CO-Induced apoptotic death of colorectal cancer cells by a luminescent photoCORM grafted on biocompatible carboxymethyl chitosan.

A photo-active luminescent rhenium carbonyl complex namely, [Re(CO)3(phen)(pyAl)](CF3SO3) was grafted on a biocompatible carboxymethyl chitosan (CMC) matrix through Schiff base condensation reaction. The light-induced CO delivery from ReCMC has been shown to eradicate human colorectal adenocarcinoma cells (HT-29) very efficiently in a dose-dependent fashion. The onset of CO-induced apoptosis was realized by caspase-3,-7 detection aided by fluorescence confocal microscopy. ReCMC represents a unique example of a biocompatible and biodegradable antineoplastic agent that could find its use in cancer photopharmacology.

Design and construction of a silver(I)-loaded cellulose-based wound dressing: trackable and sustained release of silver for controlled therapeutic delivery to wound sites.

Although application of silver nitrate and silver sulfadiazine have been shown to be effective in thwarting infections at burn sites, optimization of the delivery of bioactive silver (Ag(+)) remains as an obstacle due to rapid precipitation and/or insolubility of the silver sources. To circumvent these shortcomings, we have designed a silver(I) complex [Ag(ImD)2]ClO4 (ImD = dansyl imidazole) that effectively increases the bioavailability of Ag(+) and exhibits MIC values of 2.3 and 4.7 µg/mL against E. coli and S. aureus, respectively. This fluorescent silver complex has been incorporated within a robust hydrogel derived from carboxymethyl cellulose that allows slow release of silver. A complete occlusive dressing has finally been constructed with the Ag(ImD)CMC (1% Ag loaded) pad sealed between a sterile mesh gauze (as bottom layer) and a rayon-based surgical tape (as the top layer). Such construction has afforded a dressing that displays sustained delivery of silver onto a skin and soft tissue infection model and causes effective eradication of bacterial loads within 24 h. The transfer of the bioactive silver complex is readily visualized by the observed fluorescence that overlays precisely with the kill zone. The latter feature introduces a unique feature of therapeutic trackability to this silver-donating occlusive dressing.

Supramolecular Assembly of Ag(I) Centers: Diverse Topologies Directed by Anionic Interactions.

Ag(I)-Ag(I) interactions in supramolecular structures have been achieved through the use of structural support from the ligand frames. In structures involving simple ligands like pyridine, strong π-π interaction leads to spatial ordering of the individual [Ag(L)2]+ units. In such structures anions also play a crucial role in dictating the final arrangement of the [Ag(L)2]+ synthons. In order to determine whether the anions can solely dictate the arrangement of the [Ag(L)2]+ synthons in the supramolecular structure, four Ag(I) complexes of 4-pyridylcarbinol (PyOH), namely, [Ag(PyOH)2]X (X = NO3- (1), BF4- (2), CF3SO3- (3), and ClO4- (4)) have been synthesized and structurally characterized. Gradual transformation of the extended structures observed in 1-3 eventually merges into a unique linear alignment of the [Ag(PyOH)2]+ units in 4 along the c axis, a feature that results in strong argentophilic interactions. Complex 4 is sensitive to light and is inherently less stable than the other three analogues. The structural variations in this set of extended assemblies are solely dictated by the anions, since π-π interaction between the substituted pyridine ligands is significantly diminished due to disposition of the -CH2OH substituent at the 4 position and H-bonding throughout the structure.

Conversion of azomethine moiety to carboxamido group at cobalt(III) center in model complexes of Co-containing nitrile hydratase.

The Co(III) complex of the Schiff base ligand N-2-mercaptophenyl-2'-pyridylmethyl-enimine (PyASH), namely, [Co(PyAS)(2)]Cl (1), has been synthesized via an improved method and its structure has been determined by X-ray crystallography. The two deprotonated ligands are arranged in mer configuration around the Co(III) center and the overall coordination geometry is octahedral. The coordinated azomethine function in 1 is rapidly converted into carboxamido group upon reaction with OH(-). The product is the bis carboxamido complex (Et(4)N)[Co(PyPepS)(2)] (2), reported by us previously. Reaction of H(2)O(2) with 1 in DMF affords [Co(PyASO(2))(PyPepSO(2))] (3), a species with mixed imine and carboxamido-N donor centers as well as S-bound sulfinates. Further reaction with H(2)O(2) in the presence of NaClO(4) converts 3 into the previously reported bis carboxamido/sulfinato complex Na[Co(PyPepSO(2))(2)] (4). The reaction conditions for the various transformation reactions for complexes 1-4 and the structure of 3 are also reported. The mechanism of the -CH=NR + [O] --> -C(=O)NHR transformation has been discussed. The reactions reported here provide convenient alternate routes for the syntheses of Co(III) complexes with coordinated carboxamide, thiolate, and/or sulfinate donors as models for the Co-site in the Co-containing nitrile hydratase(s).

A synthetic analogue of the active site of Fe-containing nitrile hydratase with carboxamido N and thiolato S as donors: synthesis, structure, and reactivities.

As part of our work on models of the iron(III) site of Fe-containing nitrile hydratase, a designed ligand PyPSH(4) with two carboxamide and two thiolate donor groups has been synthesized. Reaction of (Et(4)N)[FeCl(4)] with the deprotonated form of the ligand in DMF affords the mononuclear iron(III) complex (Et(4)N)[Fe(III)(PyPS)] (1) in high yield. The iron(III) center is in a trigonal bipyramidal geometry with two deprotonated carboxamido nitrogens, one pyridine nitrogen, and two thiolato sulfurs as donors. Complex 1 is stable in water and binds a variety of Lewis bases at the sixth site at low temperature to afford green solutions with a band around 700 nm. The iron(III) centers in these six-coordinate species are low-spin and exhibit EPR spectra much like the enzyme. The pK(a) of the water molecule in [Fe(III)(PyPS)(H(2)O)](-) is 6.3 +/- 0.4. The iron(III) site in 1 with ligated carboxamido nitrogens and thiolato sulfurs does not show any affinity toward nitriles. It thus appears that at physiological pH, a metal-bound hydroxide promotes hydration of nitriles nested in close proximity of the iron center in the enzyme. Redox measurements demonstrate that the carboxamido nitrogens prefer Fe(III) to Fe(II) centers. This fact explains the absence of any redox behavior at the iron site in nitrile hydratase. Upon exposure to limited amount of dioxygen, 1 is converted to the bis-sulfinic species. The structure of the more stable O-bonded sulfinato complex (Et(4)N)[Fe(III)(PyP[SO(2)](2))] (2) has been determined. Six-coordinated low-spin cyanide adducts of the S-bonded and the O-bonded sulfinato complexes, namely, Na(2)[Fe(III)(PyP[SO(2)](2))(CN)] (4) and (Et(4)N)(2)[Fe(III)(PyP[SO(2)](2))(CN)] (5), afford green solutions in water and other solvents. The iron(II) complex (Et(4)N)(2)[Fe(II)(PyPS)] (3) has also been isolated and structurally characterized.

Syntheses, structures, and reactivity of low spin iron(III) complexes containing a single carboxamido nitrogen in a [FeN5L] chromophore.

A new pentacoordinate ligand based on TPA (tris-(2-pyridylmethyl)amine), namely, N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide (PaPy(3)H), has been synthesized. The iron(III) complexes of this ligand, namely, [Fe(PaPy(3))(CH(3)CN)](ClO(4))(2) (1), [Fe(PaPy(3))(Cl)]ClO(4) (2), [Fe(PaPy(3))(CN)]ClO(4) (3), and [Fe(PaPy(3))(N(3))]ClO(4) (4), have been isolated and complexes 1-3 have been structurally characterized. These complexes are the first examples of monomeric iron(III) complexes with one carboxamido nitrogen in the first coordination sphere. All four complexes are low spin and exhibit rhombic EPR signals around g = 2. The solvent bound species [Fe(PaPy(3))(CH(3)CN)](ClO(4))(2) reacts with H(2)O(2) in acetonitrile at low temperature to afford [Fe(PaPy(3))(OOH)](+) (g = 2.24, 2.14, 1.96). When cyclohexene is allowed to react with 1/H(2)O(2) at room temperature, a significant amount of cyclohexene oxide is produced along with the allylic oxidation products. Analysis of the oxidation products indicates that the allylic oxidation products arise from a radical-driven autoxidation process while the epoxidation is carried out by a distinctly different oxidant. No epoxidation of cyclohexene is observed with 1/TBHP.

Structure-spectroscopy correlation in distorted five-coordinate Cu(II) complexes: a case study with a set of closely related copper complexes of pyridine-2,6-dicarboxamide ligands.

Eight Cu(II) complexes with the [Cu(dmppy)] moiety (dmppyH(2) = tridentate ligand N,N'-dimethylpyridine-2,6-dicarboxamide; H's are dissociable amide protons) and ligands like pyridine, water, N-methylimidazole, substituted and unsubstituted o-phenanthroline, and bipyridine have been isolated and structurally characterized. The basal angles of these structurally related five-coordinate Cu(II) complexes (and two previously reported ones) correlate well with the EPR hyperfine splitting parameter A( parallel). However, the values of the parameter tau which provides a measure of the degree of square pyramid versus trigonal bipyramid geometry adopted by these complexes do not correlate linearly with the A( parallel) values. It is evident that out-of-plane distortions and ligand strain make calculation of tau inconsistent in certain sets of five-coordinate Cu(II) complexes. Structure-spectroscopy correlation involving tau is not feasible in such cases.

Co(III)-alkylperoxo complexes: syntheses, structure-reactivity correlations, and use in the oxidation of hydrocarbons.

Crystalline [LCo(III)-OOR] complexes with strong-field ligands, L, afford ROO(*) and RO(*) radicals upon mild heating in solution. This fact allows oxidation of hydrocarbons by these complexes under mild conditions. The extent of hydrocarbon oxidation by discrete [LCo(III)-OOR] complexes depends on the nature of L, the solvent, the temperature, and the presence of other M(II) ions. Such systems are catalytic in the presence of excess ROOH.

Monomeric and dimeric copper(II) complexes of a novel tripodal peptide ligand: structures stabilized via hydrogen bonding or ligand sharing.

The novel tripodal ligand N-(bis(2-pyridyl)methyl)-2-pyridinecarboxamide (Py3AH) affords monomeric and dimeric copper(II) complexes with coordinated carboxamido nitrogens. Although many chloro-bridged dimeric copper(II) complexes are known, [Cu(Py3A)(Cl)] (1) remains monomeric and planar with a pendant pyridine and does not form either a chloro-bridged dimer or the ligand-shared dimeric complex [Cu(Py3A)(Cl)]2 (4) in solvents such as CH3CN. When 1 is dissolved in alcohols, square pyramidal alcohol adducts [Cu(Py3A)(Cl)(CH3OH)] (2) and [Cu(Py3A)(Cl)(C2H5OH)] (3) are readily formed. In 2 and 3, the ROH molecules are bound at axial site of copper(II) and the weak axial binding of the ROH molecule is strengthened by intramolecular hydrogen bonding between ROH and the pendant pyridine nitrogen. Two ligand-shared dimeric species [Cu(Py3A)(Cl)]2 (4) and [Cu(Py3A)]2(ClO4)2 (5) have also been synthesized in which the pendant pyridine of one [Cu(Py3A)] unit completes the coordination sphere of the other [Cu(Py3A)] neighbor. These ligand-shared dimers are obtained in aqueous solutions or in complete absence of chloride in the reaction mixtures.

Co(III) complexes with coordinated carboxamido nitrogens and thiolato sulfurs as models for Co-containing nitrile hydratase and their conversion to the corresponding sulfinato species.

Recent spectroscopic data suggest that the Co(III) site in Co-containing nitrile hydratase is ligated to carboxamido nitrogens and thiolato sulfurs and most possibly one or more of the bound thiolates exist as sulfenato and/or sulfinato groups. The absence of any Co(III) complex with such coordination makes it quite difficult to predict the reactivity of this kind of Co(III) site. In this paper, the Co(III) complexes of two designed ligands PyPepSH2 (1) and PyPepRSH2 (2) have been reported. The two complexes, namely, (Et4N)[Co(PyPepS)2] (3) and Na[Co(PyPepRS)2] (4) are the first examples of Co(III) complexes with carboxamido nitrogens and thiolato sulfurs as donors. The average Co(III)-Namido and Co(III)-S distances in these complexes lie in the range 1.90-1.92 and 2.22-2.24 A, respectively. Reaction of H2O2 with both complexes readily affords Na[Co(PyPepSO2)2] (5) and Na[Co(PyPepRSO2)2] (6), species in which the thiolato sulfurs are converted to sulfinato (SO2) groups. Such conversion also occurs when solutions of 3 and 4 are exposed to dioxygen in the presence of activated charcoal. These reactions are clean and the S --> SO2 transformation does not introduce significant changes in the metric parameters of these complexes. The reactivity of 3 and 4 indicates that the bound Cys-sulfurs around the biological Co(III) site could be oxidized to sulfinato groups.

Cleavage of tRNA by Fe(II)-bleomycin.

We have investigated the action of the chemotherapeutic agent Fe(II)-bleomycin on yeast tRNA(Phe), an RNA of known three-dimensional structure. In the absence of Mg2+ ions, the RNA is cleaved preferentially at two major positions, A31 and G53, both of which are located at the terminal base pairs of hairpin loops, and coincide with the location of tight Mg2+ binding sites. A fragment of the tRNA (residues 47-76) containing the T stem-loop is also cleaved specifically at G53. Cleavage of both the intact tRNA and the tRNA fragment is abolished in the presence of physiological concentrations of Mg2+ (> 0.5 mM). Since Fe(II) is not displaced from bleomycin under these conditions, we infer that tight binding of Mg2+ to tRNA excludes productive interactions between Fe(II)-bleomycin and the RNA. These results also show that loss of cleavage is not due to Mg(2+)-dependent formation of tertiary interactions between the D and T loops. In contrast, cleavage of synthetic DNA analogs of the anticodon and T stem-loops is not detectably inhibited by Mg2+, even at concentrations as high as 50 mM. In addition, the site specificities observed in cleavage of RNA and DNA differ significantly. From these results, and from similar findings with other representative RNA molecules, we suggest that the cleavage of RNA by Fe(II)-bleomycin is unlikely to be important for its therapeutic action.

Oxygen transfer reactions by synthetic analogues of iron-bleomycin.

Synthetic analogues of the iron-bleomycins, namely [Fe(PMA)]2+ and [Fe(PMA)]+, have been studied as oxotransfer agents. Oxygen transfer has been observed using iodosobenzene (PhIO), hydrogen peroxide, and dioxygen as oxygen sources. The primary substrates were cis- and trans-stilbene. The products were determined to be cis- and trans-stilbene oxide, benzaldehyde, and deoxybenzoin. These products were recovered in ratios similar to those reported for the iron-bleomycins, albeit in lower yields. Iron complexes of simpler analogues are inactive as oxotransfer agents. This study provides further support that PMAH is an accurate model of the metal binding region of bleomycin.

Radical-induced DNA damage by a synthetic analogue of copper(II)-bleomycin.

The capacity of free-radical formation and concomitant radical-induced DNA damage by [Cu(PMA)]X (X = ClO4-, BF4-), the first crystalline synthetic analogue of Cu(II)-BLM, have been studied. In phosphate buffer, [Cu(PMA)]+ can be reduced by thiols like DTT to a Cu(I) species that can be reoxidized with dioxygen. During oxygenation of the Cu(I) analogue, appreciable amounts of .OH radical are formed. This fact has been established by spin-trapping experiments. Effects of SOD and catalase as well as pH on the ESR signal of the spin adduct have identified a set of reactions that eventually leads to .OH radical formation by [Cu(PMA)]+. Interestingly, the same reaction scheme has been proposed earlier for Cu(II)-BLM. In the presence of DTT and dioxygen, [Cu(PMA)]+ inflicts significant damage to plasmid DNA. That the damage does not arise from iron contamination has been established. The DNA damage is hindered when SOD or catalase is present in the incubation mixture. Less strand scission is observed at higher pH. Modulation of DNA cleavage efficiency with systematic variations in DTT concentration indicates that the conflicting reports on the ability of Cu(II)-BLM to mediate in vitro DNA damage might have resulted from the different DTT concentrations used by different groups.

Alteration and activation of sequence-specific cleavage of DNA by bleomycin in the presence of the antitumor drug cis-diamminedichloroplatinum(II).

The antitumor drug cis-diamminedichloroplatinum(II) dramatically alters the sequence-specific cleavage of the bleomycin A2-iron(II)-O2 system. Preferred bleomycin cleavage sites adjacent to oligo(dG) regions on two restriction fragments of plasmid pBR322 DNA were masked by pretreatment with cis-diamminedichloroplatinum(II). trans-Diamminedichloroplatinum(II), which is inactive as an antitumor drug, showed similar but not identical behavior. The DNA-cleaving activity of bleomycin was substantially modified by cis-diamminedichloroplatinum(II), and a number of specific new cutting sites in guanine-rich parts of the sequence were activated by both isomers of the platinum complex. The results further emphasize the possibility that the synergism found when cis-diamminedichloroplatinum(II) and bleomycin are used in combination chemotherapy may be due to interactions at the level of DNA-drug binding.

Fluorine-19 chemical shifts as structural probes of metal-sulfur clusters and the cofactor of nitrogenase.

Several properties of the FeMo-cofactor (co) of nitrogenase in N-methylformamide solution at ambient temperature have been investigated by means of 19F NMR spectroscopy. With C6H5CF3 reference signals the magnetic moment per Mo atom was found to be approximately equal to 3.9 BM, consistent with S = 3/2 ground state identified by other spectroscopic methods at low temperature. Reaction of FeMo-co with 1.0 eq of RFS- (RF = p-C6H4CF3, p-C6H4F) afforded isotropically shifted signals indicative of binding to a paramagnetic cluster. By comparison with the spectra of Fe-S and Fe-Mo-S species derivatized with RFS-, including the cubane-type MoFe3S4 clusters with S = 3/2 ground states, it was concluded that the essential FeMo-co cluster structure remains intact and a Fe atom is the probable thiolate binding site. An interaction of FeMo-co with C6H5S- had been detected earlier by low temperature EPR spectroscopy. The binding site assignment is based on large observed isotropic shifts (ca. -12ppm) compared to the much smaller values found for Mo-SRF ligands in MoFe3S4 clusters and anticipated in FeMo-co on the basis of recent spectroscopic results. Isotropic 19F shifts have proven extremely sensitive to electronic and structural features of Fe-S and Fe-Mo-S clusters. The inclusion of a 19F NMR label in FeMo-co should prove of utility in further investigation of cofactor properties and reactions.