propoxy group compared to the previously studied complex of 1,3-bis-N1-(1,5,9-triazacyclododecyl)-propane (4). Detailed kinetic studies of the cleavage of 2 including k(obs) vs [catalyst] plots and (s)(s)pH-rate profiles were performed for each system along with potentiometric titration experiments to determine the acid dissociation constants for the catalytically relevant groups. The results show that inclusion of the phenoxy bridging group in 7:Zn(II)2 reduces the second-order catalytic rate constant (k2(cat)) for cleavage of 2 by a factor of 160 relative to that of 6:Zn(II)2, while the incorporation of a propoxy group in 8:Zn(II)2 reduces its efficacy by 3.7 x 10(4) times relative to 4:Zn(II)2. Energetics calculations reveal that 6:Zn(II)2 offers a 3.7 kcal/mol greater stabilization of the reaction transition state for the cleavage of 2 than does 7:Zn(II)2 and that 4:Zn(II)2 affords 6.5 kcal/mol greater transition state stabilization than does 8:Zn(II)2. The analyses show that the reduction in the transition state stabilization experienced with the complexes having permanently bridging oxyanion groups stems almost entirely from a weaker binding of the phosphate and catalyst, and a reduced catalytic rate constant. These results indicate that the presence of a bridging oxyanion ligand between the metal centers, a common structural element required for the successful formation of many small molecule dinuclear catalysts that show cooperative activity in water, significantly impairs the catalytic efficiency for cleavage of 2.

Effect of Zinc (II) on the interactions of bovine serum albumin with flavonols bearing different number of hydroxyl substituent on B-ring

The impact of Zn(2+) ion on interactions of flavonols galangin (Gal), kaempferol (Kae), quercetin (Que) and myricetin (Myr) with bovine serum albumin (BSA) in aqueous solution were studied by fluorescence quenching technique. The results exhibited that Zn(2+) ion affected significantly the interactions and the effect was distinct for the flavonol bearing different number of B-ring hydroxyl. Each flavonol can quench the fluorescence of BSA, displaying a quenching extent of Myr>Que>Kae>Gal, which is in good agreement with the number variation of the B-ring hydroxyl. The presence of Zn(2+) ion promoted the quenching for the flavonols, exhibiting an extent of Que>Myr>Kae>Gal. The values of K(a) for Kae, Que and Myr decreased whereas K(SV) and k(q) for Gal, Kae and Que increased with the number of B-ring hydroxyl. The type of BSA fluorescence quenching for Gal, Kae and Que hardly changed but the preference of static quenching increased. The values of K(SV) and k(q) for Myr remarkably decreased and the fluorescence quenching of BSA alternatively occurred via both static and dynamic type instead of only one (static or dynamic). The results suggest the key role of the B-ring hydroxyl and the distinct effect of its number in the interactions. Each flavonol may capture the BSA-bound Zn(II) in the solution, forming Zn(II)-flavonol complex that is possibly responsible for BSA fluorescence quenching. The B-ring hydroxyl could establish hydrogen bonds with BSA in the absence of Zn(2+) and act as donors for chelating in the presence of Zn(2+). The formation of dinuclear Zn(II)-Myr complex together with the hydrogen bonds between the free B-ring hydroxyl and BSA may contribute to the exceptional behavior of Myr.

A new chemosensing ensemble for fluorescent recognition of pyrophosphate in water at physiological pH

A novel chemosensing ensemble that exhibits sensitive and selective recognitions of pyrophosphate in 100% aqueous solution at physiological pH has been developed. The chemosensing ensemble was constructed by a dinuclear Zn(II) complex of 2,6-bis[(bis(2-benzimidazolylmethyl)amino)methyl]- p-cresol and sodium fluorescein, the receptor–indicator pair is able to highly selectively discriminate pyrophosphate from phosphate and other anions in water at physiological pH.

Metal-Assisted Oxazolidine/Oxazine Ring Formation in Dinuclear Zinc(II) Complexes: Synthesis, Structural Aspects, and Bioactivity

Three novel dinuclear Zn(II) complexes of phenol-based compartmental macrocyclic ligands have been synthesized and characterized by routine physicochemical techniques as well as by X-ray single-crystal structure analysis. The dinuclear macrocyclic complexes 1, 2, and 3 were obtained through a 1:2 condensation reaction of 2,6-diformyl-4-methylphenol and N-(hydroxyalkyl) ethylenediamine (L(1), L(2), and L(3), respectively) in the presence of zinc(II) acetate, followed by the addition of thiocyanate anion [L(1) = N-(2-hydroxyethyl)ethylenediamine, L(2) = N-(3-hydroxypropyl)ethylenediamine, and L(3) = N-(2-hydroxypropyl)ethylenediamine]. The synthesized 18-membered macrocycles are noted to be structurally unique, and their formation proceeds with the generation of two oxazolidine side rings in complexes 1 and 3 and two oxazine side rings in 2, along with the creation of four new chiral centers in each case. Complexes 1 and 2 are characterized by a butterfly-like arrangement with the SCN ligands situated on the same side with respect to the Zn(2)O(2) moiety, whereas the centrosymmetric complex 3 exhibits a stepped arrangement with parallel methyl-phenoxy fragments (spaced at ca. 1.5 A) and trans located SCN ligands with respect to the Zn(2)O(2) core. The formation of these unusual macrocycles is considered to be zinc-mediated. Preliminary studies with the complexes show that all of them exhibit an inhibitory effect, on the cell proliferation of human stomach cancer cell line AGS, though with different degrees, where complex 3 shows the highest efficiency.

Dinuclear Zn(II) catalysts as biomimics of RNA and DNA phosphoryl transfer enzymes: changing the medium from water to alcohol provides enzyme‐like rate enhancements

AbstractPhosphodiesters are notoriously hydrolytically inert compounds that are demonstrated to have large accelerations of P‐OR cleavage promoted by transition and lanthanide metal ions in methanol and ethanol media. This review commentary describes recent findings of how a simple mononuclear and a dinuclear Zn(II) complex promote the cleavage of a series of RNA models and DNA models in alcohol media. The discussion centers on the analysis of the mechanisms of cleavage, energetics of the catalytic process, on recent findings of electrophilic assistance of leaving group departure, and the observation of a rapid hydrolytic reaction of a DNA model promoted by the dinuclear Zn(II) complex in ethanol containing less than 2% water. Copyright © 2009 John Wiley & Sons, Ltd.

Syntheses, crystal structures, and water adsorption behaviors of jungle-gym-type porous coordination polymers containing nitro moieties

NO 2 containing dicarboxylate bridging ligands, nitroterephthalate (bdc-NO 2) and 2,5-dinitroterephthalate (bdc-(NO 2) 2), afford porous coordination polymers, {[Zn 2(bdc-NO 2) 2(dabco)]· solvents} n ( 2⊃ solvents) and {[Zn 2(bdc-(NO 2) 2) 2(dabco)]· solvents} n ( 3⊃ solvents). Both compounds form jungle-gym-type regularities, where a 2D square grid composed of dinuclear Zn 2 units and dicarboxylate ligands is bridged by dabco molecules to extend the 2D layers into a 3D structure. In 2⊃ solvents and 3⊃ solvents, a rectangle pore surrounded by eight Zn 2 corners contains two and four NO 2 moieties, respectively. Thermal gravimetry (TG) and X-ray powder diffraction (XRPD) measurements reveal that both compounds maintain the frameworks regularities without guest molecules and with solvents such as MeOH, EtOH, i-PrOH, and Me 2CO. Adsorption measurements reveal that dried 2 and 3 adsorb H 2O molecules to be {[Zn 2(bdc-NO 2) 2(dabco)]·4H 2O} n ( 2⊃4H 2O) and {[Zn 2(bdc-(NO 2) 2) 2(dabco)]·6H 2O} n ( 3⊃6H 2O), showing the pore hydrophilicity enhancement caused by NO 2 group introduction.

Syntheses, structures, absorption and emission properties of a tetraiminodiphenol macrocyclic ligand and its dinuclear Zn(II) and Pb(II) complexes

Synthesis of a Robson type macrocyclic ligand [H 4L](ClO 4) 2 ( 1) obtained on condensation of 2,6-diformyl-4-methylphenol and 2,2′-dimethyl-1,3-diaminopropane, template synthesis of a dinuclear lead(II) complex [Pb II 2L(NO 3) 2] ( 2), synthesis of a dinuclear zinc(II) complex [Zn II 2L(NO 3)(H 2O)](ClO 4) ( 3) through metal substitution reaction and synthesis of another dinuclear zinc(II) complex [Zn II 2L(H 2O) 2](ClO 4) 2·(H 2O) 2 ( 4) obtained directly from 1 are described in the present study. Crystal structure determinations of 1 and 3 have been carried out. Both the compounds 1 and 3 crystallize in the orthorhombic system with the space groups Fdd2 and P2 12 12, respectively. Spectrophotometric and spectrofluorometric titrations of 1 with triethylamine as well as with zinc(II) acetate are also reported.

ChemInform Abstract: Highly Enantioselective 1,4‐Addition of Diethyl Phosphite to Enones Using a Dinuclear Zn Catalyst.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Cleavage of models for RNA mediated by a diZn(II) complex of bis[1,4-N1,N1′(1,5,9-triazacyclododecanyl)]butane in methanol and ethanol

The cleavage of seven RNA model 2-hydroxypropyl aryl phosphates (1) catalyzed by a dinuclear Zn(II) complex of bis[1,4-N1,N1′(1,5,9-triazacyclododecanyl)]butane (4) was studied in methanol and ethanol at 25 °C under pH controlled conditions. The results are compared with what was reported earlier for the dinuclear Zn(II) complex of the lower homologue bis[1,3-N1,N1′(1,5,9-triazacyclododecanyl)]propane (3). In methanol, the higher homologue exhibits saturation binding with substrates having poor aryloxy leaving groups. With good leaving groups there is an observed linear dependence of kobs versus complex concentration without saturation binding over the catalyst concentration range investigated. In ethanol, strong saturation binding between the active form of the catalyst ((RO–):Zn(II)2:4) and all substrates is observed, the results observed in both solvents being similar to what was reported for the lower (RO–):Zn(II)2:3 homologue. Energetics calculations are presented for the (RO–):Zn(II)2:4-catalyzed cleavage of each substrate in both solvents to assess the catalytic efficiency via the ΔΔG‡ for catalyst binding a transition state comprising [RO–:1]‡ or its kinetic equivalent.

Dinuclear Zn(II) Complex Promotes Cleavage and Isomerization of 2-Hydroxypropyl Alkyl Phosphates by a Common Cyclic Phosphate Intermediate

The kinetics and cleavage products of 2-hydroxypropyl p-nitrophenyl phosphate were determined in methanol containing the di-Zn(II) complex of bis-1,3-N1,N1'-(1,5,9-triazacyclododecyl)propane (4). Time-dependent 1H NMR spectra of the reaction mixture at sspH 9.8 +/- 0.1 show that the catalytic reaction proceeds via a cyclic phosphate (4-methylethylene phosphate, 2) that is subsequently cleaved into a kinetic mixture of two isomeric products, 2-hydroxypropyl methyl phosphate (3) and 1-hydroxypropan-2-yl methyl phosphate (3a), in a 29/71 ratio. In the presence of 4, the kinetic mixture of 3/3a is transformed into a thermodynamic mixture of 72/28 3/3a. The time-dependent 1H NMR spectra of 4 and a 22/78 mixture of 3/3a in CD3OH show that the formation of the thermodynamic mixture occurs on the same time scale as replacement of the P-OCH3 group of the 3/3a starting materials with OCD3. Detailed kinetic studies indicate that the dominant process for loss of the OCH3 group and equilibration of 3/3a is via a 4-catalyzed process where each of the isomers cyclizes to methylethylene phosphate (2), which subsequently reforms the 3/3a thermodynamic mixture. The kcatmax for 4-catalyzed cyclization of 3 and three other 2-hydroxypropyl O-alkyl phosphates (alkyl = CF3CH2- (6a), CH2FCH2- (6b), and CH3CH2- (6c)) has been determined, and the Brønsted plot comprising the log kcatmax vs leaving group sspKa that includes several previously studied 2-hydroxypropyl aryl phosphates is linear, following the expression log kcatmax = (-0.85 +/- 0.02) sspKa + (12.8 +/- 0.4). The betalg value of -0.85 suggests that the catalyzed cleavage of the P-OAr/OR bond has progressed to about 45% in the transition state. The combined results are analyzed in terms of two possible processes involving either a concerted reaction leading to the cyclic phosphate 2 from which the thermodynamic mixture of 3/3a is formed or a stepwise one involving a transient phosphorane whose predominant fate is to eliminate methoxide and proceed to 2 rather than partitioning between 3, 3a, and 2.

Synthesis, Electrochemical Properties and Spectroscopic Studies of Mono and Dinuclear Zn(II) Polypyridyl Complexes with Phenylcyanamide Ligands

Several novel mononuclear Zn(II) complexes, [Zn(bpy)L2], where bpy=2,2’-bipyridine and L=monoanions of phenylcyanamide (pcyd), 4-methylphenylcyanamide (4-Mepcyd), 3,5-dimethylphenylcyanamide(3,5-Me2pcyd),4-methoxyphenylcyanamide(4-MeOpcyd), 3,5-dimeth-oxyphenylcyanamide (3,5-MeO2pcyd), 3-chlorophenylcyanamide (3-Clpcyd), 2,3-dichlorophenyl-cyanamide (2,3-Cl2pcyd), 4-bromophenylcyanamide (4-Brpcyd), 1-naphthylcyanamide (1-ncyd), azophenylcyanamide (apcyd) and a dinuclear Zn(II) complex, [{Zn(terpy)}2(μ-adicyd)](PF6)2, where terpy = 2,2´:6´,2´´-terpyridine and adicyd = azodi (phenylcyanamide) dianion have been synthesized and characterized by elemental analysis, UV-vis, IR, 1H-NMR spectroscopic techniques and cyclicvoltammetry.The presence of only one sharp and intense absorption band for υ(N=C=N) around 2100-2180 cm-1 for all the monomer complexes provide evidence that both cyanamide legend are equivalent in the solid state and coordinated end-on by nitride nitrogen to Zn(II). This is also indicated by 1H-NMR spectra of the complexes. UV-vis spectra show one MLCT band that are associated with Zn(II)-pyridyl chromophores(t2g6 →π*). Blue shift of this band with increasing the polarity of solvent from CH3OH to DMSO is the reason for this assignment. The Zn(II) ion is redox innocent therefore, two irreversible oxidation peaks at positive potential and one reduction couple at the negative potential are assigned to sequential oxidation of two phenylcyanamide ligands and reduction of bipyridine, respectively.

Highly Enantioselective 1,4‐Addition of Diethyl Phosphite to Enones Using a Dinuclear Zn Catalyst

Zinc benefits: The first catalytic asymmetric phospha-Michael addition of enones has been developed. Under mild reaction conditions, the gamma-oxo phosphonates could be obtained in high yields (up to 99%) with excellent enantioselectivities (93-99% ee; see scheme). The strategy makes the asymmetric synthesis of biologically important phosphonate compounds more accessible.

Structure and Mechanism of Copper- and Nickel-Substituted Analogues of Metallo-β-lactamase L1

In an effort to further probe metal binding to metallo-beta-lactamase L1 (mbetal L1), Cu- (Cu-L1) and Ni-substituted (Ni-L1) L1 were prepared and characterized by kinetic and spectroscopic studies. Cu-L1 bound 1.7 equiv of Cu and small amounts of Zn(II) and Fe. The EPR spectrum of Cu-L1 exhibited two overlapping, axial signals, indicative of type 2 sites with distinct affinities for Cu(II). Both signals indicated multiple nitrogen ligands. Despite the expected proximity of the Cu(II) ions, however, only indirect evidence was found for spin-spin coupling. Cu-L1 exhibited higher k(cat) (96 s(-1)) and K(m) (224 microM) values, as compared to the values of dinuclear Zn(II)-containing L1, when nitrocefin was used as substrate. The Ni-L1 bound 1 equiv of Ni and 0.3 equiv of Zn(II). Ni-L1 was EPR-silent, suggesting that the oxidation state of nickel was +2; this suggestion was confirmed by (1)H NMR spectra, which showed relatively sharp proton resonances. Stopped-flow kinetic studies showed that ZnNi-L1 stabilized significant amounts of the nitrocefin-derived intermediate and that the decay of intermediate is rate-limiting. (1)H NMR spectra demonstrate that Ni(II) binds in the Zn(2) site and that the ring-opened product coordinates Ni(II). Both Cu-L1 and ZnNi-L1 hydrolyze cephalosporins and carbapenems, but not penicillins, suggesting that the Zn(2) site modulates substrate preference in mbetal L1. These studies demonstrate that the Zn(2) site in L1 is very flexible and can accommodate a number of different transition metal ions; this flexibility could possibly offer an organism that produces L1 an evolutionary advantage when challenged with beta-lactam-containing antibiotics.

A novel threefold-interpenetrating primitive cubic network based on a dinuclear Zn2node

In the mixed-ligand metal-organic polymeric compound poly[[mu(2)-1,4-bis(imidazol-1-yl)benzene](mu(2)-terephthalato)dizinc(II)], [Zn(2)(C(8)H(4)O(4))(2)(C(12)H(10)N(4))](n) or [Zn(2)(bdc)(2)(bib)](n) [H(2)bdc is terephthalic acid and bib is 1,4-bis(imidazol-1-yl)benzene], the asymmetric unit contains one Zn(II) ion, with two half bdc anions and one half bib molecule lying around inversion centers. The Zn(II) ion is in a slightly distorted tetrahedral environment, coordinated by three carboxylate O atoms from three different bdc anions and by one bib N atom. The crystal structure is constructed from the secondary building unit (SBU) [Zn(2)(CO(2))(2)N(2)O(2)], in which the two metal centers are held together by two bdc linkers with bis(syn,syn-bridging bidentate) bonding modes. The SBU is connected by bdc bridges to form a two-dimensional grid-like (4,4)-layer, which is further pillared by the bib ligand. Topologically, the dinuclear SBU can be considered to be a six-connected node, and the extended structure exhibits an elongated primitive approximately cubic framework. The three-dimensional framework possesses a large cavity with dimensions of approximately 10 x 13 x 17 A in cross-section. The potential porosity is filled with mutual interpenetration of two identical equivalent frameworks, generating a novel threefold interpenetrating network with an alpha-polonium topology [Abrahams, Hoskins, Robson & Slizys (2002). CrystEngComm, 4, 478-482].

Base moiety selectivity in cleavage of short oligoribonucleotides by di- and tri-nuclear Zn(II) complexes of azacrown-derived ligands

Cleavage of 6-mer oligoribonucleotides by the dinuclear Zn2+ complex of 1,3-bis[(1,5,9-triazacyclododecan-3-yl)oxymethyl]benzene (L1) and the trinuclear Zn2+ complex of 1,3,5-tris[(1,5,9-triazacyclododecan-3-yl)oxymethyl]benzene (L3) has been studied. The dinuclear complex cleaves at sufficiently low concentrations ([(Zn2+)2L1] < or = 0.1 mmol L(-1)) the 5'NpU3' and 5'UpN3' bonds (N = G, C, A) much more readily than the other phosphodiester bonds, but leaves the 5'UpU3' site intact. The trinuclear (Zn2+)3L3 complex, in turn, cleaves the 5'UpU3' bond more readily than any other linkages, even faster than the 5'NpU3' and 5'UpN3' sites. Somewhat unexpectedly, the 5'UpNpU3' site is cleaved only slowly by both the di- and tri-nuclear complex. The base-moiety selectivity remains qualitatively similar, though slightly less pronounced, when the hexanucleotides are closed to hairpin loops by three additional CG-pairs of 2'-O-methylribonucleotides. Phosphodiester bonds within a double helical stem are not cleaved, not even the 5'UpU3' sites. Guanine base also becomes recognized by (Zn2+)2L1 and (Zn2+)3L3, but the affinity to G is clearly lower than to U. The trinuclear cleaving agent, however, cleaves the 5'GpG3' bond only 35% less readily than the 5'UpU3' bond.

SPEED LIMITS FOR ARTIFICIAL RIBONUCLEASES

There are four major catalytic roles for natural and artificial nucleases that catalyze the hydrolytic cleavage of RNA. For metal ion complexes that act as artificial nucleases, the most important role is the stabilization of the phosphorane-like transition state. In keeping with this role the best catalysts, including Ln(III) complexes and dinuclear Zn(II) complexes, interact strongly with dianionic phosphate esters as crude transition state models. Two other catalytic modes, alignment of the hydroxyl nucleophile for in-line attack and activation of the 2′-hydroxyl, are not important for metal ion catalyzed cleavage of simple RNA analogs. This may impose a speed limit for metal ion catalysis, although additional catalytic roles may be operative in the cleavage of structured RNA.

Biomimetic Cleavage of RNA Models Promoted by a Dinuclear Zn(II) Complex in Ethanol. Greater than 30 kcal/mol Stabilization of the Transition State for Cleavage of a Phosphate Diester

The cleavage of a series of seven substituted aryl 2-hydroxypropyl phosphates (1a-g) promoted by a dinuclear Zn(II) complex (3:Zn(II)2:(-OCH2CH3)) was investigated in ethanol at pH 9.0 +/- 0.2 and 25 degrees C. The kinetics for appearance of the product phenols follow very strong saturation behavior for all substrates where the dissociation constant of the bound complex has an upper limit of Km = 3 x 10(-7) M and the k(cat)(max corr.) values (corrected for triflate inhibition) range from 168 to 3 s(-1). A partial s(s)pH/log k(cat)(max corr). profile for the 3:Zn(II)2:(-OCH2CH3)-catalyzed reaction of le (3-methoxyphenyl 2-hydroxypropyl phosphate) is bell-shaped, plateauing from 7.9-10, and is fit to a two kinetically important ionizations having s(s)pKa values of 7.22 and 10.9. The Brønsted plot of log (k(cat)(max corr.)) vs. the s(s)pKa values for the phenols shows a break at about 14.3 with two beta(lg), values of -1.12 and 0.0. This is analyzed in terms of a change in rate limiting step from cleavage of the phosphate to a conformational change where the binding of the phosphate changes from one P-O- ----Zn(II) interaction to a Zn(II)----O-P-O---Zn(II) double activation. An energetics calculation comparing the ethoxide promoted cleavage of 1a-g with the 3:Zn(ll)2:(-OEt) promoted reaction indicates that the complex, 3:Zn(II)2, stabilizes the ethoxide plus substrate transition state for the cleavage of 1a-g by between 33 and 36 kcal/mol. The origins of the large stabilization are discussed in terms of the effect of the medium on the various rate and equilibrium constants involved.

Investigation of the MSO 4 · xH 2O (M = Zn, x = 7; M = Cd, x = 8/3)/methyl 2-pyridyl ketone oxime reaction system: A novel Cd(II) coordination polymer versus mononuclear and dinuclear Zn(II) complexes

The reactions of methyl 2-pyridyl ketone oxime, (py)C(Me)NOH, with MSO 4 · xH 2O (M = Zn, x = 7; M = Cd, x = 8/3), in the absence of an external base, have been investigated. The synthetic study has led to the two new complexes [Zn(SO 4){(py)C(Me)NOH}(H 2O) 3] · H 2O ( 1 · H 2O) and [Zn 2(SO 4) 2{(py)C(Me)NOH} 4] · (py)C(Me)NOH [ 2 · (py)C(Me)NOH], and the coordination polymer [Cd(SO 4){(py)C(Me)NOH}(H 2O)] n · [Cd(SO 4){(py)C(Me)NOH}(H 2O) 2] n ( 3). In the three complexes the organic ligand chelates through its nitrogen atoms. The sulfate anion in 1 · H 2O is monodentate; the complex molecule is the mer isomer considering the positions of the aqua ligands. The Zn II centers in 2 · (py)C(Me)NOH are bridged by two syn, anti η 1:η 1:μ 2 SO 4 2 - ligands; each metal ion has the cis– cis– trans disposition of the coordinated sulfate oxygen, pyridyl nitrogen and oxime nitrogens, respectively. The molecular structure of 3 is unique consisting of two different linear and ladder – type chains. π–π stacking interactions and/or hydrogen bonds lead to the formation of interesting supramolecular architectures in the three complexes. The thermal decomposition of complex 3 has been studied. Characteristic vibrational (IR, Raman) bands are discussed in terms of the nature of bonding and the structures of the three complexes.

Enzyme-like Acceleration for the Hydrolysis of a DNA Model Promoted by a Dinuclear Zn(II) Catalyst in Dilute Aqueous Ethanol

The rates and products of cleavage of methyl (2-chloro-4-nitrophenyl) phosphate (2) promoted by a dinuclear Zn(II) complex (3) of 1,3-bis-N,N'(1,5,9-triazacyclododecyl)propane along with 1 equiv of ethoxide were investigated in ethanol solution containing small amounts of water (8 mM <or= [H2O] <or= 2.1 M) at 25 degreesC and sspH 7.9. The kinetics of decomposition of 5 x 10(-5) M 2 in the presence of varying [3] (0.03 mM <or= [3] <or= 0.12 mM) in ethanol containing 28 mM water show very strong saturation binding which is analyzed to give fitted values for the complex dissociation constant (Kd) and maximum catalytic rate constant (kcatmax) of 3.2 x 10(-8) M and 1.47 x 10(-3) s(-1). Product analysis indicates that the reaction in ethanol containing 28 mM and 2.1 M water produces 46% and 93% of the hydrolysis product (methyl phosphoric acid), with the remainder being ethanolysis product (ethyl methyl phosphate). Analysis of the kinetics of the reaction at 28 mM water indicates that second order rate constant for the catalytic reaction, given as kcatmax/Kd, is 4.6 x 104 M(-1) s(-1), is a factor of 8.4 x 10(10) larger than the second order rate constant (k2EtO) for the ethoxide promoted reaction of 2 in ethanol (5.5 +/- 0.3) x 10(-7) M(-1) s(-1). A more detailed analysis that considers the relative acidities of water and ethanol in ethanol solvent indicates that at sspH 7.9, the ethanolysis is accelerated by 2.3 x 10(14) times relative to the computed ethoxide reaction at that sspH, while the hydrolysis is accelerated by >or=1.6 x 10(17) times relative to the background hydroxide reaction, suggesting that complex 3 promotes the hydrolysis at least 1000 times more effectively than ethanolysis.

Cleavage of a RNA analog containing uridine by a bifunctional dinuclear Zn(II) catalyst

The macrocyclic ligand, 1,4-bis((1-oxa-4,7,10-triazacyclododecan-7-yl)methyl)benzene ( L1) is prepared. L1 binds two Zn(II) ions at neutral pH to form Zn 2( L1) as studied by using pH-potentiometric titrations. Zn 2( L1) binds two uridines at pH 7.0, I = 0.100 M (NaCl) and the mononuclear analog Zn( L2) ( L2 = 1-oxa-4,7,10-triazacyclododecane) binds a single uridine; dissociation constants for both complexes are in the millimolar range. Both complexes promote the cleavage of a simple RNA analog lacking a nucleobase ( HpPNP = 2-hydroxypropyl-4-nitrophenylphosphate), and a uridine containing RNA analog UpPNP (uridine-3′-4-nitrophenylphosphate). Plots of the first-order rate constant for cleavage of HpPNP as a function of Zn( L2) concentration from 0.5 mM to 20.0 mM are linear, consistent with weak complexation to substrate K d > 20 mM. In contrast, first-order rate constants for cleavage of UpPNP by Zn( L2) or Zn 2( L1) over similar concentration ranges exhibit a downward curvature, consistent with the formation of a complex between catalyst and UpPNP. Comparison of second-order rate constants ( k 2 = k cat/ K d) shows that the dinuclear complex Zn 2( L1) is a better catalyst than Zn( L2) for both HpPNP and UpPNP cleavage.