The Effects of Dexamethasone in Diabetic Patients Undergoing Primary Total Joint Arthroplasty.

Dexamethasone reduces postoperative pain and nausea, with no evidence of increased rate of infection in total joint arthroplasty. However, the effects of dexamethasone on diabetic patients undergoing total joint arthroplasty remain relatively unexplored. The purpose of this study was to examine the effects of dexamethasone on postoperative blood glucose levels, prosthetic joint infections (PJIs), and 90-day hospital returns in diabetic patients following total joint arthroplasty. Retrospective analysis was performed on 228 adult patients with a diagnosis of diabetes who underwent primary total joint arthroplasty. Patients were stratified by intraoperative dexamethasone administration. In total, 173 (75.9%) patients received intraoperative dexamethasone, with no differences in demographic variables compared with patients who did not receive dexamethasone. There was no significant difference in PJIs or 90-day hospital returns. Patients who received dexamethasone had significantly increased blood glucose concentration on Postoperative Day 1 and were significantly more likely to have blood glucose levels exceeding 180 g/dl. Although postoperative blood glucose levels were significantly increased, it is unclear what effects, if any, transient hyperglycemia may have on outcomes. The outcomes of this study support perioperative administration of dexamethasone in diabetic patients.

Design and synthesis of antifreeze glycoproteins and mimics.

Antifreeze glycoproteins are an important class of biological antifreezes that have potential applications in many areas of medicine, agriculture and industry in which ice crystal growth is damaging. While the synthesis of antifreeze glycoproteins as pure glycoforms has recently been achieved by using ligation and polymerisation strategies, the routine production of large quantities of pure glycoforms remains challenging. A range of C-linked analogues that are readily produced by solid-phase synthesis have delivered novel compounds that are not biological antifreezes, but are potent, non-cytotoxic, ice-recrystallisation inhibitors. Structure-activity studies, the identification of cyclic antifreeze glycoproteins and conformational studies have provided further insight into the requirements for antifreeze activity. These results, coupled with significant advances in approaches to the routine synthesis of different glycoproteins and mimics, present opportunities for the design and synthesis of novel ice-growth-inhibiting and antifreeze compounds.

Applications of type I antifreeze proteins: studies with model membranes & cryoprotectant properties.

Antifreeze proteins (AFPs) and antifreeze glycoproteins (AFGPs), found in the body fluids of many species of polar fish allow them to survive in waters colder than the equilibrium freezing point of their blood and other internal fluids. Despite their structural diversity, all AF(G)Ps kinetically depress the temperature at which ice grows in a non-colligative manner and hence exhibit thermal hysteresis. AF(G)Ps also share the ability to interact with and protect mammalian cells and tissues from hypothermic damage (e.g., improved storage of human blood platelets at low temperatures), and are able to stabilize or disrupt membrane composition during low temperature and freezing stress (e.g., cryoprotectant properties in stabilization of sperm and oocytes). This review will summarize studies of AFPs with phospholipids and plant lipids, proposed mechanisms for inhibition of leakage from membranes, and cryoprotectant studies with biological samples. The major focus will be on the alpha-helical type I antifreeze proteins, and synthetic mutants, that have been most widely studied. For completeness, data on glycoproteins will also be presented. While a number of models to explain stabilization and destabilization of different lipid systems have been proposed, it is currently not possible to predict whether a particular AFP will stabilize or destabilize a given lipid system. Furthermore the relationship between the antifreeze property of thermal hysteresis and membrane stabilization is unknown. This lack of detailed knowledge about how AFPs function in the presence of different types of materials has hampered progress toward the development of antifreezes for cold storage of cells, tissues, and organs.

Diffusion NMR studies on fish antifreeze proteins and synthetic analogues.

Pulsed field gradient spin echo NMR spectroscopy was used to measure diffusion coefficients of the alpha-helical type I antifreeze protein from the winter flounder, two synthetic derivatives in which the four Thr residues were replaced with Val and Ala, respectively, and the low molecular weight fraction antifreeze glycoprotein. Under the conditions studied, the natural type I antifreeze protein and low molecular weight glycoprotein gave diffusion values that were consistent with the presence of monomeric protein in solution. While significant aggregation of the Ala analogue was observed (2-10 mM), there was no evidence for aggregation in the Val analogue (1-3 mM). These results are compared with previously reported solubility and thermal hysteresis data and the implications for the design of synthetic antifreeze proteins are discussed.

Genotoxicity and transmission electron microscopy studies of molybdocene dichloride.

The cytotoxic effects of molybdocene dichloride (Cp(2)MoCl(2)) were investigated in V79 Chinese hamster lung cells using the micronucleus assay. Cp(2)MoCl(2) produced significant genotoxic damage whereby 0.2micronuclei/1000 binucleated cells were induced per muM of Cp(2)MoCl(2). Transmission electron microscopic analysis of thin-sectioned cells treated with Cp(2)MoCl(2) (300microM) showed distinct morphological alterations of the nuclei, condensation of chromatin, and a high incidence of polynucleated cells. Implications for the mechanism of antitumor action of molybdocene dichloride are discussed.

Organometallic anticancer agents: cellular uptake and cytotoxicity studies on thiol derivatives of the antitumor agent molybdocene dichloride.

The effect of aqueous solubility, charge, and lability of four thiol derivatives of the antitumor metallocene molybdocene dichloride (Cp(2)MoCl(2)) on the cell uptake and cytotoxicity against V79 Chinese hamster lung cells has been determined. Addition of 4-thiol-2,3,5,6-tetrafluorobenzoic acid, 1-thio-beta-d-glucose, and 1-thio-2,3,4,5-tetraacetyl-beta-d-glucose to aqueous solutions of molybdocene dichloride afforded the corresponding metallocenes in which the deprotonated thiols are coordinated to the metal center. These metallocenes were studied, along with the previously reported glutathione derivative Cp(2)Mo(GS)(2), which has been proposed to be formed from molybdocene dichloride in blood plasma. In contrast to Cp(2)MoCl(2) which rapidly loses the chloride ligands to form a positively charged aquated complex at pH 7, the thiol derivatives are stable to ligand hydrolysis in 50 mM salt at 37 degrees C for 24 h. Cytotoxicity values determined by a modified 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay gave an IC(50) value of 350 microM for molybdocene dichloride with V79 cells, with similar values obtained with human breast MCF-7 (620 microM) and ovarian 2008 (700 microM) cell lines. The water-soluble thiol derivatives were not cytotoxic, while the acetylated sugar derivative was insoluble in water or aqueous dimethyl sulfoxide. Cell uptake experiments in which the molybdenum content in cells treated with each metallocene for 24 h was measured by graphite furnace atomic absorption spectroscopy showed that the fluorinated aromatic derivative was most efficiently transported into cells, followed by molybdocene dichloride, with the lowest uptake observed for Cp(2)Mo(GS)(2) and the glucose derivative. The cell uptake results do not correlate with overall charge of the complexes or the measured IC(50) values. The distinct cytotoxicity and cell uptake profiles of Cp(2)MoCl(2) compared with Cp(2)Mo(GS)(2) show that while rapid coordination of Cp(2)MoCl(2) to glutathione occurs in water at pH 7, significant deactivation of molybdocene dichloride by conversion to Cp(2)Mo(GS)(2) does not occur in cells.

The effect of hydrophobic analogues of the type I winter flounder antifreeze protein on lipid bilayers.

The effect of four synthetic analogues of the 37-residue winter flounder type I antifreeze protein (AFP), which contain four Val, Ala or Ile residues in place of Thr residues at positions 2, 13, 24 and 37 and two additional salt bridges, on the binary lipid system prepared from a 1:1 mixture of the highly unsaturated DGDG and saturated DMPC has been determined using FTIR spectroscopy. In contrast to the natural protein, which increases the thermotropic phase transition, the Thr, Val and Ala analogues decreased the thermotropic phase transitions of the liposomes by 2.2 degrees Celsius, 3.4 degrees Celsius and 2.4 degrees Celsius, while the Ile analogue had no effect on the transition. Experiments performed using perdeuterated DMPC showed that the Ala and Thr peptides interacted preferentially with the DGDG in the lipid mixture, while the Val peptide showed no preference for either lipid. The results are consistent with interactions involving the hydrophobic face of type I AFPs and model bilayers, i.e. the same face of the protein that is responsible for antifreeze properties. The different effects correlate with the helicity of the peptides and suggest that the solution conformation of the peptides has a significant role in determining the effects of the peptides on thermotropic membrane phase transitions.

9-hydroxyellipticine and derivatives as chemotherapy agents.

The hydroxy group in 9-hydroxyellipticines increases the apparent affinity for DNA, stabilisation of toposiomerase II-DNA cleavable complex, oxidation to reactive quinone-imine intermediates, phosphorylation of p53 suppressor proteins and cytotoxicity relative to the parent ellipticines. Recent studies have focused on the mechanism of inhibition of phosphorylation of the mutant type of p53 protein, structural characterisation of the drug-DNA complex, the synthesis of carbohydrate derivatives and calculations of physical parameters, including dipole moments, as potential screens to allow identification of new active derivatives. Derivatisation at the 2- and 9-positions has lead to significant improvements in the in vivo activity of the 9-hydroxyellipticine derivatives and has provided important insights into the mechanism of action of these compounds.

The effect of metal ions on the electrochemistry of the antitumor antibiotic streptonigrin.

The effect of transition metal ions on the electrochemistry of 6-methoxy-5,8-quinolinedione (L1), 7-amino-6-methoxy-5,8-quinolinedione (L2) and the antitumor antibiotic streptonigrin (SN) was studied. In 10% methanol/water, the one-electron reduction of quinones L1 and L2 to the corresponding semiquinones is shifted to more positive potentials upon addition of one equivalent of Zn(II), Ni(II), Co(II) or Cd(II) and is consistent with formation of a 1:1 complex involving the quinone(N) and adjacent quinone(O). Similar results are observed for Cu(II) and Mn(II), but the redox chemistry is also complicated by metal-based redox chemistry. The addition of further equivalents of M(II) results in a number of different coordination and electrochemical processes including formation of 1:1 and 2:1 complexes of the quinone, semiquinone and dianion. Under similar conditions, the 1:1 SN 2,2'-bipyridyl metal complex undergoes a reversible one-electron reduction to the semiquinone. The redox potential of the quinone in SN was shifted positive in the presence of the metal ions, but both the magnitude of the shift, and the relative influence of the metals was different to ligands L1 and L2. The changes in redox chemistry of SN compared with L1 and L2 are consistent with the formation of the 2,2-bipyridyl complexes in which there is weaker coordination to the quinone(O) in ring A of SN. These results suggest that in vivo, metal ions such as Zn(II), Cu(II) and Mn(II) facilitate the initial reduction of streptonigrin to the semiquinone by capturing the semiquinone after SN is reduced by biological reductants.

Platinum(II) and palladium(II) metallomacrocycles derived from cationic 4,4'-bipyridinium, 3-aminopyrazinium and 2-aminopyrimidinium ligands.

A series of cationic, ditopic N-donor ligands based on 4,4'-bipyridine (4,4'-bipy), 3-aminopyrazine (apyz) and 2-aminopyrimidine (apym), each incorporating two positively-charged N-heterocycles linked by a conformationally-flexible spacer unit, have been synthesised and treated with palladium(II) or platinum(II) precursors [M(2,2'-bipy)(NO(3))(2)] (M = Pd(II) or Pt(II)) to form highly cationic metallocyclic species. Treatment of 1,6-bis(4,4'-bipyridinium)hexane nitrate with [M(2,2'-bipy)(NO(3))(2)] in aqueous solution, followed by the addition of KPF(6), resulted in the formation of the [2+2] species [M(2)(2,2'-bipy)(2){4,4'-bipy(CH(2))(6)4,4'-bipy}(2)](PF(6))(8). Treatment of [Pd(PhCN)(2)Cl(2)] with 1,3-bis(4,4'-bipyridinium)propane hexafluorophosphate in MeCN afforded [Pd(2)Cl(4){4,4'-bipy(CH(2))(3)4,4'-bipy}(2)](PF(6))(4). When the cationic apyz or apym ligands were used in aqueous solution, the analogous metallomacrocycles did not form. Instead, deprotonation of the exocyclic amino group occurred upon coordination of the ligand to afford a tetranuclear [4+2] species in the case of platinum(II), with Pt(II)...Pt(II) bonding supported by strong UV-vis absorption at lambda = 428 nm which was assigned to a metal-metal-to-ligand charge transfer (MMLCT) band. Thus, treatment of 1,6-bis(3-aminopyrazinium)hexane nitrate with [Pt(2,2'-bipy)(NO(3))(2)], followed by the addition of KPF(6), led to the formation of the red species [Pt(4)(2,2'-bipy)(4){apyz(CH(2))(6)apyz-2H}(2)](PF(6))(8). No related products could be identified with palladium(II), consistent with the low propensity for this metal ion to form strong Pd(II)...Pd(II) bonding interactions.

ESI-MS and thermal melting studies of nanoscale platinum(II) metallomacrocycles with DNA.

The hydrophilic, long-chain diamine PEGda (O,O'-bis(2-aminoethyl)octadeca(ethylene glycol)), when complexed with cis-protected Pt(II) ions afforded water-soluble complexes of the type [Pt(N,N)(PEGda)](NO(3))(2) (N,N = N,N,N',N'-tetramethyl-1,2-diaminoethane (tmeda), 1,2-diaminoethane (en), and 2,2'-bipyridine (2,2'-bipy)) featuring unusual 62-membered chelate rings. Equimolar mixtures containing either the 16-mer duplex DNA D2 or the single-stranded D2a and [Pt(N,N)(PEGda)](2+) were analyzed by negative-ion ESI-MS. Analysis of D2-Pt(II) mixtures showed the formation of 1 : 1 adducts of [Pt(en)(PEGda)](2+), [Pt(tmeda)(PEGda)](2+) and the previously-described metallomacrocycle [Pt(2)(2,2'-bipy)(2){4,4'-bipy(CH(2))(4)4,4'-bipy}(2)](8+) with D2; the dinuclear species bound to D2 most strongly, consistent with its greater charge and aromatic surface area. D2 formed 1 : 2 complexes with the acyclic species [Pt(2,2'-bipy)(Mebipy)(2)](4+) and [Pt(2,2'-bipy)(NH(3))(2)](2+). Analyses of D2a-Pt(II) mixtures gave results similar to those obtained with D2, although fragmentation was more pronounced, indicating that the nucleobases in D2a play more significant roles in mediating the decomposition of complexes than those in D2, in which they are paired in a complementary manner. Investigations were also conducted into the effects of selected platinum(II) complexes on the thermal denaturation of calf thymus DNA (CT-DNA) in buffered solution. Both [Pt(2)(2,2'-bipy)(2){4,4'-bipy(CH(2))(6)4,4'-bipy}(2)](8+) and [Pt(2,2'-bipy)(Mebipy)(2)](4+) stabilized CT-DNA. In contrast, [Pt(tmeda)(PEGda)](2+) and [Pt(en)(PEGda)](2+) (as well as free PEGda) caused negligible changes in melting temperature (ΔT(m)), suggesting that these species interact weakly with CT-DNA.

Synthesis of homogeneous antifreeze glycopeptides via a ligation-desulfurisation strategy.

Homogeneous glycopeptide analogues of fish antifreeze glycoproteins of discrete oligomeric length have been synthesised using a native chemical ligation-desulfurisation strategy.

Inclusion complexes of the antitumour metallocenes Cp(2)MCl(2) (M = Mo, Ti) with cucurbit[n]urils.

The encapsulation of the aquated forms of molybdocene dichloride and titanocene dichloride by cucurbit[n]uril (Q[n], where n = 7 and 8) at different pD values has been studied by (1)H NMR spectroscopy and molecular modelling. (1)H NMR titration experiments indicate that both metallocenes form 1 : 1 host-guest complexes with both Q[7] and Q[8]. In these complexes, both the cyclopentadienyl ligands and metal centre are positioned deep within the cucurbituril cavity. In vitro cell proliferation studies using the cancer cell lines MCF-7 and 2008 showed that the encapsulated molybdocene complex was more active than the corresponding free metallocene, with GI(50) values of 210 and 400 muM respectively. However, unexpectedly the encapsulation of Cp(2)MoCl(2(aq))at pD 7 catalysed significant degradation of the cucurbituril framework in the presence of oxygen. Encapsulation of Cp(2)TiCl(2(aq)) by Q[7] greatly slowed the protonolysis of the cyclopentadienyl ligands in aqueous phosphate buffer (pD 7), while encapsulation in Q[8] only slightly retarded the hydrolytic degradation of the metallocene.

A solid-state NMR study of the interaction of fish antifreeze proteins with phospholipid membranes.

Fish antifreeze proteins and glycoproteins (AF(G)Ps) prevent ice crystal growth and are able to protect mammalian cells and tissues from hypothermic damage in the sub-zero Polar oceans. This protective mechanism is not fully understood, and further data is required to explain how AF(G)Ps are able to stabilize lipid membranes as they pass through their phase transition temperatures. Solid-state NMR spectroscopy was used as a direct method to study the interaction of the 37-residue alpha-helical type I AFP, TTTT, and the low molecular weight fraction glycoprotein, AFGP8, with dimyristoylphosphatidylcholine membranes above and below the gel-fluid phase transition temperature. In contrast to previous studies in fluid phase bilayers these experiments have provided direct information regarding both the mobility of the phosphate headgroups and perturbation of the acyl chains at a range of temperatures under identical conditions on the same sample. At 5 degrees C changes in the 2H and 31P spectra and a dramatic increase in the 31P T1 relaxation times were consistent with a significant disruption of the membrane by TTTT. Heating to 30 degrees C appeared to expel the peptide from the lipid and re-cooling showed that the interaction of TTTT was not reversible. By contrast, 31P spectra of the membranes with AFGP8 were consistent with interaction with the phosphate headgroups at both 5 and 30 degrees C. Although both peptides interact with the phospholipid bilayer surface, which may stabilize the membrane at lower temperatures, the longer 31P T1 values and the 2H NMR data obtained for TTTT compared with AFGP8 suggest that TTTT causes a greater reduction of phosphate headgroup mobility and has a greater effect on the lipid acyl chains at 5 degrees C.

Design and synthesis of alpha-helical peptides and mimetics.

The alpha-helix is the most abundant secondary structural element in proteins and is an important structural domain for mediating protein-protein and protein-nucleic acid interactions. Strategies for the rational design and synthesis of alpha-helix mimetics have not matured as well as other secondary structure mimetics such as strands and turns. This perspective will focus on developments in the design, synthesis and applications of alpha-helices and mimetics, particularly in the last 5 years. Examples where synthetic compounds have delivered promising biological results will be highlighted as well as opportunities for the design of mimetics of the type I alpha-helical antifreeze proteins.

Antitumour bis(cyclopentadienyl) metal complexes: titanocene and molybdocene dichloride and derivatives.

This Perspective will focus on recent developments in the field of antitumour metallocenes structurally related to titanocene dichloride. Despite extensive testing of titanocene dichloride which culminated in phase I and II clinical trials, further trials have been abandoned. While DNA has been implicated as the major target related to anticancer activity, identification of the active species and mechanism of action has been poorly understood and hence the design of second generation titanocene derivatives has not been possible. Recent mechanistic studies have provided a plausible mechanism for delivery of Ti to cancer cells via transferrin mediated endocytosis. This mechanism requires the presence of labile Cp-Ti bonds that hydrolyse on a time scale to deliver Ti to transferrin. A large range of titanocene derivatives in which the cyclopentadienyl rings have been substituted by both electron withdrawing and donating groups, including aromatic, alkyl and cyclic amines, have been prepared and tested for activity in the last 5 years. These results have shown that subtle structural effects can have a significant effect on biological activity and that biological activity is highly cell line dependent. However, the biological chemistry and cellular studies required to determine the mechanism of action of these new titanocenes have not been reported. In contrast, the bioorganometallic chemistry and cellular studies of molybdocene dichloride have implicated interaction with cellular thiols as the key reaction related to biological activity. Tailoring of the pseudohalide ligands by tuning the strength of the Mo-S bonds provides the opportunity to enhance cell uptake. Further research is required to establish the origin of antitumour activity.

Intracellular mapping of the distribution of metals derived from the antitumor metallocenes.

The intracellular distribution of transition metals in V79 Chinese hamster lung cells treated with subtoxic doses of the organometallic anticancer complexes Cp(2)MCl(2), where Cp is eta (5) -cyclopentadienyl and M is Mo, Nb, Ti, or V, has been studied by synchrotron-based X-ray fluorescence (XRF). While significantly higher concentrations of Mo and Nb were found in treated cells compared with control cells, distinct differences in the cellular distribution of each metal were observed. Analysis of thin sections of cells was consistent with some localization of Mo in the nucleus. Studies with a noncytotoxic thiol derivative of molybdocene dichloride showed an uneven distribution of Mo in the cells. For comparison, the low levels of Ti and V in cells treated with the more toxic titanocene and vanadocene complexes, respectively, resulted in metal concentrations at the detection limit of XRF. The results agree with independent chemical studies that have concluded that the biological chemistry of each of the metallocene dihalides is unique.

Solution structure of a recombinant type I sculpin antifreeze protein.

We have determined the solution structure of rSS3, a recombinant form of the type I shorthorn sculpin antifreeze protein (AFP), at 278 and 268 K. This AFP contains an unusual sequence of N-terminal residues, together with two of the 11-residue repeats that are characteristic of the type I winter flounder AFP. The solution conformation of the N-terminal region of the sculpin AFP has been assumed to be the critical factor that results in recognition of different ice planes by the sculpin and flounder AFPs. At 278 K, the two repeats units (residues 11-20 and 21-32) in rSS3 form a continuous alpha-helix, with the residues 30-33 in the second repeat somewhat less well defined. Within the N-terminal region, residues 2-6 are well defined and helical and linked to the main helix by a more flexible region comprising residues A7-T11. At 268 K the AFP is overall more helical but retains the apparent hinge region. The helical conformation of the two repeats units is almost identical to the corresponding repeats in the type I winter flounder AFP. We also show that while tetracetylated rSS3 has antifreeze activity comparable to the natural AFP, its overall structure is the same as that of the unacetylated peptide. These data provide some insight into the structural determinants of antifreeze activity and should assist in the development of models that explain the recognition of different ice interfaces by the sculpin and flounder type I AFPs.

Coordination chemistry of the antitumor metallocene molybdocene dichloride with biological ligands.

The relative affinity of molybdocene dichloride (Cp(2)MoCl(2)) for the thiol, amino, carboxylate, phosphate(O) and heterocyclic(N) donor ligands present in amino acids and nucleotides, has been studied in aqueous solutions at pH 2-7, using (1)H, (13)C and (31)P NMR spectroscopy. Molybdocene dichloride forms the highly water soluble, air-stable complexes Cp(2)Mo(Cys)(2) and Cp(2)Mo(GS)(2) with cysteine and glutathione respectively, via coordination of the deprotonated thiol groups. While coordination to the imidazole nitrogen in histidine was observed, no evidence for coordination of the amino or carboxylate groups in the amino acids cysteine, histidine, alanine or lysine to Cp(2)MoCl(2) was detected. Competition experiments with dAMP, ribose monophosphate and histidine showed preferential coordination to the cysteine thiol over the phosphate(O) and heterocyclic(N) groups. Cp(2)Mo(Cys)(2) is stable in the presence of excess dAMP or ribose monophosphate and Cys displaces coordinated histidine, dAMP or ribose monophosphate to give Cp(2)Mo(Cys)(2). These results provide further evidence against interaction with DNA as the key interaction that is related to the antitumor activity of molybdocene dichloride. The implications of these results for the biological activity of the antitumor metallocene and the likely species formed in vivo are discussed.

Ruthenium complexes of analogues of the antitumor antibiotic streptonigrin.

The complexes Ru(L1-CH3)(CO)3Cl, RuL2(CO)2Cl2, and RuL3(CO)2Cl2 (L1= 6-methoxy-5,8-quinolinedione, L2 = 7-amino-6-methoxy-5,8-quinolinedione, L3 = 6,6'-dimethoxycarbonyl-2,2'-bipyridine) were prepared by reaction of L1-L3 with the tricarbonyldichlororuthenium(II) dimer. L1-L3 act as bidentates through the ortho oxygen atoms, the pyridyl nitrogen and the adjacent quinone oxygen, and the bipyridyl nitrogens, respectively. RuL3(CO)2Cl2 is characterized by X-ray crystallography. 15N NMR correlation spectra give upfield shifts of around 60 ppm for the pyridyl nitrogens that are coordinated to the metal, while 13C NMR correlation spectra give a downfield shift of 10 ppm for the quinone carbonyl group that is coordinated to the metal. The electrochemistry of RuL2(CO)2Cl2 is examined, and the implications for the formation of metal complexes of the antitumor antibiotic streptonigrin, which cleaves DNA in the presence of metal ions, are discussed.