The ruthenium(II) complex cis-[RuCl₂(dmso)₄] is known for its antiproliferative properties. This has stimulated the discovery of a wide group of new ruthenium complexes considered as potential anticancer drugs. The stability of these ruthenium complexes under physiological conditions and their interaction with protein amino acid residues are particularly important because of their intravenous administration. The studies presented here are devoted to the stability of cis-[RuCl₂(dmso)₄] under physiological pH conditions and its reactivity towards L-methionine. Immediate dissociation of one dmso ligand from the parent complex after dilution in water leads to the formation of fac-[RuCl₂(H₂O)(dmso)₃], which under basic conditions undergoes stepwise dissociation of chloride ions without further dmso release. The hydrolysis of fac-[RuCl₂(H₂O)(dmso)₃] at pH 7.4 and also base-catalyzed hydrolysis were investigated in detail by application of kinetic and spectroscopic (UV/Vis, NMR) measurements. The combined experimental and theoretical study revealed that L-methionine coordinates to fac-[RuCl(OH)(H₂O)(dmso)₃] by substituting a water ligand with simultaneous dmso release. Detailed NMR characterization of the product indicated that methionine coordinates through the NH₂ group rather than the thioether moiety. This conclusion was further supported by theoretical calculations with the application of ETS–NOCV analysis.

The kinetics of reduction of the mer-[Ru^III(pic)₃] complex (pic^– = picolinato) by ascorbic acid (AscH₂) leading to formation of a red ruthenium(II) species have been studied spectrophotometrically by using both conventional mixing and stopped-flow methods. The reaction was followed as a function of the reductant concentration over a wide pH range (1.0–7.4). Electron transfer proceeds by an outer-sphere mechanism involving three protolytic forms of ascorbic acid, AscH₂, AscH^– and Asc^2–, for which specific rate constants have been determined. The Gibbs' energy of activation was found to correlate linearly with the HOMO energies of the protolytic forms of the reductant. The mer-[Ru^III(pic)₃] complex is too sparingly soluble in water to inhibit the growth of the Escherichia coli (ATCC 8739) strain. Its cytotoxicity against non-tumorigenic cells precludes its potential use as an anticancer agent.

High-valent iron-oxo species have been invoked as reactive intermediates in catalytic cycles of heme and nonheme enzymes. The studies presented herein are devoted to the formation of compound II model complexes, with the application of a water soluble (TMPS)Fe^III(OH) porphyrin ([meso-tetrakis(2,4,6-trimethyl-3-sulfonatophenyl)porphinato]iron(III) hydroxide) and hydrogen peroxide as oxidant, and their reactivity toward selected organic substrates. The kinetics of the reaction of H₂O₂ with (TMPS)Fe^III(OH) was studied as a function of temperature and pressure. The negative values of the activation entropy and activation volume for the formation of (TMPS)Fe^IVO(OH) point to the overall associative nature of the process. A pH-dependence study on the formation of (TMPS)Fe^IVO(OH) revealed a very high reactivity of OOH⁻ toward (TMPS)Fe^III(OH) in comparison to H₂O₂. The influence of N-methylimidazole (N-MeIm) ligation on both the formation of iron(IV)-oxo species and their oxidising properties in the reactions with 4-methoxybenzyl alcohol or 4-methoxybenzaldehyde, was investigated in detail. Combined experimental and theoretical studies revealed that among the studied complexes, (TMPS)Fe^III(H₂O)(N-MeIm) is highly reactive toward H₂O₂ to form the iron(IV)-oxo species, (TMPS)Fe^IVO(N-MeIm). The latter species can also be formed in the reaction of (TMPS)Fe^III(N-MeIm)₂ with H₂O₂ or in the direct reaction of (TMPS)Fe^IVO(OH) with N-MeIm. Interestingly, the kinetic studies involving substrate oxidation by (TMPS)Fe^IVO(OH) and (TMPS)Fe^IVO(N-MeIm) do not display a pronounced effect of the N-MeIm axial ligand on the reactivity of the compound II mimic in comparison to the OH⁻ substituted analogue. Similarly, DFT computations revealed that the presence of an axial ligand (OH⁻ or N-MeIm) in the trans position to the oxo group in the iron(IV)-oxo species does not significantly affect the activation barriers calculated for CH dehydrogenation of the selected organic substrates.

For the exploration of the intrinsic reactivity of two key active species in the catalytic cycle of horseradish peroxidase (HRP), Compound I (HRP-I) and Compound II (HRP-II), we generated in situ [Fe^IVO(TMP^+.)(2-MeIm)]⁺ and [Fe^IVO(TMP)(2-MeIm)]⁰ (TMP=5,10,15,20-tetramesitylporphyrin; 2-MeIm=2-methylimidazole) as biomimetics for HRP-I and HRP-II, respectively. Their catalytic activities in epoxidation, hydrogen abstraction, and heteroatom oxidation reactions were studied in acetonitrile at −15 °C by utilizing rapid-scan UV/Vis spectroscopy. Comparison of the second-order rate constants measured for the direct reactions of the HRP-I and HRP-II mimics with the selected substrates clearly confirmed the outstanding oxidizing capability of the HRP-I mimic, which is significantly higher than that of HRP-II. The experimental study was supported by computational modeling (DFT calculations) of the oxidation mechanism of the selected substrates with the involvement of quartet and doublet HRP-I mimics (^2,4Cpd I) and the closed-shell triplet spin HRP-II model (³Cpd II) as oxidizing species. The significantly lower activation barriers calculated for the oxidation systems involving ^2,4Cpd I than those found for ³Cpd II are in line with the much higher oxidizing efficiency of the HRP-I mimic proven in the experimental part of the study. In addition, the DFT calculations show that all three reaction types catalyzed by HRP-I occur on the doublet spin surface in an effectively concerted manner, whereas these reactions may proceed in a stepwise mechanism with the HRP-II mimic as oxidant. However, the high desaturation or oxygen rebound barriers during CH bond activation processes by the HRP-II mimic predict a sufficient lifetime for the substrate radical formed through hydrogen abstraction. Thus, the theoretical calculations suggest that the dissociation of the substrate radical may be a more favorable pathway than desaturation or oxygen rebound processes. Importantly, depending on the electronic nature of the oxidizing species, that is, ^2,4Cpd I or ³Cpd II, an interesting region-selective conversion phenomenon between sulfoxidation and H-atom abstraction was revealed in the course of the oxidation reaction of dimethylsulfide. The combined experimental and theoretical study on the elucidation of the intrinsic reactivity patterns of the HRP-I and HRP-II mimics provides a valuable tool for evaluating the particular role of the HRP active species in biological systems.

The present study focuses on the formation and reactivity of hydroperoxo–iron(III) porphyrin complexes formed in the [Fe^III(tpfpp)X]/H₂O₂/HOO⁻ system (TPFPP=5,10,15,20-tetrakis(pentafluorophenyl)-21H,23H-porphyrin; X=Cl⁻ or CF₃SO₃⁻) in acetonitrile under basic conditions at −15 °C. Depending on the selected reaction conditions and the active form of the catalyst, the formation of high-spin [Fe^III(tpfpp)(OOH)] and low-spin [Fe^III(tpfpp)(OH)(OOH)] could be observed with the application of a low-temperature rapid-scan UV/Vis spectroscopic technique. Axial ligation and the spin state of the iron(III) center control the mode of OO bond cleavage in the corresponding hydroperoxo porphyrin species. A mechanistic changeover from homo- to heterolytic OO bond cleavage is observed for high- [Fe^III(tpfpp)(OOH)] and low-spin [Fe^III(tpfpp)(OH)(OOH)] complexes, respectively. In contrast to other iron(III) hydroperoxo complexes with electron-rich porphyrin ligands, electron-deficient [Fe^III(tpfpp)(OH)(OOH)] was stable under relatively mild conditions and could therefore be investigated directly in the oxygenation reactions of selected organic substrates. The very low reactivity of [Fe^III(tpfpp)(OH)(OOH)] towards organic substrates implied that the ferric hydroperoxo intermediate must be a very sluggish oxidant compared with the iron(IV)–oxo porphyrin π-cation radical intermediate in the catalytic oxygenation reactions of cytochrome P450.

We present for the first time Gutmann donor and acceptor numbers for a series of 36 different ionic liquids that include 26 distinct anions. The donor numbers were obtained by ²³Na NMR spectroscopy and show a strong dependence on the anionic component of the ionic liquid. The donor numbers measured vary from −12.3 kcal mol⁻¹ for the ionic liquid containing the weakest coordinative anion [emim][FAP] (1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate), which is a weaker donor than 1,2-dichloroethane, to 76.7 kcal mol⁻¹ found for the ionic liquid [emim][Br], which exhibits a coordinative strength in the range of tertiary amines. The acceptor numbers were measured by using ³¹P NMR spectroscopy and also vary as a function of the anionic and cationic component of the ionic liquid. The data are presented and correlated with other solvent parameters like the Kamlet–Taft set of parameters, and compared to the donor numbers reported by other groups.

The colourless room-temperature ionic liquid 1-ethyl-3-methylimidazolium perchlorate, [emim][ClO₄], was revisited for reasons of its hydrophilic character and its weak coordinating ability. In this study, [emim][ClO₄] was synthesized from the cheaper ionic liquid 1-ethyl-3-methylimidazolium ethyl sulfate, [emim][EtSO₄], by way of direct and indirect membrane-assisted anion metathesis. Both synthetic methods, as alternatives to the metathesis of silver perchlorate and [emim] halides, are compared and discussed in detail. In addition, some physicochemical parameters were determined to characterize the ionic liquid. With respect to the hazardous nature of organic perchlorates, the thermal stability as well as the impact and friction sensitivity of [emim][ClO₄] were tested according to the UN Test Series 3a–3d.

Catalytic mechanisms of carboxypeptidase A (CPA) are well known for their diversity and the relative inaccessibility for a decisive comprehension. Recent encouraging attempts through modern computational techniques promoted new challenges for the complementary experimental endeavors. In this work, we have applied the stopped-flow technique and the method of reaction progress curve fitting to extract kinetic parameters for the CPA-catalyzed hydrolyses of smaller (typical) peptide and ester substrates, known for their strong activating/inhibiting impact, thus to which the traditional method of “initial rates” is not applicable. Our approach that innately implies the overall constancy of the affecter (substrate plus “active” product) concentration, made it possible to rigorously determine the physically meaningful “effective” values for the catalytic and Michaelis constants under diverse experimental conditions including variable temperature and urea or trimethylamine N-oxide concentrations. Analysis of the obtained results allowed for: (i) the further substantiation of diverse mechanistic patterns for archetypal specific peptide and ester substrates, (ii) testing and disclosure of intrinsic links between the stabilizing/destabilizing and activating/inhibiting effects for the important model enzyme, CPA, and (iii) tentative explanation of a distinct activating/inhibiting impact of these substrates through the strong specific interaction of their benzyl (Bz) moiety with the substrate binding S₃ subsite of CPA. We have demonstrated that stabilization of CPA either through the interaction with an extra Bz moiety (belonging to another substrate or to the product) leads to the increase of its catalytic power with respect to the specific peptide substrate and to its decrease with respect to the counterpart ester substrate. We conjecture that the catalytic mechanisms operating in these two cases include: (a) the “promoted water” mechanism for the peptide substrate that, seemingly, provides the almost “perfect induced fit” (low-barrier conformational adaptation), and (b) presumably, the “anhydride intermediate” mechanism for the ester substrate that, anyway, requires substantial conformational rearrangement (in fact, “partial or local unfolding”) of the protein environment in the course of the rate-determining step. © 2011 Wiley Periodicals, Inc. Biopolymers 95: 852–870, 2011.

Detailed kinetic studies on the ligand-substitution reactions of the very labile [Pd(terpy)Cl]⁺ (terpy = 2,2′:6′,2″-terpyridine) complex have been performed for neutral and anionic nucleophiles in several imidazolium-based ionic liquids. The detailed substitution mechanisms derived from the obtained rate and activation parameters differ from those expected on the basis of data aquired in aqueous medium, since the selected ionic liquids significantly affect the nucleophilic attack of the entering ligand on the complex. The rate constant for the substitution reaction with thiourea as entering nucleophile is much larger in water, viz. (7.8 ± 0.2) × 10⁵M^–1 s^–1, than in the ionic liquids [emim][NTf₂] and [emim][EtOSO₃], viz. (2.5 ± 0.1) × 10⁴ and (1.8 ± 0.1) × 10³M^–1 s^–1 at 25 °C, respectively, which can be accounted for in terms of the protective interaction of the anionic components of the ionic liquids with the electrophilic metal centre.

Substitution reactions of the complexes cis-[Pt(NH₃)₂Cl₂], [Pt(SMC)Cl₂]^–, [Pt(en)Cl₂], and [Pt(dach)Cl₂], where SMC = S-methyl-L-cysteine, en = ethylenediamine and dach = 1,2-diaminocyclohexane, with selected biologically important ligands, viz. guanosine-5′-monophosphate (5′-GMP), L-histidine and 1,2,4-triazole, were studied. All reactions were studied in aqueous 25 mM Hepes buffer in the presence of 5 mM NaCl at pH = 7.2 under pseudo-first-order conditions as a function of concentration at 310 K by using UV/Vis spectrophotometry. Two consecutive reaction steps, which both depend on the nucleophile concentration, were observed in all cases. The second-order rate constants for both reaction steps indicate a decrease in the order [Pt(SMC)Cl₂]^– > cis-[Pt(NH₃)₂Cl₂] > [Pt(en)Cl₂] > [Pt(dach)Cl₂]. DFT calculations (B3LYP/LANL2DZp) showed that the Pt–N7(Guo) adduct is more stable than the Pt–S(thioether) adduct for the studied complexes cis-[Pt(NH₃)₂Cl₂], [Pt(SMC)Cl₂]^–, and [Pt(en)Cl₂]. The calculations collectively support the experimentally observed substitution of thioethers bound to Pt^II complexes by N7(5′-GMP). Finally, this result could be the first to clearly show how much the Pt–N7(Gua) adduct is more stable than the Pt–S(thioether).

The water-exchange mechanisms of [Zn(H₂O)₄(L)]²⁺⋅2 H₂O (L=imidazole, pyrazole, 1,2,4-triazole, pyridine, 4-cyanopyridine, 4-aminopyridine, 2-azaphosphole, 2-azafuran, 2-azathiophene, and 2-azaselenophene) have been investigated by DFT calculations (RB3LYP/6-311+G**). The results support limiting associative reaction pathways that involve the formation of six-coordinate intermediates [Zn(H₂O)₅(L)]²⁺⋅H₂O. The basicity of the coordinated heterocyclic ligands shows a good correlation with the activation barriers, structural parameters, and stability of the transition and intermediate states.

The dinuclear Pt^II complexes bis{[(1R,2R)-(–)-1,2-diaminocyclohexane]chloroplatinum(II)}(μ-1,8-octanediamine), bis{[(1R,2R)-(–)-1,2-diaminocyclohexane]chloroplatinum(II)}(μ-1,10-decanediamine) and bis{[(1R,2R)-(–)-1,2-diaminocyclohexane]chloroplatin(II)}[μ-1,4-bis(3-pyridyl)buta-1,3-diyne] were synthesized. Acid-base titrations and concentration and temperature dependent measurements of the reactions with chloride and thiourea were performed to study the influence of the nature of the bridging ligand on the thermodynamic and kinetic properties of the complexes. The reactions with chloride and thiourea were followed under pseudo-first-order conditions by stopped-flow and UV/Vis spectrophotometry. The results indicate that the bridging ligand has an influence on the reactivity and stability of the complexes towards nucleophiles as well as on possible electronic interactions between the two Pt^II centres. The experimental results are discussed in reference to structures obtained by DFT (BP86/LACVP*) calculations. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The kinetics and mechanism of the complex-formation reactions of [Pd(AEP)(H₂O)]²⁺, where AEP stands for 1-(2-aminoethyl)piperazine, with biologically relevant ligands were studied as a function of selected nucleophiles and pH. The reactivity of the ligands follows the sequence L-methionine > guanosine-5′-monophosphate > glycine > inosine >> glutathione. The substitution reactions with glutathione showed two reaction steps in which the first step involves coordination through nitrogen and depends on the nucleophile concentration, whereas the second step involves intramolecular isomerization from N- to S-bonded glutathione and does not depend on the nucleophile concentration. The stoichiometry and stability constants of the formed complexes are also reported, and the concentration distribution of the various complex species was evaluated as a function of pH. The results are discussed in terms of the mechanism of antitumor activity of related platinum complexes.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Chlorophyll a and pheophytin a have been treated with various metal salts in solution. As a result, metallo derivatives of photosynthetic pigments were formed, or the chlorine ring was oxidized. These studies showed that both effects can be caused by Cu^II ions depending on the redox potential of its complexes formed in specific media. In this report the results of studies on metal insertion and exchange reactions are presented. According to the common use of the “acetate method” in the synthesis of metallotetrapyrroles, and the fact that acetates coordinated to Cu^II ions protect chlorophylls from oxidative degradation, Cu^II acetate monohydrate was selected as main reagent. The metalation and transmetalation processes were investigated with spectroscopic (UV/Vis absorption and emission) and kinetic (conventional and high-pressure) techniques. It turned out that differences in solvent properties can significantly affect not only the rates (metalation), but also the course (transmetalation) of the reaction. The most pronounced effect was found for the transmetalation reaction in acetonitrile, which was terminated at the very initial stage. As shown elsewhere, some special agents/factors, such as excess of acetate, are required to facilitate the dissociation of Mg²⁺ and to push the reaction forward towards formation of the Cu^II derivative of chlorophyll. The mechanisms of two other reactions are proposed on the basis of the determined activation parameters, which correspond to the general mechanisms for metalation and transmetalation of tetrapyrroles with sitting-atop and bimetallic intermediates, respectively. The bimetallic complex of chlorophyll in methanol is probably the most stable complex of this type observed ever. It confirms that spontaneous exchange of Mg²⁺ by Cu^II occurring in plants must result from specific conditions and/or involve contribution from other components of the photosystem.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The present study focuses on the oxidation of the water-soluble and water-insoluble iron(III)–porphyrin complexes [Fe^III(TMPS)] and [Fe^III(TMP)] (TMPS=meso-tetrakis(2,4,6-trimethyl-3-sulfonatophenyl)porphyrinato, TMP=meso-tetrakis(2,4,6-trimethylphenyl)porphyrinato), respectively, by meta-chloroperoxybenzoic acid (m-CPBA) in aqueous methanol and aqueous acetonitrile solutions of varying acidity. With the application of a low-temperature rapid-scan UV/Vis spectroscopic technique, the complete spectral changes that accompany the formation and decomposition of the primary product of OO bond cleavage in the acylperoxoiron(III)–porphyrin intermediate [(P)Fe^IIIOOX] (P=porphyrin) were successfully recorded and characterized. The results clearly indicate that the OO bond in m-CPBA is heterolytically cleaved by the studied iron(III)–porphyrin complexes independent of the acidity of the reaction medium. The existence of two different oxidation products under acidic and basic conditions is suggested not to be the result of a mechanistic changeover in the mode of OO bond cleavage on going from low to high pH values, but rather the effect of environmental changes on the actual product of the OO bond cleavage in [(P)Fe^IIIOOX]. The oxoiron(IV)–porphyrin cation radical formed as a primary oxidation product over the entire pH range can undergo a one- or two-electron reduction depending on the selected reaction conditions. The present study provides valuable information for the interpretation and improved understanding of results obtained in product-analysis experiments.

Kinetic and mechanistic studies on the formation of an oxoiron(IV) porphyrin cation radical bearing a thiolate group as proximal ligand are reported. The SR complex, a functional enzyme mimic of P450, was oxidized in peroxo-shunt reactions under different experimental conditions with variation of solvent, temperature, and identity and excess of oxidant in the presence of different organic substrates. Through the application of a low-temperature rapid-scan stopped-flow technique, the reactive intermediates in the SR catalytic cycle, such as the initially formed SR acylperoxoiron(III) complex and the SR high-valent iron(IV) porphyrin cation radical complex [(SR^.+)Fe^IVO], were successfully identified and kinetically characterized. The oxidation of the SR complex under catalytic conditions provided direct spectroscopic information on the reactivity of [(SR^.+)Fe^IVO] towards the oxidation of selected organic substrates. Because the catalytically active species is a synthetic oxoiron(IV) porphyrin cation radical bearing a thiolate proximal group, the effect of the strong electron donor ligand on the formation and reactivity/stability of the SR high-valent iron species is addressed and discussed in the light of the reactivity pattern observed in substrate oxygenation reactions catalyzed by native P450 enzyme systems.

The iron(III) meso-tetramesitylporphyrin complex is a good biomimetic to study the catalytic reactions of cytochrome P450. All of the three most discussed reactive intermediates concerning P450 catalysis (namely, Cpd 0, Cpd I, and Cpd II) can be selectively produced, identified, and stabilized for many minutes in solution at low temperature by choosing appropriate reaction conditions. In this way, their reactivity against various substrates was determined by utilizing low-temperature rapid-scan UV/Vis spectroscopy. Since all reactive intermediates are derived from a single model complex, the results of these kinetic measurements provide for the first time a full comparability of the determined rate constants for the three intermediates. The rate constants reveal a significant dependence of the reactivity on the type of reaction (e.g., oxygenation, hydrogen abstraction, or hydride transfer), which closely correlates with the chemical nature of Cpds 0, I, and II. The detailed knowledge of the reactivity of these intermediates provides a valuable tool to evaluate their particular role in biological systems.

The effect of temperature and pressure on the water exchange reaction of [Fe^II(NTA)(H₂O)₂]⁻ and [Fe^II(BADA)(H₂O)₂]⁻ (NTA = nitrilotriacetate; BADA = β-alanindiacetate) was studied by ¹⁷O NMR spectroscopy. The [Fe^II(NTA)(H₂O)₂]⁻ complex showed a water exchange rate constant, k_ex, of (3.1 ± 0.4) × 10⁶ s⁻¹ at 298.2 K and ambient pressure. The activation parameters ΔH^≠, ΔS^≠ and ΔV^≠ for the observed reaction are 43.4 ± 2.6 kJ mol⁻¹, + 25 ± 9 J K⁻¹ mol⁻¹ and + 13.2 ± 0.6 cm³ mol⁻¹, respectively. For [Fe^II(BADA)(H₂O)₂]⁻, the water exchange reaction is faster than for the [Fe^II(NTA)(H₂O)₂]⁻ complex with k_ex = (7.4 ± 0.4) × 10⁶ s⁻¹ at 298.2 K and ambient pressure. The activation parameters ΔH^≠, ΔS^≠ and ΔV^≠ for the water exchange reaction are 40.3 ± 2.5 kJ mol⁻¹, + 22 ± 9 J K⁻¹ mol⁻¹ and + 13.3 ± 0.8 cm³ mol⁻¹, respectively. The effect of pressure on the exchange rate constant is large and very similar for both systems, and the numerical values for ΔV^≠ suggest in both cases a limiting dissociative (D) mechanism for the water exchange process. Copyright © 2008 John Wiley & Sons, Ltd.