Nonenzymatic hydrolysis reactions of adenosine 5'-triphosphate and its related compounds. III: Catalytic aspects of some cobalt(III) complexes in ATP-hydrolysis.

Trichlorodiethylenetriaminecobalt (III), [CoCl3dien], which is provided with three good leaving ligands and, hence, capable of binding ATP in a characteristic mode, accelerated effectively and specifically hydrolysis of ATP to ADP and Pi. A kinetic study of the reaction indicated that the rate of hydrolysis was first order with respect to the concentration of ATP in the presence of an excess of [CoCl3dien]. The rate constant was calculated to be 1.05 X 10(-2) min-1 at pH 4.0 (50 degrees C), corresponding to a catalysis of the hydrolysis of ATP by a factor of 150. The complex possessing one good leaving ligand, chlorotetraethylenepentaminecobalt(III), and that having two of them in trans-position, dichlorobis(dimethylglyoximato)cobalt(III) only slightly enhanced the hydrolysis of ATP. Dichloro-cis-alpha- and dichloro-cis-beta-triethylenetetraminecobalt(III) complexes, which have two good leaving ligands and allow chelation of ATP in their coordination sphere, exhibited fairly good activities, although the hydrolysis reactions of ATP occurred in two modes as ATP leads to ADP + Pi and ATP leads to AMP + PPi. The mechanism of ATP-hydrolysis reaction with [CoCl3dien] was also discussed on the basis of the kinetic data.     

Metal complexes with schiff bases obtained from salicylaldehyde derivatives and 2-aminoethylpyridine

Cobalt(II), cobalt(III), nickel(II), copper(II) and palladium(II) complexes with N-2-(2-pyridyl)ethylring-substituted salicylideneiminates (abbreviated as X-Sal-2-Epy) were synthesized. In addition to Co^III (H-Sal-2-Epy)₃, the complexes of the formula M^II(X-Sal-2-Epy)₂·nH₂O were obtained in crystals. The cobalt(III) complex is diamagnetic and has an electronic absorption spectrum typical of the six-coordinate, octahedral cobalt(III) complex. The cobalt(II) complexes in the solid state show electronic spectra typical of the six-coordinate cobalt(II) complexes. Electronic spectra also indicate that the nickel(II) complexes in the solid state and in non-donor solvents are six-coordinate, octahedral. In the cobalt(II) and nickel(II) complexes, the ligand X-Sal-2-Epy functions as terdentates, while in the cobalt(III) complex it acts as a bidentate ligand. The results are compared with those reported previously for related ligands.     

Di-organocobalt complexes of macrocyclic ligands

Three new di-metallorganic cobalt complexes of the type trans-(Bz)₂Co(chel), where Bz is a benzyl group σ-bonded to cobalt atom and chel is an equatorial chelating system constituted by an amino-oximic ligand and its conjugated base, were synthesised. The protonated and the unprotonated ligands interact through an O-H ? O bridge stabilising the entire structure. The complexes differ in the equatorial moiety which is derived from the following ligands: HLN-py=3-[(2-pyridyl)ethylimino]-butan-2-one oxime), HLN-Ph=3-[(2-phenyl)ethylimino]-butan-2-one oxime and the analogous HLN-PhCl=3-[(2-chlorophenyl)ethylimino]-butan-2-one oxime. Two of these compounds, namely those derived from HLN-py and HLN-PhCl were structurally characterised by means X-ray diffractometry. Data reveal that each complex is characterised by the presence of two unusually long cobalt-carbon bonds which are 2.120(4) Å (mean value) in complex with HLN-py ligand and 2.119(4) Å (mean value) in complex with HLN-PhCl. These data are consistent with a strong mutual trans-influence exerted by one ligand on the other.     

Modification of the heme·heme oxygenase system with iron and cobalt tetrasulfonated phthalocyanines

Modification of heme·heme oxygenase by iron(III) and cobalt(II) tetrasulfonated phthalocyanines has been performed. New compounds have been isolated and their properties have been investigated by difference spectroscopy, electrophoresis, molecular weight estimation, electron paramagnetic resonance (EPR) and carboxymethylation at histidyl groups. Spectrophotometric titration data indicate the ratio of the reagents in this process to be 1:1. The visible absorption spectra show the main peak at 650 nm for the iron compound and 682 nm for the cobalt one. Electrophoresis and molecular weight estimation show both complexes to be monomers. Cobalt(II) tetrasulfonated phthalocyanine, under aerobic conditions with heme oxygenase protein, undergoes autooxidation to the cobalt(III) complex, as has been proved by EPR and spectroscopic data. Iron and cobalt phthalocyanine modified heme·heme oxygenase with excess dithionite is reduced at the phthalocyanine ligand. In the presence of oxygen, the reduction product transforms into oxygenated Fe(III)Lheme oxygenase or Co(III)heme oxygenase, respectively. Reduction of the iron(III) model complex with ascorbic acid under anaerobic conditions leads to degradation of the phthalocyanine moiety, while Co(III)heme oxygenase with ascorbic acid is reduced to Co(II)Lheme oxygenase. As has been shown by carboxymethylation of the heme oxygenase protein at the histidine residues, the predominant binding site of both phthalocyanine complexes is the heme-binding histidyl residue. There is evidence that there is a second binding site with lower affinity towards Co(II)L on the heme oxygenase protein. Iron and cobalt tetrasulfonated phthalocyanines are not able to displace heme from the heme·heme oxygenase complex. In this reaction the iron complex undergoes degradation and the cobalt one gives a hybrid compound with heme·heme oxygenaseHeme oxygenase protein complexes with iron and cobalt tetrasulfonated phthalocyanines do not exhibit activity in their oxidative degradation.     

Synthesis, crystal structure and DNA-binding studies of the Ln(III) complex with 6-hydroxychromone-3-carbaldehyde benzoyl hydrazone

A novel 6-hydroxy chromone-3-carbaldehyde benzoyl hydrazone ligand (L) and its Ln(III) complexes, [Ln=La(1) and Sm(2)], have been prepared and characterized. The crystal and molecular structures of complexes 1 and 2 were determined by single-crystal X-ray diffraction. Antioxidative activity tests in vitro showed that L and its complexes have significant antioxidative activity against hydroxyl free radicals from the Fenton reaction and also oxygen free radicals, and that the effect of the La(III) complex 1 is stronger than that of mannitol and the other compounds. The compounds were tested against tumor cell lines including HL-60 and A-549. The data shows that the suppression rate of complexes 1 and 2 against the tested tumor cells are superior to the free ligand (L). The interactions of complexes 1 and 2, and L, with calf thymus DNA were investigated by UV-visible (UV-vis), fluorescence, denaturation experiments and viscosity measurements. Experimental results indicated that complexes 1 and 2, and L can bind to DNA via the intercalation mode, and that the binding affinity of complex 1 is higher than that of complex 2 and of free ligand (L). The intrinsic binding constants of complexes 1 and 2, and L were (7.62+/-0.56)x10(6), (3.70+/-0.47)x10(6) and (2.41+/-0.46)x10(6)M(-1), respectively.     

Cytotoxicity of some platinum(IV) complexes with ethylenediamine-N,N'-di-3-propionato ligand

The cytotoxicities of two platinum(IV) complexes of formula [PtX2(eddp)].nH2O (eddp=ethylenediamine-N,N'-di-3-propionate, X=chloro [I] or bromo [II], n=1 or 1.24) are reported. The complexes have been obtained by direct reaction of potassium hexahaloplatinate(IV) with H2eddp.2HCl followed by addition of a base (LiOH). The crystal and molecular structure has confirmed that the complex with bromo ligands, similarly to the complex with chloro ligands previously reported, has trans configuration of the halogens. In both the chloro and bromo complexes there appear to be intramolecular N-H...X interactions which account for a narrowing of the corresponding X-Pt-N angles below 90degrees. The trans isomer (configuration index OC-6-13, two nitrogens and two oxygens of eddp bound in the equatorial plane) is the only one obtained in the reaction of hexahaloplatinate(IV) with the eddp ligand while a similar reaction performed with ethylenediamine-N,N'-diacetate (edda) affords exclusively the symmetrical cis-isomer (configuration index OC-6-33, equatorial nitrogen and axial oxygen atoms of edda). The longer chain of the propionato groups (as compared to the acetato ones) is responsible for such a change in preferred configuration. NMR data have revealed a very large diastereotopic splitting of the propionato methylene protons to the nitrogens (0.88 ppm). The trans disposition of the halogen ligands in the compounds with eddp leads to deactivation of platinum(IV) complexes in comparison to those with edda having cis disposition of the leaving chlorides (human ovarian cancer cell line A2780, IC50 [muM] of 92.6 +/- 12 and 30.3 +/- 7.5 for [I] and [II], respectively).     

Tc(NX)Y3(Me2PhP)2 complexes (X = O or S; Y = Cl or Br). Preparation,characterization and EPR studies

The d⁵-‘low-spin’ Tc(II) complexes tribromonitrosyl-bis(dimethylphenylphosphine)technetium(II) and tribromo-thionitrosyl-bis(dimethylphenylphosphine)technetium(II) were prepared by ligand exchange starting from the analogous chloro compounds. The complexes were characterized chemically and IR, UVVis and EPR spectroscopically.In the room temperature EPR spectra a well-resolved ⁹⁹Tc hyperfine splitting is observed inidicating a ground state of the unpaired electron which is well separated from the other orbit states. The general features of the spectra at low temperatures are characteristic for an axially symmetric spin Hamiltonian. Analysis of the ⁹⁹Tc and ³¹P hfs (hfs = hyperfine splittings) shows a marked covalency of the Tcligand bonds. A comparison is given between the chloro and bromo, as well as between the nitrosyl and thionitrosyl complexes.     

Polymer–cobalt(III) complexes: structural analysis of metal chelates on DNA interaction and comparative cytotoxic activity

A new series of pendant-type polymer-cobalt(III) complexes, [Co(LL)₂(BPEI)Cl]²⁺, (where BPEI?=?branched polyethyleneimine, LL?=?dipyrido[3,2-a:2′,3′-c](6,7,8,9-tetrahydro)phenazine (dpqc), dipyrido[3,2-d:2′,3′-f]quinoxaline (dpq) and imidazo[4,5-f]1,10-phenanthroline (ip)) each with three different degrees of coordination have been synthesized and characterized. Studies to know the mode and strength of interaction between these polymer–metal complexes and calf thymus DNA have been performed by UV–Visible absorption and emission techniques. Among these series, each polymer metal complex having higher binding strength with DNA has been selected to test against human cancer/normal cell lines. On the basis of these spectral studies, it is proposed that our polymer–metal complexes bind with DNA mainly through intercalation along with some electrostatic binding. The order of binding strength for the complexes with ligand, dpqc?>?dpq?>?ip. The analysis of the results suggests that polymer–cobalt(III) complexes with higher degree of coordination effectively binds with DNA due to the presence of large number of positively charged cobalt(III) chelates in the polymer chain which cooperatively act to increase the overall binding strength. These polymer–cobalt(III) complexes with hydrophobic ligands around the cobalt(III) metal centre favour the base stacking interactions via intercalation. All the complexes show very good anticancer activities and increasing of binding strength results in higher inhibition value. The polymer–cobalt(III) complex with dpqc ligand possess two fold increased anticancer activity when compared to complexes with other ligands against MCF-7 cells. Besides, the complexes were insensitive towards the growth of normal cells (HEK-293) at the IC₅₀ concentration.     

Synthesis of rhodium(III), iridium(III) and osmium(III) complexes with tris(2-aminoethanethiolato)cobalt(III)

Polynuclear sulfur bridged complexes where the neutral complex tris(2-aminoethanethiolato)cobalt(III) acts as a tridentate ligand to rhodium(III), iridium(III) and osmium(III) have been prepared. These complexes have been characterized by electronic spectroscopy, vibrational spectroscopy and nuclear magnetic resonance spectroscopy. Along with the previously prepared complexes of iron(III), ruthenium(III) and cobalt(III), these complexes form two series of complexes with the group 8 and group 9 elements from all three transition series.     

Some complexes of cobalt(III) and iron(III) are radiosensitizers of hypoxic EMT6 cells

The radiosensitizing potential in hypoxic EMT6 cells of several complexes of Co(III) and Fe(III) has been examined. The cytotoxicity of each of the agents toward oxygenated and hypoxic EMT6 cells was tested over the concentration range of 1 to 500 micron for 1-h drug exposure. There was no statistically significant difference between the cytotoxicity of these complexes toward oxygenated and hypoxic cells. Based on these findings, 100 micron was selected as the drug concentration for the initial assessment of radiosensitizing potential. The radiation survival of EMT6 cells in the presence of 100 microM drug for a series of Co(III) complexes in which the number of nitro ligands was varied showed that the hexanitro and the triamine-trinitro complexes are very effective radiosensitizers. The trans-tetrammine dinitro complex was a more effective radiosensitizer than the corresponding cis-dinitro complex. The diethylenetriamine and 1,10-phenanthroline complexes were very effective radiosensitizers, producing dose-modifying factors of 2.4. The trans-tetrammine dichloro complex was moderately effective, giving a dose-modifying factor of 1.9. On the other hand, the hexammine and triammine tricyano complexes and the trans-dinitro complex with negatively charged acetylacetonate ligands were ineffective as radiosensitizers in this system. Finally, three complexes with cyclopentadienyl ligands were examined. The ferricenium salt itself was a moderately effective radiosensitizer, giving a dose-modifying factor of 2.0. However, both the dimethylferricenium salt and the analogous cobalt complex were ineffective. The FSaIIC fibrosarcoma was used to study radiosensitizing potential in vivo. The trans-tetramminedinitro complex was administered at doses of 100, 200, or 300 mg/kg as a single ip injection 1 h prior to irradiation or as three daily ip injections. There was increasing dose modification with increasing drug dosage. With a fractionated radiation protocol in which five daily fractions of 2, 3, or 4 Gy were administered to the tumor-bearing limb with ip drug injections of 100 or 200 mg/kg given 1 h prior to irradiation, a dose-modifying effect of 1.6 was observed with 5 X 200 mg/kg of the drug.     

A nucleophilic reaction of acetylacetonatocobalt(III) complexes with nitriles in Lewis acidic conditions

Under Lewis acidic conditions, tris(acetylacetonato)cobalt(Co(acac)₃) reacted with ethoxymethylenemalononitrile (ETMN), analogous derivatives and general aliphatic nitriles to give a new type of β-imino-cobalt(III) complexes. A possible mechanism for this reaction is proposed. In the case of ethyl acetocyanoacetate, an acylated γ-acetylimino-cobalt(III) complex was isolated. The structure of the complex was explained supporting the proposed mechanism. The crystal structures of several complexes were determined by X-ray diffraction. One of the complexes showed prominent crystal dichroic properties in the first absorption band region. Benzoyl cyanide and phthalonitrile did not react with Co(acac)₃ under the same conditions.     

Coordination of different ligands to copper(II) and cobalt(III) metal centers enhances Zika virus and dengue virus loads in both arthropod cells and human keratinocytes

Trace elements such as copper and cobalt have been associated with virus-host interactions. However, studies to show the effect of conjugation of copper(II) or cobalt(III) metal centers to thiosemicarbazone ligand(s) derived from either food additives or mosquito repellent such as 2-acetylethiazole or citral, respectively, on Zika virus (ZIKV) or dengue virus (serotype 2; DENV2) infections have not been explored. In this study, we show that four compounds comprising of thiosemicarbazone ligand derived from 2-acetylethiazole viz., (E)-N-ethyl-2-[1-(thiazol-2-yl)ethylidene]hydrazinecarbothioamide (acetylethTSC) (compound 1), a copper(II) complex with acetylethTSC as a ligand (compound 2), a thiosemicarbazone ligand-derived from citral (compound 3) and a cobalt(III) complex with a citral-thiosemicarbazone ligand (compound 4) increased DENV2 and ZIKV replication in both mosquito C6/36 cells and human keratinocytes (HaCaT cells). Treatment of both cell lines with compounds 2 or 4 showed increased dengue viral titers at all three tested doses. Enhanced dengue viral plaque formation was also noted at the tested dose of 100 μM, suggesting higher production of infectious viral particles. Treatment with the compounds 2 or 4 enhanced ZIKV and DENV2 RNA levels in HeLa cell line and primary cultures of mouse bone marrow derived dendritic cells. Also, pre- or post treatments with conjugated compounds 2 or 4 showed higher loads of ZIKV or DENV2 envelope (E) protein in HaCaT cells. No changes in loads of E-protein were found in ZIKV-infected C6/36 cells, when compounds were treated after infection. In addition, we tested bis(1,10-phenanthroline)copper(II) chloride ([Cu(phen)₂]Cl₂, (compound 5) and tris(1,10-phenanthroline)cobalt(III) chloride ([Co(phen)₃]Cl₃, (compound 6) that also showed enhanced DENV2 loads. Also, we found that copper(II) chloride dehydrate (CuCl₂·2H₂O) or cobalt(II) chloride hexahydrate (CoCl₂·6H₂O) alone had no effects as “free” cations. Taken together, these findings suggest that use of Cu(II) or Co(III) conjugation to organic compounds, in insect repellents and/or food additives could enhance DENV2/ZIKV loads in human cells and perhaps induce pathogenesis in infected individuals or individuals pre-exposed to such conjugated complexes.

Carbohydrate-derived alkylcobalt carbonyl:([(1,2:5,6-di-O,O-isopropylidene-alpha-D-glucofuranos-3-yl)oxycarbonyl]methyl)cobalt tricarbonyl triphenylphosphene

A carbohydrate-derivative alkylcobalt carbonyl,([(1,2:5,6-di-O,O-isopropylidene-alpha-D-glucofuranos-3-yl)oxycarbonyl]methyl)cobalt tricarbonyl triphenylphosphene(3) was prepared and characterized by IR, NMR, and CD spectra, as well as by X-ray diffraction. The supramolecular chemistry in the crystalline P2(1)2(1)2(1) (chiral) phase and in solution was analyzed and compared. The bulky carbohydrate-based ligand stabilized one chiral conformation, which, however, is less ordered than for analogous compounds with more flexible and less bulky chiral groups. Intermolecular, H-bond interactions are more important in the P2(1)2(1)2(1) phase of complex 3 than at other analogous compounds.     

Cytotoxicity and DNA binding properties of a chloro glycylhistidinate gold(III) complex (GHAu)

The chloro glycylhistidinate gold(III) complex (GHAu) is shown to be fairly cytotoxic towards the established A2780 ovarian carcinoma human cell line either sensitive or resistant to cisplatin. Remarkably, GHAu is far more cytotoxic than the corresponding zinc(II), palladium(II), platinum(II) and cobalt(II) complexes implying that cytotoxicity is essentially to be ascribed to the presence of a gold(III) center. Circular dichroism (CD) spectra, atomic absorption measurements and DNA melting profiles suggest that GHAu in vitro is able to bind DNA, the presumed target for several antitumor metal complexes, and to modify its conformation, even if the observed changes are generally small. Implications of these findings for the mechanism of action of cytotoxic gold(III) complexes are discussed.     

Synthesis, spectroscopy and electrochemistry of some polynuclear complexes of aminoethanethiol

Polynuclear S-bridged complexes of the general formula {[Co₂L₆]M}ⁿ⁺ with M = Co(III), Cd(II), Pb(II), Ni(II), Zn(II) and Hg(II) were prepared from [Co(2-aminoethanethiolate)₃] and the appropriate metal salt. Proton and ¹³C NMR spectra are consistent with structures previously proposed for these species with ¹³C chemical shifts dependent on the bridging metal ion. Electrochemical studies are consistent with a model in which an S-bonded ML₆ moiety (i.e., the bridging metal ion and the six aminoethanethiolate ligands) acts as a ‘dodecadentate’ ligand bonded to two Co³⁺ ions. Reduction of the terminal cobalt ions in these trinuclear complexes is observed in the range −0.75 to −1 V vs. SCE on mercury, gold or glassy carbon working electrodes. For complexes with relatively labile bridging ions, the electrode reaction is irreversible, presumably due to rapid decomposition of the labile cobalt(II) product. For the tricobalt(III) derivative, however, the electrode reaction is reversible consistent with other recent observations on cage or otherwise stereorestrictive ligand systems [1].     

X-ray and NMR study of the fate of the Co(1,10-phenanthroline-5,6-diketone)3 ion in aqueous solution: supramolecular motifs in the packing of 1,10-phenanthroline-5,6-diketone and 1,10-phenanthroline-5,6-diol complexes

X-ray structures are presented of three new cobalt complexes prepared from Co(III) and N,N^′-1,10-phenanthroline-5,6-dione. The cis-aqua-chloro-bis(N,N^′-1,10-phenanthroline-5,6-dione)cobalt(II) nitrate trihydrate (3) and the cis-aqua-bromo-bis(N,N^′-1,10-phenanthroline-5,6-dione)cobalt(II) trifluoro-methanesulfonate tetrahydrate (4), crystalize in the same space group with a similar arrangement of the complex ions. However, on the molecular scale there are important differences. The cobalt complex in 3 has a typical high-spin geometry whereas in 4 the cobalt complex displays a Jahn-Teller distortion characteristic for low-spin compounds. The third structure is di(N,N^′-1,10-phenanthroline-5,6-diol)(N,N^′-1,10-phenanthroline-5,6-dione)cobalt(III) bromide hexahydrate (5). NMR studies of the hydration of the Co(III)(1,10-phenanthroline-5,6-dione)₃ ³⁺ ion in water and DMSO are also presented. The various possible transformations of the N,N^′-1,10-phenanthroline-5,6-dione ligand are discussed.     

Syntheses and crystal structures of mono-, di- and trinuclear cobalt complexes of a salen type ligand

Three cobalt complexes containing the salen type ligand, bis(salicylidene)-meso-1,2-diphenylethylenediaminato (mdpSal²⁻), are reported. The complexes differ in nuclearity and include the mononuclear, Co(mdpSal) (1), which contains a Co(II) metal center bound to one mdpSal⁻² ligand frame in a square planar geometry. The second complex is the dinuclear [Co(mdpSal)Cl]₂ (2) in which both cobalt ions have been oxidized to the +3 oxidation state. The overall geometry of complex 2 is an edge-sharing bioctahedron with the coordination sphere around each cobalt metal center consisting of one mdpSal⁻² ligand and one Cl⁻ ion. The shared edge between the Co(III) ions contains two bridging phenolate groups, one from each ligand frame. Complex 3 is a linear, mixed valence, trinuclear species, [Co(mdpSal)(OAc)(μ-OAc)]₂Co, with the oxidation states of the metal centers assigned as Co(III)-Co(II)-Co(III). The terminal Co(III) centers are equivalent with the central Co(II) lying on the inversion center of the molecule. Each cobalt ion in 3 adopts an octahedral geometry with the terminal Co(III) ions being bound to one mdpSal²⁻ ligand each. All phenolate groups bridge to the central Co(II). The coordination sphere about each metal center in the trinuclear complex is completed by four acetate groups, two of which bind in a μ-fashion bridging from the terminal Co(III) metal centers to the central Co(II). The complexes have been characterized by X-ray crystallography as well as UV-Vis and IR spectroscopy.     

Synthesis, characterization and antibacterial properties of some trivalent metal complexes with [(2-hydroxy-1-naphthaldehyde)-3-isatin]-bishydrazone

Complexes of manganese(III), iron(III) and cobalt(III) with a bishydrazone, formed by condensation of isatin monohydrazone with 2-hydroxy-1-naphthaldehyde, have been synthesized. The spectral data reveal that the ligand acts as monobasic tridentate, coordinating through the deprotonated naphtholate oxygen, azomethine nitrogen, and carbonyl oxygen. Molar conductance values adequately support the electrolytic nature of the complexes. On the basis of the above observations the complexes have been formulated as [M(NIB)2]X where M = Mn(III), Fe(III) or Co(III); X = Cl-, NO3(-); or OAc-; HNIB = [(2-hydroxy-1-naphthaldehyde)-3-isatin]-bishydrazone. Based on electronic spectral data and magnetic moment values, an octahedral geometry has been proposed. The iron(III) complex has been subjected to thermal decomposition studies. The ligand and the metal complexes have been screened for their antibacterial activity and it has been observed that the complexes are more potent bactericides than the ligand.     

Structural and thermodynamic study on aldose reductase: nitro-substituted inhibitors with strong enthalpic binding contribution

To prevent diabetic complications derived from enhanced glucose flux via the polyol pathway the development of aldose reductase inhibitors (ARIs) has been established as a promising therapeutic concept. In order to identify novel lead compounds, a virtual screening (VS) was performed successfully suggesting carboxylate-type inhibitors of sub-micromolar to micromolar affinity. Here, we combine a structural characterization of the binding modes observed by X-ray crystallography with isothermal titration calorimetry (ITC) measurements providing insights into the driving forces of inhibitor binding, particularly of the first leads from VS. Characteristic features of this novel inhibitor type include a carboxylate head group connected via an alkyl spacer to a heteroaromatic moiety, which is linked to a further nitro-substituted aromatic portion. The crystal structures of two enzyme-inhibitor complexes have been determined at resolutions of 1.43 A and 1.55 A. Surprisingly, the carboxylic group of the most potent VS lead occupies the catalytic pocket differently compared to the interaction geometry observed in almost all other crystal structures with structurally related ligands and obtained under similar conditions, as an interstitial water molecule is picked up upon ligand binding. The nitro-aromatic moiety of both leads occupies the specificity pocket of the enzyme, however, adopting a different geometry compared to the docking prediction: unexpectedly, the nitro group binds to the bottom of the specificity pocket and provokes remarkable induced-fit adaptations. A peptide group located at the active site orients in such a way that H-bond formation to one nitro group oxygen atom is enabled, whereas a neighbouring tyrosine side-chain performs a slight rotation off from the binding cavity to accommodate the nitro group. Identically constituted ligands, lacking this nitro group, exhibit an affinity drop of one order of magnitude. In addition, thermodynamic data suggest a strongly favourable contribution to binding enthalpy in case the inhibitor is equipped with a nitro group at the corresponding position. To further investigate this phenomenon, we determined crystal structures and thermodynamic data of two similarly constituted IDD-type inhibitors addressing the specificity pocket with either a nitro or halogen-substituted aromatic moiety. As these data suggest, the nitro group provokes the enthalpic contribution, in addition to the H-bond mentioned above, by accepting two "non-classical" H-bonds donated by the aromatic tyrosine side-chain. In summary, this study provides the platform for further structure-guided design hypotheses of novel drug candidates with higher affinity and selectivity.