Europium luminescence of EF-hand helix-turn-helix chimeras: impact of pH and DNA-binding on europium coordination

A series of Eu(III) metallopeptides, designed on the basis of the structural similarity of the helix-turn-helix and EF-hand motifs, have been studied by Eu(III) (7)F(0) --> (5)D(0) excitation spectroscopy. The impact of EF-hand ligand set differences on the hydration number and Eu(III) coordination environment are compared among the peptides. The conditional binding affinities were determined by Eu titration (P3, log K(a) = 6.0 +/- 0.4; P3W, log K(a) = 5.9 +/- 0.2; P5b, log K(a) = 5.3 +/- 0.1). Two similar coordination environments occur in each case, consistent with structural flexibility about the metal site. The coordination environments are consistent with 8- or 9-coordinate Eu(III), including six peptide-based ligands and two to three water molecules (P3, q = 1.9 +/- 0.2; P3W, q = 2.3 +/- 0.2; P4a, q = 1.9 +/- 0.3; P5b, q = 2.6 +/- 0.2). The Eu(III) (7)F(0) --> (5)D(0) excitation spectra are pH-dependent, as reported for several EF-hand proteins (oncomodulin, parvalbumin). A higher energy transition occurs at pH > 6, and has been assigned to deprotonation of coordinated water. The pK(a) leading to this new transition is dependent on Eu(III) Lewis acidity, which varies with the inner and outer sphere ligand set. The noncoordinating ninth position of the Eu-binding loop, which is poised to make second-sphere contacts to the coordinated water, stabilizes the deprotonated form of the coordinated solvent more effectively when it is Thr (P5b) than Asp (P3W). Upon DNA-binding by the metallopeptides, the pK(a) of the pH-dependent peak increases, but no new DNA-dependent transitions are observed. This indicates no DNA-based Eu(III) ligands are introduced, such as phosphate oxygen atoms of the DNA backbone. The hydration number decreases in the presence of DNA (P3W + DNA, q = 1.9 +/- 0.2; P5b + DNA, q = 1.7 +/- 0.2), indicating that DNA-binding by the metallopeptides organizes rather than compromises the Eu-binding site within the peptide.     

Double-strand hydrolysis of plasmid DNA by dicerium complexes at 37 degrees C.

Significant effort has been made to develop synthetic metal complexes that hydrolyze DNA. Here we report a new dicerium complex, Ce(2)(HXTA) (HXTA = 5-methyl-2-hydroxy-1,3-xylene-alpha,alpha-diamine-N,N,N',N'-tetraacetic acid), which can hydrolyze DNA at pH 8 and 37 degrees C. This complex hydrolyzes DNA restriction fragments to give products with high regioselectivity, affording >90% 5'-OPO(3) and 3'-OH ends, like the products of DNA hydrolyzing enzymes. Ce(2)(HXTA) also hydrolyzes Litmus 29 plasmid DNA to afford both nicked and linear DNA. Analysis of the relative amounts of supercoiled, nicked, and linear DNA present show that there is one double-strand cleavage per ten single-strand cleavages, indicating that the linear DNA formed cannot be the result of two random single-strand cleavage events. The kinetics of nicked and linear DNA formation are comparable, both being associated with apparent first-order rate constants of approximately 1 x 10(-)(4) s(-)(1) for complex concentrations of 10(-)(5)-10(-)(4) M. These observations suggest that similar factors affect the hydrolysis of the first and second DNA strands and that cleaving the phosphodiester bond is likely the rate determining step in both cases. This is the first detailed study of a metal complex shown to mimic DNA hydrolases in their capability to effect double-strand DNA hydrolysis regioselectively at the 3'-O-P bond.     

Of folding and function: understanding active-site context through metalloenzyme design

In the emerging field of biomolecular design, the introduction of metal-binding sites into loop or turn regions of known protein scaffolds has been utilized to create unique metalloprotein and metallopeptide systems for study. This Forum Article highlights examples of the modular-turn-substitution approach to design and the range of structural and mechanistic questions to which this tool can be applied. Examples from the authors' laboratory are given to show that lanthanide-binding metallopeptides, and now a full metallohomeodomain, can be generated by modular substitution of a Ca-binding EF-hand loop into the unrelated scaffold, the engrailed helix-turn-helix motif. We have previously shown that these peptides bind trivalent Ln(III) ions and promote DNA and phosphate hydrolysis, the targeted function. Here, a series of chimeric peptides are presented that differ only in the ninth loop position [given in parentheses; Peptides P3N (Asn), P3E (Glu), P3A (Ala), and P3W(D) (Asp]. This residue, a putative second-shell ligand stabilizing a coordinated water, was found to influence not only metal affinity but also peptide folding. The affinity for Tb(III) was determined by Trp-Tb fluorescence resonance energy transfer and followed the order Ka = P3W(D) > P3A approximately P3E > P3N. However, circular dichroism (CD) titrations with EuCl3 showed that only P3W(D) and P3N folded to any extent upon metal binding, indicating that the Asp/Asn side chains stabilize the central loop structure and thus propagate folding of the peripheral helices, whereas neither Ala nor Glu appears to be interacting with the metal to organize the loop. Finally, we investigated the longer range context of a given loop substitution by cloning and expressing a lanthanide-binding homeodomain (C2), whose loop insertion sequence is analogous to that of peptide P3W(D). We find by CD that apo-C2 has a significant helical structure (approximately 25% alphahelicity), which increases further upon the addition of Tb(III) (approximately 32% alpha helicity). The protein's Tb(III) affinity is similar to that of the chimeric peptides. However, unlike previously reported metallopeptides, we find that EuC2 does not appreciably promote phosphate or DNA cleavage, which suggests a difference in metal accessibility in the context of the full domain. We have demonstrated that substituting beta turns with metal-binding turns does not necessarily require homologous parental scaffolds or small flexible peptides but rather relies on the structural similarity of the motifs flanking the turn.     

DNA nuclease activity of Rev-coupled transition metal chelates

Artificial nucleases containing Rev-coupled metal chelates based on combinations of the transition metals Fe(2+), Co(2+), Ni(2+), and Cu(2+) and the chelators DOTA, DTPA, EDTA, NTA, tripeptide GGH, and tetrapeptide KGHK have been tested for DNA nuclease activity. Originally designed to target reactive transition metal chelates (M-chelates) to the HIV-1 Rev response element mRNA, attachment to the arginine-rich Rev peptide also increases DNA-binding affinity for the attached M-chelates. Apparent K(D) values ranging from 1.7 to 3.6 μM base pairs for binding of supercoiled pUC19 plasmid DNA by Ni-chelate-Rev complexes were observed, as a result of electrostatic attraction between the positively-charged Rev peptide and negatively-charged DNA. Attachment of M-chelates to the Rev peptide resulted in enhancements of DNA nuclease activity ranging from 1-fold (no enhancement) to at least 13-fold (for Cu-DTPA-Rev), for the rate of DNA nicking, with second order rate constants for conversion of DNA(supercoiled) to DNA(nicked) up to 6 × 10(6) M(-1) min(-1), and for conversion of DNA(nicked) to DNA(linear) up to 1 × 10(5) M(-1) min(-1). Freifelder-Trumbo analysis and the ratios of linearization and nicking rate constants (k(lin)/k(nick)) revealed concerted mechanisms for nicking and subsequent linearization of plasmid DNA for all of the Rev-coupled M-chelates, consistent with higher DNA residency times for the Rev-coupled M-chelates. Observed rates for Rev-coupled M-chelates were less skewed by differing DNA-binding affinities than for M-chelates lacking Rev, as a result of the narrow range of DNA-binding affinities observed, and therefore relationships between DNA nuclease activity and other catalyst properties, such as coordination unsaturation, the ability to consume ascorbic acid and generate diffusible radicals, and the identity of the metal center, are now clearly illustrated in light of the similar DNA-binding affinities of all M-chelate-Rev complexes. This work paints a clearer picture of the factors governing DNA nuclease activity by redox active M-chelates than was previously possible. The results demonstrate enhancement of DNA cleavage by use of a targeting sequence, but also clearly underscore that significant orientational factors are required for optimal reactivity at the metal center. Moreover, the studies confirm high selectivity for the target HIV RRE RNA at the most likely dosage concentrations, lending further support to the feasibility of designing and applying targeted catalytic metallodrugs.     

Efficient visible light induced nuclease activity of a ternary mono-1,10-phenanthroline copper(II) complex containing 2-(methylthio)ethylsalicylaldimine

Ternary Schiff base copper(II) complex [CuL(phen)](ClO(4)), where HL is 2-(methylthio)ethylsalicylaldimine and phen is 1,10-phenanthroline, has been prepared and structurally characterized by X-ray crystallography. The complex shows a CuN(3)OS coordination in a square-pyramidal (4 + 1) geometry with the sulfur as an equatorial ligand. The complex is an avid binder to double-stranded DNA in the minor groove and exhibits both photonuclease and chemical nuclease activity. When exposed to UV light of 312 nm (96 W) or visible light of 532 nm (125 W) under aerobic conditions, the complex causes significant cleavage of supercoiled pUC19 DNA in the absence of any externally added reducing agent or H(2)O(2).     

Factors influencing the DNA nuclease activity of iron, cobalt, nickel, and copper chelates

A library of complexes that included iron, cobalt, nickel, and copper chelates of cyclam, cyclen, DOTA, DTPA, EDTA, tripeptide GGH, tetrapeptide KGHK, NTA, and TACN was evaluated for DNA nuclease activity, ascorbate consumption, superoxide and hydroxyl radical generation, and reduction potential under physiologically relevant conditions. Plasmid DNA cleavage rates demonstrated by combinations of each complex and biological co-reactants were quantified by gel electrophoresis, yielding second-order rate constants for DNA(supercoiled) to DNA(nicked) conversion up to 2.5 × 10(6) M(-1) min(-1), and for DNA(nicked) to DNA(linear) up to 7 × 10(5) M(-1) min(-1). Relative rates of radical generation and characterization of radical species were determined by reaction with the fluorescent radical probes TEMPO-9-AC and rhodamine B. Ascorbate turnover rate constants ranging from 3 × 10(-4) to 0.13 min(-1) were determined, although many complexes demonstrated no measurable activity. Inhibition and Freifelder-Trumbo analysis of DNA cleavage supported concerted cleavage of dsDNA by a metal-associated reactive oxygen species (ROS) in the case of Cu(2+)(aq), Cu-KGHK, Co-KGHK, and Cu-NTA and stepwise cleavage for Fe(2+)(aq), Cu-cyclam, Cu-cyclen, Co-cyclen, Cu-EDTA, Ni-EDTA, Co-EDTA, Cu-GGH, and Co-NTA. Reduction potentials varied over the range from -362 to +1111 mV versus NHE, and complexes demonstrated optimal catalytic activity in the range of the physiological redox co-reactants ascorbate and peroxide (-66 to +380 mV).     

Double-strand DNA cleavage by copper complexes of 2,2'-dipyridyl with guanidinium/ammonium pendants

Two ligands with guanidinium/ammonium groups were synthesized and their copper complexes, [Cu(L1)Cl2](ClO4)2.H2O (1) and [Cu(L2)Cl2](ClO4)2 (2) (L1 = 5,5'-di[1-(guanidyl)methyl]-2,2'-bipyridyl cation and L2 = 5,5'-di[1-(amino)methyl]-2,2'-bipyridyl cation), were prepared to serve as nuclease mimics. X-Ray analysis revealed that Cu(II) ion in 1 has a planar square CuN2Cl2-configuration. The shortest distance between the nitrogen of guanidinium and copper atoms is 6.5408(5) A, which is coincident with that of adjacent phosphodiesters in DNA (ca. 6 A). In the absence of reducing agent, supercoiled plasmid DNA cleavage by the complexes were performed and their hydrolytic mechanisms were demonstrated with radical scavengers and T4 ligase. The pseudo-Michaelis-Menten kinetic parameters (kcat, KM) were calculated to be 4.42 h(-1), 7.46 x 10(-5) M for 1, and 4.21 h(-1), 1.07 x 10(-4) M for 2, respectively. The result shows that their cleavage efficiency is about 10-fold higher than the simple analogue [Cu(bipy)Cl2] (3) (0.50 h(-1), 3.5 x 10(-4) M). The pH dependence of DNA cleavage by 1 and its hydroxide species in solution indicates that mononuclear [Cu(L1)(OH)(H2O)]3+ ion is the active species. Highly effective DNA cleavage ability of is attributed to the effective cooperation of the metal moiety and two guanidinium pendants with the phosphodiester backbone of nucleic acid.     

Influence of stereochemistry and redox potentials on the single- and double-strand DNA cleavage efficiency of Cu(II) and Ni(II) Lys-Gly-His-derived ATCUN metallopeptides

The DNA cleavage chemistry of a series of metallopeptides based on the amino-terminal Cu and Ni (ATCUN) binding motif of proteins has been studied. Specifically, the impact of the positioning of charged Lys side chains and their stereochemistry on metal reduction potentials and DNA cleavage reactivity have been quantitatively evaluated. Both Cu and Ni metallopeptides show a general increase in reactivity toward DNA with an increasing number of Lys residues, while a corresponding decrease in complex reduction potential reflects the enhanced sigma-donor character of the Lys side chain relative to that of Gly. Placement of Lys at the first position in the tripeptide ligand sequence resulted in a greater increase in DNA cleavage reactivity, relative to placement at the second position, while a switch from an l-Lys to a d-Lys typically resulted in enhanced reactivity, as well as perturbations of reduction potential. In the case of Cu peptides, reactivity was enhanced with both increasing positive charge density on the peptide and stabilization of the Cu3+ state. However, for Ni peptides, while the general trends are the same, the correlation with redox behavior was less pronounced. Most likely these differences in specific trends for the Cu and Ni complexes reflect the distinct coordination preferences for Cu3+/2+ and Ni3+/2+ oxidation states, and the consequent distinct positioning of metal-associated reactive oxygen species, as well as the orientation of the DNA-associated complex. Thus, the amino acid composition and stereochemistry of ATCUN metallopeptides can tune the intrinsic reactivities of these systems (their ability to promote formation and activity of metal-associated ROS) as well as their overall structural features, and both of these aspects appear to influence their reactivity and efficiency of DNA strand scission.     

Europium(III) reduction and speciation within a Wells-Dawson heteropolytungstate

The redox speciation of Eu(III) in the 1:1 stoichiometric complex with the alpha-1 isomer of the Wells-Dawson anion, [alpha-1-P 2W 17O 61] (10-), was studied by electrochemical techniques (cyclic voltammetry and bulk electrolysis), in situ XAFS (X-ray absorption fine structure) spectroelectrochemistry, NMR spectroscopy ( (31)P), and optical luminescence. Solutions of K 7[(H 2O) 4Eu(alpha-1-P 2W 17O 61)] in a 0.2 M Li 2SO 4 aqueous electrolyte (pH 3.0) show a pronounced concentration dependence to the voltammetric response. The fully oxidized anion and its reduced forms were probed by Eu L 3-edge XANES (X-ray absorption near edge structure) measurements in simultaneous combination with controlled potential electrolysis, demonstrating that Eu(III) in the original complex is reduced to Eu(II) in conjunction with the reduction of polyoxometalate (POM) ligand. After exhaustive reduction, the heteropoly blue species with Eu(II) is unstable with respect to cluster isomerization, fragmentation, and recombination to form three other Eu-POMs as well as the parent Wells-Dawson anion, alpha-[P 2W 18O 62] (6-). EXAFS data obtained for the reduced, metastable Eu(II)-POM before the onset of Eu(II) autoxidation provides an average Eu-O bond length of 2.55(4) A, which is 0.17 A longer than that for the oxidized anion, and consistent with the 0.184 A difference between the Eu(II) and Eu(III) ionic radii. The reduction of Eu(III) is unusual among POM complexes with Lindqvist and alpha-2 isomers of Wells-Dawson anions, that is, [Eu(W 5O 18) 2] (9-) and [Eu(alpha-2-As 2W 17O 61) 2] (17-), but not to the Preyssler complex anion, [EuP 5W 30O 110] (12-), and fundamental studies of materials based on coupling Eu and POM redox properties are still needed to address new avenues of research in europium hydrometallurgy, separations, and catalysis sciences.     

Diastereoselective DNA cleavage recognition by Ni(II) x Gly-Gly-His-derived metallopeptides

Site-selective DNA cleavage by diastereoisomers of Ni(II) x Gly-Gly-His-derived metallopeptides was investigated through high-resolution gel analyses and molecular dynamics simulations. Ni(II) x L-Arg-Gly-His and Ni(II) x D-Arg-Gly-His (and their respective Lys analogues) targeted A/T-rich regions; however, the L-isomers consistently modified a subset of available nucleotides within a given minor groove site, while the D-isomers differed in both their sites of preference and their ability to target individual nucleotides within some sites. In comparison, Ni(II) x L-Pro-Gly-His and Ni(II) x D-Pro-Gly-His were unable to exhibit a similar diastereoselectivity. Simulations of the above systems, along with Ni(II) x Gly-Gly-His, indicated that the stereochemistry of the amino-terminal amino acid produces either an isohelical metallopeptide that associates stably at individual DNA sites (L-Arg or L-Lys) or, with D-Arg and D-Lys, a noncomplementary metallopeptide structure that cannot fully employ its side chain nor amino-terminal amine as positional stabilizing moieties. In contrast, amino-terminal Pro-containing metallopeptides of either stereochemistry, lacking an extended side chain directed toward the minor groove, did not exhibit a similar diastereoselectivity. While the identity and stereochemistry of amino acids located in the amino-terminal peptide position influenced DNA cleavage, metallopeptide diastereoisomers containing L- and D-Arg (or Lys) within the second peptide position did not exhibit diastereoselective DNA cleavage patterns; simulations indicated that a positively charged amino acid in this location alters the interaction of the metallopeptide equatorial plane and the minor groove leading to an interaction similar to Ni(II) x Gly-Gly-His.     

DNA cleavage promoted by Cu2+ complex of cyclen containing pyridine subunit

The tetraazamacrocycle crown ether (cyclen) containing two pyridine subunits was prepared by a modified procedure and the interaction of its metal complexes with DNA was studied by agarose gel electrophoresis analysis. The results indicate that the Cu²⁺ complex as nuclease model can promote the hydrolysis of phosphodiester bond of supercoiled DNA. The rate of degradation of the supercoiled DNA (form I) to nicked DNA (form II) obtained at physiological condition in the presence of 2.14 mM Cu²⁺ complex is 2.31 × 10^–3 min⁻¹. The dependence of the rate of supercoiled DNA cleavage from the complex concentration shows an unusual profile and a hydrolytic cleaving mechanism of two monometallic complexes through cooperation from two-point binding to DNA is proposed.

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Graphical abstract DNA cleavage promoted by metal complex of cyclen containing pyridine subunit Ying Li, Xiao-Min Lu, Xin Sheng, Guo-Yuan Lu*, Ying Shao and Qiang Xu* The copper complex of tetraazamacrocycle crown ether (cyclen) containing two pyridine subunits can promote the hydrolysis of phosphodiester bond of supercoiled DNA and a hydrolytic mechanism of two monometallic complexes through cooperation from two-point binding to DNA is proposed.

     

Synthesis, DNA-binding, cleavage, and cytotoxic activity of new 1,7-dioxa-4,10-diazacyclododecane artificial receptors containing bisguanidinoethyl or diaminoethyl double side arms

Novel 1,7-dioxa-4,10-diazacyclododecane artificial receptors with two pendant aminoethyl (3) or guanidinoethyl (4) side arms have been synthesized. Spectroscopy, including fluorescence and CD spectroscopy, of the interactions of 3, 4, and their copper(II) complexes with calf thymus DNA indicated that the DNA binding affinity of these compounds follows the order Cu(2+)-4>Cu(2+)-3>4>3, and the binding constants of Cu(2+)-3 are Cu(2+)-4 are 7.2x10(4) and 8.7x10(4) M(-1), respectively. Assessment by agarose gel electrophoresis of the plasmid pUC 19 DNA cleavage activity in the presence of the receptors showed that the complexes Cu(2+)-3 and Cu(2+)-4 exhibit powerful supercoiled DNA cleavage efficiency. Kinetic data of DNA cleavage promoted by Cu(2+)-3 and Cu(2+)-4 under physiological conditions fit to a saturation kinetic profile with kmax values of 0.865 and 0.596 h(-1), respectively, which give about 10(8)-fold rate acceleration over uncatalyzed supercoiled DNA. This acceleration is due to efficient cooperative catalysis of the copper(II) center and the functional (diamino or bisguanidinium) groups. In-vitro cytotoxic activities toward murine melanoma B16 cells and human leukemia HL-60 cells were also examined: Cu(2+)-4 shows the highest activity with IC(50) values of 1.62x10(-4) and 1.19x10(-5) M, respectively.     

Correlated electrons in the Eu-exchanged Preyssler anion [EuP(5)W(30)O(110)](n-)

The Eu-encrypted Preyssler anion, [EuP5W30O110]n-, demonstrates unusual electronic properties as a function of applied potenial. The Preyssler anion itself, when exchanged with a nonredox active trivalent ion such as La3+, is electroactive in acidic solution, reversibly accepting up to 10 electrons under reducing conditions. Encrypted Eu, which is trivalent at rest potential, is reduced concomitantly with the framework. The details of this reduction are probed with Eu L3-XANES, coulometry, and magnetic susceptibility. The results are interpreted in terms of a bonding interaction between the Eu f-states and the W-O framework band, facilitated by the crossing of the framework Fermi level with the localized states. Such behavior has been previously observed in intermediate-valent and correlated-electron systems. The observation of such behavior in an isolated cluster opens the door for new avenues of research into correlated-electron behavior.     

Hydrolytic cleavage of DNA by quercetin manganese(II) complexes

Quercetin manganese(II) complexes were investigated focusing on its DNA hydrolytic activity. The complexes successfully promote the cleavage of plasmid DNA, producing single and double DNA strand breaks. The amount of conversion of supercoiled form (SC) of plasmid DNA to the nicked circular form (NC) depends on the concentration of the complex as well as the duration of incubation of the complexes with DNA. The maximum rate of conversion of the supercoiled form to the nicked circular form at pH 7.2 in the presence of 100 μM of the complexes is found to be 1.32 × 10⁻⁴ s⁻¹. The hydrolytic cleavage of DNA by the complexes was supported by the evidence from free radical quenching, thiobarbituric acid-reactive substances (TBARS) assay and T4 ligase ligation.     

Unraveling the aflatoxin-FAPY conundrum: structural basis for differential replicative processing of isomeric forms of the formamidopyrimidine-type DNA adduct of aflatoxin B1

Aflatoxin B1 (AFB) epoxide forms an unstable N7 guanine adduct in DNA. The adduct undergoes base-catalyzed ring opening to give a highly persistent formamidopyrimidine (FAPY) adduct which exists as a mixture of forms. Acid hydrolysis of the FAPY adduct gives the FAPY base which exists in two separable but interconvertible forms that have been assigned by various workers as functional, positional, or conformational isomers. Recently, this structural question became important when one of the two major FAPY species in DNA was found to be potently mutagenic and the other a block to replication [Smela, M. E.; Hamm, M. L.; Henderson, P. T.; Harris, C. M.; Harris, T. M.; Essigmann, J. M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 6655-6660]. NMR studies carried out on the AFB-FAPY bases and deoxynucleoside 3',5'-dibutyrates now establish that the separable FAPY bases and nucleosides are diastereomeric N5 formyl derivatives involving axial asymmetry around the congested pyrimidine C5-N5 bond. Anomerization of the protected beta-deoxyriboside was not observed, but in the absence of acyl protection, both anomerization and furanosyl --> pyranosyl ring expansion occurred. In oligodeoxynucleotides, two equilibrating FAPY species, separable by HPLC, are assigned as anomers. The form normally present in duplex DNA is the mutagenic species. It has previously been assigned as the beta anomer by NMR (Mao, H.; Deng, Z. W.; Wang, F.; Harris, T. M.; Stone, M. P. Biochemistry 1998, 37, 4374-4387). In single-stranded environments the dominant species is the beta anomer; it is a block to replication.     

Binuclear copper and zinc complexes possessing bio-potential ligands: synthesis,characterization, antimicrobial,SOD mimetic,DNA binding,and cleavage studies

Schiff bases (L) viz, N,N′,N′′,N′′′-tetra-3,4-dimethoxybenzalidene-3,3′-diaminobenzidine (TDBD), N,N′,N′′,N′′′-tetra-4-hydroxy-3-methoxybenzalidene-3,3′-diaminobenzidine (THMBD), and N,N′,N′′,N′′′-tetra-3-hydroxy-4-nitrobenzalidene-3,3′-diaminobenzidine (THNBD) afford binuclear [M₂LCl₄] complexes where M = Cu(II) or Zn(II). These Schiff bases and their binuclear complexes have been characterized by analytical and spectral data showing square-planar geometry on metalation with Cu²⁺. Intercalative binding of these complexes with DNA has been investigated by electronic absorption spectroscopy, viscosity measurements, cyclic voltammetry, and differential pulse voltammetry. Control DNA cleavage experiments using pUC19 supercoiled (SC) DNA and minor groove binder distamycin suggest that these synthesized complexes bind to the major groove. In the presence of a reducing agent like 3-mercaptopropionic acid (MPA), they show chemical nuclease activity. They also show an efficient photo-induced DNA cleavage on irradiation with a monochromatic UV light of 360 nm in the presence of inhibitors. Control experiments indicate the inhibition of cleavage in the presence of singlet oxygen quencher like sodium azide and the enhancement of cleavage in D₂O show the formation of singlet oxygen as reactive species. The superoxide dismutase (SOD)-mimetic activity of the synthesized complexes has been assessed for their ability to inhibit the reduction of nitroblue tetrazolium chloride (NBT). The complexes have promising SOD-mimetic activity. The antimicrobial results indicate that the complexes inhibit the growth of bacteria and fungi more than free ligands.     

Ce3+,Tb3+,Eu3+共掺杂Sr2MgSi2O7体系的白色发光和能量传递机理

通过正交试验,采用高温固相法制备了Sr2-x-y-zMgSi2O7∶xCe3+,yTb3+,zEu3+系列样品.使用X射线衍射仪和荧光光谱仪表征了样品的物相和发光性质,并讨论了Ce3+-Tb3+-Eu3+共掺杂Sr2MgSi2O7体系中的能量传递过程.实验结果表明,在327 nm波长激发下,所合成荧光粉的发射峰主要位于387 nm(蓝紫)、542nm(绿)和611 nm(红)处;分别以387,542和611 nm为监控波长,所得激发光谱显示荧光粉在327 nm处有最好的激发.在327 nm光激发下,系列样品发光进入白光区.最优化的荧光粉为Sr1.91MgSi2O7∶0.01Ce3+,0.05Tb3+,0.03Eu3+,其色坐标为(0.337,0.313),是一种潜在的发光二极管(LED)用白色荧光粉.     

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.     

Synthesis, DNA binding, photo-induced DNA cleavage and cytotoxicity studies of europium(III) complexes

Two Eu(III) complexes, [Eu(acac)(3)(dpq)] (1) and [Eu(acac)(3)(dppz)] CH(3)OH (2) {viz. acetylacetonate (acac), dipyrido[3,2-d:20,30-f]quinoxaline (dpq), dipyrido[3,2-a:20,30-c] phenazine (dppz)}, have been synthesized and their DNA binding, photo-induced DNA cleavage activity and cell cytotoxicity are studied. The complexes display significant binding propensity to the calf thymus DNA in the order: 2(dppz) >1(dpq). Cleavage experiments using pBR322 supercoiled DNA suggest major groove binding for 2 and minor groove binding for 1. The mechanistic aspects on natural light (natural light in room during the day) and UV-A (365 nm) irradiation are via a mechanistic pathway involving formation of singlet oxygen and hydroxyl radical as the reactive species. The photo-induced DNA cleavage activity of 2 is also stronger than 1. The cytotoxicity of 1 and 2 against HeLa (cervical) cancer cells show that the IC(50) value of 19.11 ± 3.56 μM and 17.95 ± 5.47 μM, respectively.