Peripheral substituents of di(pyridiumyl)porphyrins affected on their interactions with DNA

meso- and beta-Substituted di(pyridiumyl)porphyrins 3, 4, and 7 have been synthesized and their interactions with DNA have been investigated. meso-Substituted porphyrins showed the stronger effect on DNA than that of beta-substituted porphyrin. Cytotoxicity of compound 3 (IC(50)) to THP-1 tumor cell was up to 0.11 nM.     

Minor groove binding of Co(III)meso-tetrakis(N-methylpyridinium-4-yl)porphyrin to various duplex and triplex polynucleotides

The binding site and the geometry of Co(III)meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (CoTMPyP) complexed with double helical poly(dA).poly(dT) and poly(dG).poly(dC), and with triple helical poly(dA).[poly(dT)](2) and poly(dC).poly(dG).poly(dC)(+) were investigated by circular and linear dichroism (CD and LD). The appearance of monomeric positive CD at a low [porphyrin]/[DNA] ratio and bisignate CD at a high ratio of the CoTMPyP-poly(dA).poly(dT) complex is almost identical with its triplex counterpart. Similarity in the CD spectra was also observed for the CoTMPyP-poly(dG).poly(dC) and -poly(dC).poly(dG).poly(dC)(+) complex. This observation indicates that both monomeric binding and stacking of CoTMPyP to these polynucleotides occur at the minor groove. However, different binding geometry of CoTMPyP, when bind to AT- and GC-rich polynucleotide, was observed by LD spectrum. The difference in the binding geometry may be attributed to the difference in the interaction between polynucleotides and CoTMPyP: in the GC polynucleotide case, amine group protrude into the minor groove while it is not present in the AT polynucleotide.     

A pared-down version of 5,10,15,20-tetra(N-methylpyridinium-4-yl)porphyrin intercalates into B-form DNA regardless of base composition: binding studies of tri(N-methylpyridinium-4-yl)porphyrins

Positively charged N-methylpyridinium-4-yl substituents promote the binding of a porphyrin to DNA, but they also impose steric constraints. To clarify when intercalative binding is most feasible, this report describes syntheses and binding studies of two tricationic ligands: 5,10,15-tri(N-methylpyridinium-4-yl)porphyrin (H2Tri4) and 5-methyl-10,15,20-tri(N-methylpyridinium-4-yl)porphyrin (H2MeTri4). Techniques used to characterize the binding interactions include viscometry and spectroscopic studies of the absorption, emission, and circular dichroism. The striking observation is that intercalation is the only detectable binding motif when the trisubstituted porphyrin H2Tri4 combines with [poly(dA-dT)]2, [poly(dG-dC)]2, or salmon testes DNA. H2Tri4 is, however, a limiting case. Parallel studies of H2MeTri4 and the copper(II) derivative Cu(MeTri4) reveal that external binding to [poly(dA-dT)]2 becomes important when a fourth meso substituent is present, even one as small as the methyl group. Intercalation of H2Tri4 is sterically feasible because two N-methylpyridinium-4-yl substituents can reside in the major groove, though the charge alignment is not optimal. However, the presence of the fourth substituent on H2MeTri4 further destabilizes the intercalated form, and external binding becomes competitive for a flexible host like [poly(dA-dT)]2.     

Synthesis and cytotoxic potency of novel tris(1-alkylindol-3-yl)methylium salts: Role of N-alkyl substituents

Novel derivatives of tris(indol-3-yl)methane and tris(indol-3-yl)methylium salts with the alkyl substituents at the N-atoms of the indole rings were synthesized. An easy substitution of indole rings in trisindolylmethanes for other indoles under the action of acids is demonstrated, and the mechanism of substitution is discussed. To obtain trisindolylmethylium salts, the environmentally safe method of oxidation of trisindolylmethanes with air oxygen in acidic conditions was developed. Tris(1-alkylindol-3-yl)methanes and tris(1-alkylindol-3-yl)methylium salts represent three-bladed molecular propellers whose physico-chemical and biological properties strongly depend on the N-alkyl substituent. The cytotoxicity of novel compounds increased with the number of C atoms in the alkyl chains, with optimal number n = 3–5 whereas the derivatives with longer side chains were less cytotoxic. The most potent novel compounds killed human tumor cells at nanomolar-to-submicromolar concentrations, being one order of magnitude more potent than the prototype antibiotic turbomycin A [tris(indol-3-yl)methylium salt]. Apoptosis in HCT116 colon carcinoma cell line induced by tris(1-pentyl-1H-indol-3-yl)methylium methanesulfonate was detectable at concentrations tolerable by normal blood lymphocytes. Thus, N-alkyl substituted tris(1-alkylindol-3-yl)methylium salts emerge as perspective anticancer drug candidates.     

Dispersion of meso-tetrakis(N-methylpyridinium-4-yl)porphyrin on [d(A-T)n]2 oligonucleotides

The binding modes of the free-base meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (TMPyP) complexed with [d(AT)n]2 oligonucleotides (where n=3-8, corresponding to 6 to 16 AT base pairs) were studied by circular dichroism (CD). When associated with the shortest oligonucleotide, ([d(AT)3]2), a bisignate CD spectrum was produced in the Soret absorption region at the mixing ratio between 2.0 and 0.25, corresponding to one TMPyP per 0.5 to 4 oligonucleotides. Apparent bisignate CD was attributed to a stacked TMPyP along the DNA. On the other hand, when the oligonucleotide length reaches one helical turn or longer, ([d(AT)n]2, n=6,7,8), TMPyP exhibited a positive CD signal, that corresponds to the monomeric groove binding mode, at the mixing ratio below 1.0 (one TMPyP per oligonucleotide). As the mixing ratio increases, the CD signal was best accounted for by the sum of the stacked and groove-binding TMPyP. At the intermediate oligonucleotide length ([d(AT)n]2, n=4,5), the CD spectrum appeared to be the sum of the stacked and groove binding TMPyP at all mixing ratios. Therefore, it is conclusive that the full dispersion of TMPyP requires at least one helical turn of the AT sequence at a mixing ratio below 1.0. Further increase of the mixing ratio resulted in the stacking of TMPyP even at the long oligonucleotides.     

Photo-induced DNA scission by Cu(ii)-meso-tetrakis(n-N-methylpyridiniumyl)porphyrins (n = 2, 3, 4) and their binding modes to supercoiled DNA

The photo-induced cleavage of pGEM-7zf-NIS super-coiled DNA by Cu(ii)-meso-tetrakis(n-N-methylpyridiniumyl)porphyrins (n = 2, 3, 4 referred to as o-, m- and p-CuTMPyP, respectively) and their binding mode were investigated in this study. m-CuTMPyP was most efficient in cleavage than o- and p-CuTMPyP isomers. Cleavage was suppressed by N(2) bubbling, suggesting that the cleavage occurred by an oxidative cleavage mechanism. Sodium azide, an (1)O(2) quencher, and DMSO, a hydroxyl radical scavenger, inhibited cleavage, indicating that hydroxyl radicals and singlet oxygen were likely reactive species responsible for the cleavage. Reduced linear dichroism spectroscopy showed angles of o-CuTMPyP's electric transition moments, in which the periphery pyridinium ring was prevented from free rotation, of 59° and 61° with respect to the local DNA helix axis. The spectra of m- and p-CuTMPyP complexed with pGEM-7zf-NIS DNA were characterized by large signals in the Soret band, coincident with those of known intercalated porphyrins.     

Microwave-assisted synthesis of 4'-azaflavones and their N-alkyl derivatives with biological activities

4'-Azaflavone (=2-(pyridin-4-yl)-4H-1-benzopyran-4-one; 4) and 3-[(pyridin-4-yl)methyl]-4'-azaflavone (5) were synthesized by a simple environmentally friendly microwave-assisted one-pot method through the cyclization of 3-hydroxy-1-(2-hydroxyphenyl)-3-(pyridin-4-yl)propan-1-one (1), (E)-2'-hydroxy-4-azachalcone (2; chalcone=1,3-diphenylprop-2-en-1-one), and 2'-hydroxy-2-[(hydroxy)(pyridin-4-yl)methyl]-4'-azachalcone (3) under solventless conditions using silica-supported NaHSO(4), followed by treatment with base. In addition, N-alkyl-substituted 4'-azaflavonium bromides 6 and 7 were prepared from compounds 4 and 5, respectively. The antimicrobial and antioxidant activities of compounds 1-7 were tested. The N-alkyl-substituted 4'-azaflavonium bromides 6 and 7 showed high antimicrobial activity against the Gram-positive bacteria and the fungus tested, with MIC values close to those of reference antimicrobials ampicilline and fluconazole. The alkylated compounds 6 and 7 also showed a good antioxidant character in the two antioxidant methods, DPPH (=1,1-diphenyl-2-picrylhydrazyl) radical-scavenging and ferric reducing/antioxidant power (FRAP) tests.     

Binding of the Mn(III) complex of meso-tetrakis (4-N-methyl-pyridiniumyl) porphyrin to DNA. Effect of ionic strength

Interactions of the water-soluble Mn(III) complex of meso-tetrakis (4-N-methyl-pyridiniumyl) porphyrin (Mn(III)TMPyP) with DNA in aqueous solutions at low (0.01 M) and high (0.2 M) ionic strengths have been studied by optical absorption, resonance light scattering (RLS) and 1H NMR spectroscopies. Optical absorption and RLS measurements have demonstrated that in DNA solutions at low ionic strength the Mn(III)TMPyP form aggregates, which are decomposed at DNA excess. At high ionic strength the aggregation was not observed. We explain this effect by assuming that upon increase in ionic strength, Mn(III) TMPyP dislocates from the DNA sites, which produces better conditions for the porphyrin aggregation, to sites where the aggregation is hindered. The 1H NMR data demonstrated that the aggregation observed at low ionic strength reduces the paramagnetism of Mn(III)TMPyP. This phenomenon was not observed at the high ionic strength in the absence of aggregation.     

Tricationic pyridium porphyrins appending different peripheral substituents: experimental and DFT studies on their interactions with DNA

Four tricationic pyridium porphyrins appending hydroxyphenyl, methoxyphenyl, propionoxyphenyl or carboxyphenyl group at meso-20-position of porphyrin core have been synthesized and their abilities to bind and cleave DNA have been investigated. Using a combination of absorption, fluorescence, circular dichroism (CD) spectra, thermal DNA denaturation as well as viscosity measurements, their binding modes and intrinsic binding constants (K_b) to calf DNA (CT DNA) were comparatively studied and also compared with those of 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin (TMPyP). The results suggest that the K_b values of these porphyrins are greatly influenced by the number of positive charges and steric hindrance. Theoretical calculations applying the density functional theory (DFT) have been carried out and explain their DNA-binding properties reasonably. The efficiency of DNA photocleavage by these porphyrins shows high dependence on the values of K_b.     

N-(pyrimidin-4-yl) and N-(pyridin-2-yl) phenylalanine derivatives as VLA-4 integrin antagonists

The SAR studies to optimise both potency and rate of clearance in the rat for a series of pyrimidine and pyridine based VLA-4 antagonists are described.     

Dicationic pyridium porphyrins appending different peripheral substituents: synthesis and studies for their interactions with DNA

Twelve trans-dicationic pyridium porphyrins appending different peripheral substituents were synthesized and their abilities to bind and cleave DNA under irradiation have been investigated. Their binding modes to DNA were studied by UV-vis spectroscopy, circular dichroism. The apparent constants were measured by EB competitive fluorescence method and most of them were in the range of 10(4)-10(5) M(-1). We found that both the position of positive charges and steric hindrance could greatly influence their binding affinities and modes to DNA, and then affect their photocleaving abilities to DNA.     

Tris-chelated complexes of nickel(II) with bipyridine derivatives: DNA binding and cleavage,BSA binding,molecular docking,and cytotoxicity

Abstract

Two nickel(II) complexes with substituted bipyridine ligand of the type [Ni(NN)₃](ClO₄)₂, where NN is 4,4′-dimethyl-2,2′-bipyridine (dimethylbpy) (1) and 4,4′-dimethoxy-2,2′-bipyridine (dimethoxybpy) (2), have been synthesized, characterized, and their interaction with DNA and bovine serum albumin (BSA) studied by different physical methods. X-ray crystal structure of 1 shows a six-coordinate complex in a distorted octahedral geometry. DNA-binding studies of 1 and 2 reveal that both complexes sit in DNA groove and then interact with neighboring nucleotides differently; 2 undergoes a partial intercalation. This is supported by molecular-docking studies, where hydrophobic interactions are apparent between 1 and DNA as compared to hydrogen bonding, hydrophobic, and π–π interactions between 2 and DNA minor groove. Moreover, the two complexes exhibit oxidative cleavage of supercoiled plasmid DNA in the presence of hydrogen peroxide as an activator in the order of 1?>?2. In terms of interaction with BSA, the results of spectroscopic methods and molecular docking show that 1 binds with BSA only via hydrophobic contacts while 2 interacts through hydrophobic and hydrogen bonding. It has been extensively demonstrated that the nature of the methyl- and methoxy-groups in ligands is a strong determinant of the bioactivity of nickel(II) complexes. This may justify the above differences in biomolecular interactions. In addition, the in vitro cytotoxicity of the complexes on human carcinoma cells lines (MCF-7, HT-29, and U-87) has been examined by MTT assay. According to our observations, 1 and 2 display cytotoxicity activity against selected cell lines.

Communicated by Ramaswamy H. Sarma     

The role of carboxymethyl substituents in the interaction of tetracationic porphyrins with DNA

Cationic porphyrin-based compounds capable of interacting with DNA are currently under extensive investigation as prospective anticancer and anti-infective drugs. One of the approaches to enhancing the DNA-binding affinity of these ligands is chemical modification of functional groups of the porphyrin macrocycle. We analyzed the interaction with DNA of novel derivatives containing carboxymethyl and ethoxycarbonylmethyl substituents at quaternary nitrogen atoms of pyridinium groups at the periphery of the porphyrin macrocycle. The parameters of binding of 5,10,15,20-tetrakis(N-carboxymethyl-4-pyridinium)porphyrin (P1) and 5,10,15,20-tetrakis(N-ethoxycarbonylmethyl-4-pyridinium)porphyrin (P2) to double-stranded DNA sequences of different nucleotide content were determined using optical spectroscopy. The association constant of P1 interaction with calf thymus DNA (K?=?3.4?×?10(6)?M(-1)) was greater than that of P2 (K?=?2.8?×?10(5)?M(-1)). Preferential binding of P1 to GC- rather than AT-rich oligonucleotides was detected. In contrast, P2 showed no preference for particular nucleotide content. Modes of binding of P1 and P2 to GC and AT duplexes were verified using the induced circular dichroism spectra. Molecular modeling confirmed an intercalative mode of interaction of P1 and P2 with CpG islands. The carboxyl groups of the peripheral substituent in P1 determine the specific interactions with GC-rich DNA regions, whereas ethoxycarbonylmethyl substituents disfavor binding to DNA. This study contributes to the understanding of the impact of peripheral substituents on the DNA-binding affinity of cationic porphyrins, which is important for the design of DNA-targeting drugs.     

Cleavage of DNA by electrochemically activated MnIII and FeIII complexes of meso-tetrakis (N-methyl-4-pyridiniumyl)porphine

Electrochemical methods were used to activate MnIII and FeIII complexes of meso-tetrakis(N-methyl-4-pyridiniumyl)porphine (H2TMPyP) to cause cleavage of pBR322 DNA and to study their interaction with sonicated calf thymus DNA. Electrochemical reduction of MnIIITMPyP and FeIIITMPyP (at low concentrations) in the presence of O2 was required to activate these complexes. However, FeIIITMPyP at 1 x 10(-6) M produced DNA strand breakage without being electrochemically reduced. At low concentrations, FeIITMPyP was more efficient at cleaving DNA than MnIITMPyP. Reduction of O2 at a platinum electrode also produced some cleavage but to a much smaller extent. The oxidized form of MnIIITMPyP (charge 5+) has higher affinity for sonicated calf thymus (CT) DNA than the reduced form (charge 4+), as determined by the negative shift in E degrees' for the voltammetric wave in the presence of DNA. Both forms of FeIIITMPyP (charge 4+) interact with DNA to about the same extent. Differential pulse voltammetry was used to determine binding constants (K) and binding-site sizes (s) of the interaction of these metalloporphyrins with sonicated CT DNA. The data were analyzed assuming both mobile and static equilibria. MnIIITMPyP binds to DNA (5 mM Tris, 50 mM NaCl, pH 7) with K = 5 (+/- 2) x 10(6) M-1, s = 3 bp (mobile) or K = 3.6 (+/- 0.3) x 10(6) M-1, s = 4 bp (static). FeIIITMPyP at that ionic strength caused DNA precipitation. At higher ionic strength (0.1 M Tris, 0.1 M NaCl, pH 7), FeIIITMPyP associates to DNA with K = 4.4 (+/- 0.2) x 10(4) M-1, s = 5 bp (mobile) or K = 1.9 (+/- 0.1) x 10(4) M-1, s = 6 bp (static).     

Synthesis of new N-(2-(trifluoromethyl)pyridin-4-yl)anthranilic acid derivatives and their evaluation as anticancer agents

The N-(2-(trifluoromethyl)pyridin-4-yl)anthranilic acid 6 and a series of its ester and amide derivatives were synthesized and evaluated for their in vitro cytotoxic activity against human cancer cells. Ester derivatives 13 and 18 exhibited potent growth inhibitory activity with GI(50) values at nanomolar concentrations. Among amide derivatives, N-anthraniloylglycinate 19 shown moderate inhibitory activity in the full panel cancer cell line screening.     

New antiprotozoal agents: Synthesis and biological evaluation of different 4-(7-chloroquinolin-4-yl) piperazin-1-yl)pyrrolidin-2-yl)methanone derivatives

In an endeavor to develop efficacious antiprotozoal agents 4-(7-chloroquinolin-4-yl) piperazin-1-yl)pyrrolidin-2-yl)methanone derivatives (5–14) were synthesized, characterized and biologically evaluated for antiprotozoal activity. The compounds were screened in vitro against the HM1: IMSS strain of Entamoeba histolytica and NF54 chloroquine-sensitive strain of Plasmodium falciparum. Among the synthesized compounds six exhibited promising antiamoebic activity with IC₅₀ values (0.14–1.26 μM) lower than the standard drug metronidazole (IC₅₀ 1.80 μM). All nine compounds exhibited antimalarial activity (IC₅₀ range: 1.42–19.62 μM), while maintaining a favorable safety profile to host red blood cells. All the compounds were less effective as an antimalarial and more toxic (IC₅₀ range: 14.67–81.24 μM) than quinine (IC₅₀: 275.6 ± 16.46 μM) against the human kidney epithelial cells. None of the compounds exhibited any inhibitory effect on the viability of Anopheles arabiensis mosquito larvae.     

Binding of meso-tetrakis(N-methylpyridinium-4-yl)porphyrin to AT oligomers: effect of chain length and the location of the porphyrin stacking

Circular dichroism (CD) spectra of meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (TMPyP) that are associated with various duplex and triplex AT oligomers were investigated in this study. A strong positive CD was apparent for both the TMPyP complexed with duplex d[(A-T)(12)](2), d(A)(12).d(T)(12) and triplex d(A)(12).d[(T)(12)](2) at a low mixing ratio. As the mixing ratio increased, bisignate excitonic CD was produced for TMPyP complexed with duplexes, whereas the positive CD signal remained the same for the TMPyP-d(A)(12).d[(T)(12)](2) complex. This difference in the CD spectrum in the presence of duplex and triplex oligomers indicates that the moderate stacking of TMPyP occurs at the major groove of the duplex and the monomeric binding occurs in (or near) the minor groove. When TMPyP forms a complex with duplex d[(A-T)(6)](2) only excitonic CD was observed, even at a very low mixing ratio. Therefore, at least seven or more basepairs are required for TMPyP to exhibit a monomeric CD spectrum. After close analysis of the CD spectrum, the TMPyP-poly[d(A-T)(2)] complex could be explained by a combination of the CD spectrum of the monomeric, moderately stacked, and extensively stacked TMPyP.     

Binding modes of V(=O)meso-tetrakis(N-methylpyridinium-4-yl)porphyrin to various synthetic DNAs studied by polarized spectroscopy

The interactions between water-soluble cationic oxovanadyl[meso-tetrakis(4-N-methylpyridiumyl)]porphyrin (VOTMPyP) and various synthetic polynucleotide including poly[d(A-T)2], poly[d(G-C)2], and poly[d(I-C)2] were studied using absorption, circular dichroism (CD), and linear dichroism (LD) spectroscopy. When VOTMPyP formed a complex with poly[d(A-T)2] and poly[d(I-C)2], a positive CD signal at low [VOTMPyP]/[DNA] ratios (R ratios) and strong excitonic CD signals at above R > or = 0.15 were induced. The appearance of the CD spectra of the VOTMPyP-poly[d(G-C)2] complex were very different: a small negative CD at low R ratios and very small excitonic CD at high R ratios were observed. Considering the facts that the minor grooves of the former two polynucleotides resemble and the major groove of poly[d(I-C)2] is similar with that of poly[d(G-C)2], it is conclusive that VOTMPyP binds to the minor groove of all DNA at lower R ratios while they stack at the outside of DNA at higher R ratios. The binding geometry of VOTMPyP to all polynucleotides studied by LD seemed to be homogenous, irrespective of the R ratio. It has been found that VOTMPyP can have five- and six-fluxional coordination states. Comparing the absorption spectra of VOTMPyP complexed with poly[d(A-T)2] and poly[d(G-C)2], the distinctive absorptions of the five- and six-coordinated species were observed at lower R ratios which centered at 420-430 nm and 442 nm, respectively. While the six-coordinated VOTMPyP favored the poly[d(A-T)2], the five-coordinated species favored the poly[d(G-C)2] at the low R ratios. As the stacked species increased with an increasing R ratio, the six-coordinated species became the major bound species. These observations lead us to conclude that the guanine base' amino group plays a crucial role not only in determining the binding mode of VOTMPyP but also in the conversion of the six-coordinated species to the five-coordinated species.     

Valence tautomerism and macrocycle ruffling in nickel(III) porphyrins

Nonlocal density functional calculations with full geometry optimization have been carried out on the low-lying electronic states of oxidized nickel porphyrins. For [NiIII(P)(Py)2]+, the ground state corresponds to a t2g6(z2)1 configuration and the t2g6(x2-y2)1 configuration is 0.43 eV higher in energy. In contrast, the ground state of [NiIII(P)(CN)2]- corresponds to a t2g6(x2-y2)1 configuration, the t2g6(z2)1 configuration being 0.96 eV higher in energy. The results are consistent with EPR spectroscopic results on the TPP analogs of these complexes. For [Ni(P)(Py)2]+, the a2u- and a1u-type Ni(II) porphyrin cation radical states are higher in energy by 0.63 and 1.23 eV, respectively, relative to the t2g6(z2)1 Ni(III) ground state. The Ni-N(Porphyrin) distance is significantly shorter in [NiIII(P)(Py)2]+ (196 pm) than in [NiIII(P)(CN)2]- (206 pm), which is consistent with the ruffled and planar macrocycle conformations, respectively, in the two complexes.     

Imidazole derivatives of binuclear copper (II) and nickel (II) complexes incorporating bis(1,4,7-triazacyclononan-1-yl) ligands

A series of new binuclear copper (II) and nickel (II) complexes of the macrocyclic ligands bis(1,4,7-triazacyclononan-1-yl)butane (L^but) and bis(1,4,7-triazacyclononan-1-yl)-m-xylene (L^mx) have been synthesized: [Cu₂L^butBr₄] (1), [Cu₂L^but(imidazole)₂Br₂](ClO₄)₂ (2), [Cu₂L^mx(μ-OH)(imidazole)₂](ClO₄)₃ (3), [Cu₂L^but(imidazole)₄](ClO₄)₄ · H₂O (4), [Cu₂L^mx(imidazole)₄](ClO₄)₄ (5), [Ni₂ L^but(H₂O)₆](ClO₄)₄ · 2H₂O (6), [Ni₂L^but(imidazole)₆](ClO₄)₄ · 2H₂O (7) and [Ni₂L^mx (imidazole)₄(H₂O)₂](ClO₄)₄ · 3H₂O (8). Complexes 1, 2, 7 and 8 have been characterized by single crystal X-ray studies. In each of the complexes, the two tridentate 1,4,7-triazacyclononane rings of the ligand facially coordinate to separate metal centres. The distorted square-pyramidal coordination sphere of the copper (II) centres is completed by bromide anions in the case of 1 and/or monodentate imidazole ligands in complexes 2, 4 and 5. Complex 3 has been formulated as a monohydroxo-bridged complex featuring two terminal imidazole ligands. Complexes 6-8 feature distorted octahedral nickel (II) centres with water and/or monodentate imidazole ligands occupying the remaining coordination sites. Within the crystal structures, the ligands adopt trans conformations, with the two metal binding compartments widely separated, perhaps as a consequence of electrostatic repulsion between the cationic metal centres. The imidazole-bearing complexes may be viewed as simple models for the coordinative interaction of the binuclear complexes of bis (tacn) ligands with protein molecules bearing multiple surface-exposed histidine residues.