Seven-coordinate Mn(II) and Co(II) complexes of the pentadentate ligand 2,6-diacetyl-4-carboxymethyl-pyridine bis(benzoylhydrazone): Synthesis, crystal structure and magnetic properties

The metal complexation properties of a functionalized N₃O₂ donor ligand H₂L², where H₂L² stands for 2,6-diacetyl-4-carboxymethyl-pyridine bis(benzoylhydrazone), are investigated by structural and spectroscopic (IR, ESI-MS and EPR) characterization of its Mn(II) and Co(II) complexes. The ligand H₂L² is observed to react essentially in the same fashion as its unmodified parent H₂L¹ producing mixed-ligand [M(H₂L²)(Cl₂)] complexes (M = Mn^II (1), Co^II (3)) upon treatment with MCl₂. Complexes [M(HL²)(H₂O)(EtOH)]BPh₄ (M = Mn 2, M = Co 4), incorporating the supporting ligand in the partially deprotonated form (HL²)⁻, are formed by salt elimination of the [M(H₂L²)(Cl₂)] compounds with NaBPh₄. Compounds 2 and 4 are isostructural featuring distorted pentagonal-bipyramidal coordinated Mn^II and Co^II ions, with the H₂O and EtOH ligands bound in axial positions. Intermolecular hydrogen bonding interactions of the type M-OH₂?O-M involving the H₂O ligands and the carbonyl functions of the supporting ligand assembles the complexes into dimers. Temperature-dependent magnetic susceptibility measurements (2-300 K) show a substantially paramagnetic Curie behavior for the Mn²⁺ compound (2) influenced by zero-field splitting and significant orbital angular momentum contribution for 4 (high-spin Co^II). The exchange coupling across the Mn^II-OH₂?O-Mn^II bridges in 2 was found to be less than 0.1 cm⁻¹, suggesting that no significant intradimer exchange coupling occurs via this path.     

An attempt to apply ligand field theory to positronium reactions with 3d complexes

Positronium, Ps, the bound state between an electron, e, and a positron, c⁺, may have two spin states: theortho,o-Ps, and thepara,p-Ps, states, which differ for the spin orientation of the two particles. The two types of Ps atoms may be inter-converted by collision with paramagnetic compounds, such as several3d complexes. By investigating about 90 complexes of V^II, Cr^II, Cr^III, Mn^II, Fe^II, Co^II and Ni^II as a function of temperature, it was found that the rate constants k_CR of theo-Ps intop-Ps conversion reactions, CR, are linearly correlated to the delocalization β of metal electrons caused by the ligands. Therefore, if β known, k_CR may be estimated andvice versa. This suggests a new method for the experimental determination of β. The rate constants of the Ps oxidation reactions by Co^III complexes were also investigated and discussed. The paper is preceded by four sections dealing with: 1) the positron and positronium formation; 2) the positron annihilation modes; 3) the methods for measuring the Ps rate constants and establishing the Ps reaction types; 4) the application of the Smoluchowski equation to Ps reactions. Moreover, an attempt is made to ascertain the standard electrochemical potential of Ps atoms. The positron reactions and the formation of positronides are also taken into consideration. Presentata nella seduta del 13 dicembre 2002 dal Socio F. Calderazzo.     

Spectroscopic and electrochemical investigation of coordination complexes of octakis(benzylthio)tetraazaporphyrincobalt(II)

Synthesis of new bichromophoric di- and pentanuclear complexes 2-7 by datively binding (bpy)₂Ru^II, (phen)₂Ru^II and Cp (PPh₃)Ru^II units to the periphery of [Co(OBTTAP)], 1, and their spectroscopic properties are described. IR, ¹H NMR, UV-Vis, and mass spectral data were used for their characterization. Relative intensities and positions of the Soret and Q-bands absorptions in the di- and pentanuclear complexes were observed shifted vis-à-vis that in the precursor complex [Co(OBTTAP)], 1. These complexes particularly, those possessing [Co(OBTTAP)] and (bpy)₂Ru^II/(phen)₂Ru^II units, exhibited efficient inter-component electronic excitation energy transfer in their fluorescence excitation-emission spectra, that are suggestive of a high degree of inter-component electronic interaction in them. Also, the electrode activity of the complexes improved upon binding of the peripheral units and they exhibited multiple one-electron reversible oxidation waves in the cyclic voltammograms. These effects have been explained in terms of dπ(S)-dπ(Ru) interactions.     

Synthesis and pH-sensitive luminescence of bis-terpyridyl iridium(III) complexes incorporating pendent pyridyl groups

A series of iridium(III) bis-terpyridine complexes have been prepared which incorporate pendent pyridyl groups at the 4′-positions of one or both of the terpyridine (tpy) ligands. These include: three mutually isomeric homoleptic complexes, in which the nitrogen atom of the pendent pyridyl is para, meta or ortho to the C-C bond to the terpyridine; their heteroleptic analogues in which the second ligand is 4′-tolyl-terpyridine (ttpy); analogous complexes of the new ligand, 4′-(2,6-dimethylpyrid-4-yl)-terpyridine; and related complexes incorporating an additional phenyl ring interposed between the terpyridine and the pendent pyridyl group. All of the complexes are luminescent in air-equilibrated aqueous solution at room temperature. The homoleptic complexes display structured emission resembling that of unsubstituted [Ir(tpy)₂]³⁺, with luminescence lifetimes of around 1 μs under these conditions. The heteroleptic analogues give broader, red-shifted emission spectra, similar to that of [Ir(ttpy)₂]³⁺, indicating that emission in these complexes arises primarily from a lower-energy excited state associated with the 4′-tolyl-terpyridine ligand. A further red-shift for the complexes incorporating the additional phenyl ring suggests that the emissive state involves the more conjugated phenylpyridyl-appended ligand in these cases. The luminescence of all of the heteroleptic complexes investigated, except the meta-substituted system, is sensitive to the protonation state of the pendent pyridyl group, and the structure of the ligand can have a significant influence on both the magnitude of the response and the pH region over which it occurs.     

Metal complexes as optical probes for DNA sensing and imaging

Transition and lanthanide metal complexes have rich photophysical properties that can be used for cellular imaging, biosensing and phototherapy. One of the applications of such luminescent compounds is the detection and visualisation of nucleic acids. In this brief review, we survey the recent literature on the use of luminescent metal complexes (including Re^I, Ru^II, Os^II, Ir^III, Pt^II, Eu^III and Tb^III) as DNA optical probes, including examples of compounds that bind selectively to non-duplex DNA topologies such as quadruplex, i-motif and DNA mismatches. We discuss the applications of metal-based luminescent complexes in cellular imaging, including time-resolved microscopy and super-resolution techniques. Their applications in biosensing and phototherapy are briefly mentioned in the relevant sections.     

Metal–drug synergy: new ruthenium(II) complexes of ketoconazole are highly active against Leishmania major and Trypanosoma cruzi and nontoxic to human or murine normal cells

In our ongoing search for new metal-based chemotherapeutic agents against leishmaniasis and Chagas disease, six new ruthenium–ketoconazole (KTZ) complexes have been synthesized and characterized, including two octahedral coordination complexes—cis,fac-[Ru^IICl₂(DMSO)₃(KTZ)] (1) and cis-[Ru^IICl₂(bipy)(DMSO)(KTZ)] (2) (where DMSO is dimethyl sulfoxide and bipy is 2,2′-bipyridine)—and four organometallic compounds—[Ru^II(η⁶-p-cymene)Cl₂(KTZ)] (3), [Ru^II(η⁶-p-cymene)(en)(KTZ)][BF₄]₂ (4), [Ru^II(η⁶-p-cymene)(bipy)(KTZ)][BF₄]₂ (5), and [Ru^II(η⁶-p-cymene)(acac)(KTZ)][BF₄] (6) (where en is ethylenediamine and acac is acetylacetonate); the crystal structure of 3 is described. The central hypothesis of our work is that combining a bioactive compound such as KTZ and a metal in a single molecule results in a synergy that can translate into improved activity and/or selectivity against parasites. In agreement with this hypothesis, complexation of KTZ with Ru^II in compounds 3–5 produces a marked enhancement of the activity toward promastigotes and intracellular amastigotes of Leishmania major, when compared with uncomplexed KTZ, or with similar ruthenium compounds not containing KTZ. Importantly, the selective toxicity of compounds 3–5 toward the leishmania parasites, in relation to human fibroblasts and osteoblasts or murine macrophages, is also superior to the selective toxicities of the individual constituents of the drug. When tested against Trypanosoma cruzi epimastigotes, some of the organometallic complexes displayed activity and selectivity comparable to those of free KTZ. A dual-target mechanism is suggested to account for the antiparasitic properties of these complexes.     

Contrast agents for MRI: 30+ years and where are we going?

Thirty years ago, Schering filed the first patent application for a contrast agent for magnetic resonance imaging (MRI) covering the forefather of the gadolinium contrast agents and still the most widely used gadolinium probe: gadolinium(III) diethylenetriaminepentaacetate (Magnevist). To date, 11 contrast agents have been approved by the US Food and Drug Administration for intravenous use. Coordination chemists have done a great deal to move the field forward. Our understanding of lanthanide chemistry now makes possible the design of complexes with long rotational correlation times, fast or slow water-exchange rates, high thermodynamic stabilities, and kinetic inertness, leading to sensitive and nontoxic contrast agents. Chemists did not stop there. The last few decades has seen the development of novel classes of probes that yield contrast through different mechanisms, such as paramagnetic chemical exchange saturation transfer agents. Thirty years since the first patent, chemists are still leading the way. The development of high-sensitivity contrast agents for high magnetic fields, safe probes for patients with kidney disorders, and multimodal, targeted, and responsive agents demonstrates that the field of contrast agents for MRI still has much to offer.     

TmDOTA-tetraglycinate encapsulated liposomes as pH-sensitive LipoCEST agents

Lanthanide DOTA-tetraglycinate (LnDOTA-(gly)₄ ⁻) complexes contain four magnetically equivalent amide protons that exchange with protons of bulk water. The rate of this base catalyzed exchange process has been measured using chemical exchange saturation transfer (CEST) NMR techniques as a function of solution pH for various paramagnetic LnDOTA-(gly)₄ ⁻ complexes to evaluate the effects of lanthanide ion size on this process. Complexes with Tb(III), Dy(III), Tm(III) and Yb(III) were chosen because these ions induce large hyperfine shifts in all ligand protons, including the exchanging amide protons. The magnitude of the amide proton CEST exchange signal differed for the four paramagnetic complexes in order, Yb>Tm>Tb>Dy. Although the Dy(III) complex showed the largest hyperfine shift as expected, the combination of favorable chemical shift and amide proton CEST linewidth in the Tm(III) complex was deemed most favorable for future in vivo applications where tissue magnetization effects can interfere. TmDOTA-(gly)₄ ⁻ at various concentrations was encapsulated in the core interior of liposomes to yield lipoCEST particles for molecular imaging. The resulting nanoparticles showed less than 1% leakage of the agent from the interior over a range of temperatures and pH. The pH versus amide proton CEST curves differed for the free versus encapsulated agents over the acidic pH regions, consistent with a lower proton permeability across the liposomal bilayer for the encapsulated agent. Nevertheless, the resulting lipoCEST nanoparticles amplify the CEST sensitivity by a factor of ∼10⁴ compared to the free, un-encapsulated agent. Such pH sensitive nano-probes could prove useful for pH mapping of liposomes targeted to tumors.     

Hydrothermal synthesis, crystal structures and properties of new Fe, Co, Ni, and Zn complexes with 6-quinolinecarboxylate: Interplay of coordinative and noncovalent interactions

Reactions of Fe^II, Co^II, Ni^II, and Zn^II salts with 6-quinolinecarboxylic acid (HL) under the hydrothermal conditions afford three monomeric complexes [M(L)₂(H₂O)₄] (M = Fe^II for 1, Co^II for 2, and Ni^II for 3) and a 1-D polymeric species {[Zn(L)₂(H₂O)] · H₂O}_n (4). The crystal structures of the ligand HL and these four complexes have been determined by using the X-ray single-crystal diffraction technique. The results suggest that complexes 1-3 are isostructural, displaying novel 3-D pillar-layered networks through multiple intermolecular hydrogen bonds, whereas in coordination polymer 4, the 1-D comb-like coordination chains are extended to generate a hydrogen-bonded layer, which is further reinforced via aromatic stacking interactions. Solid-state properties such as thermal stability and fluorescence emission of the polymeric Zn^II complex 4 have also been investigated.     

Metal-based environment-sensitive MRI contrast agents

Interactions of paramagnetic metal complexes with their biological environment can modulate their magnetic resonance imaging (MRI) contrast–enhancing properties in different ways, and this has been widely exploited to create responsive probes that can provide biochemical information. We survey progress in two rapidly growing areas: the MRI detection of biologically important metal ions, such as calcium, zinc, and copper, and the use of transition metal complexes as smart MRI agents. In both fields, new imaging technologies, which take advantage of other nuclei (¹⁹F) and/or paramagnetic contact shift effects, emerge beyond classical, relaxation-based applications. Most importantly, in vivo imaging is gaining ground, and the promise of molecular MRI is becoming reality, at least for preclinical research.     

Synthesis of palladium(II) 3d-metal(II) paddlewheel acetate-bridged heterodimetallic complexes: Unexpected catalysis by water molecules

A study of the reaction kinetics between the trinuclear palladium(II) acetate Pd₃(μ-OOCMe)₆ (1) and the mononuclear 3d-metal (Ni^II, Co^II, Cu^II) acetates in acetic acid under water-specified conditions revealed a fairly complicated reaction mechanism triggered by the primary hydrolytic cleavage of an acetate bridge in molecule 1. The isolated reaction products, as established by X-ray diffraction study, are 1D polymeric complexes {Pd(μ-OOCMe)₄M(OH₂)(HOOCMe)₂}_n (M = Ni^II, Co^II, Cu^II, Mn^II, Zn^II) built of the Pd^II-based paddlewheel units [Pd(μ-OOCMe)₄M] and linked trough the H-bonded H₂O and MeCOOH molecules.     

Synthesis and Relaxometric Characterization of New Poly[N,N‐bis(3‐aminopropyl)glycine] (PAPGly) Dendrons Gd‐Based Contrast Agents and Their in Vivo Study by Using the Dynamic Contrast‐Enhanced MRI Technique at Low Field (1 T)

The synthesis of poly[N,N‐bis(3‐aminopropyl)glycine] (PAPGly) dendrons Gd‐based contrast agents (GdCAs) via an orthogonal protection of the different functional groups and an activation/coupling strategy wherein a specific number of synthetic steps add a generation to the existing dendron has been described. The aim of this protocol is to build up two different generations of dendrons ( G‐0 or dendron's core, and G‐1 ) with peripheral NH₂ groups to conjugate a 1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetic acid (DO3A) derivative and afterwards to chelate with Gd³⁺ paramagnetic ions. These complexes, which have a well‐defined molecular weight, are of relevance to MRI as an attempt to gain higher ¹H relaxivity by slowing down the rotation of molecule compared to monomeric Gd(III) complexes used as contrast agents and to increase the number of paramagnetic centers present in one molecular structure. From the study of their water ¹H longitudinal relaxation rate at different magnetic fields (NMRD, Nuclear Magnetic Relaxation Dispersion) and by evaluating the variable temperature ¹⁷O‐NMR data we determined the parameters characterizing the water exchange rate and the rotational correlation time of each complex, both affecting ¹H relaxivity. Furthermore, these two novel PAPGly GdCAs were objects of i) an in vivo study to determine their biodistributions in healthy C57 mice at several time points, and ii) the Dynamic Contrast‐Enhanced MRI (DCE‐MRI) approach to assess their contrast efficiency measured in the tumor region of C57BL/6 mice transplanted subcutaneously with B16‐F10 melanoma cells. The aim of the comparison of these two dendrons GdCAs, having different molecular weights (MW), is to understand how MW and relaxivity may influence the contrast enhancement capabilities in vivo at low magnetic field (1 T). Significant contrast enhancement was observed in several organs (vessel, spleen and liver), already at 5 min post‐injection, for the investigated CAs. Moreover, these CAs induced a marked contrast enhancement in the tumor region, thanks to the enhanced permeability retention effect of those macromolecular structures.     

Metal-directed assembly of 1-D and 2-D coordination polymers with fluconazole and dicyanamide co-ligand

The coordination chemistry of a flexible poly(triazolyl)alkane derivative, fluconazole (HFlu), with a series of transition metal ions and dicyanamide (dca) anionic co-ligand has been explored to afford six new metal-organic coordination polymers. Complexes [M(HFlu)₂(dca)₂]_n (M = Mn^II for 1, Fe^II for 2, Co^II for 3, Zn^II for 5, and Cd^II for 6) have the isostructural 1-D double-chain array via bridging fluconazole, whereas [Cu₃(Flu)₂(dca)₄(CH₃OH)₂]_n (4) shows an unusual 2-D layered metal-organic framework with dimeric Cu^II subunits. Notably, both types of coordination patterns are extended into distinct 3-D supramolecular networks via hydrogen-bonding interactions. This result indicates that the choice of metal ion has a significant effect on these polymeric structures as well as the binding modes of the ligands, which is discussed in detail. The Zn^II and Cd^II complexes 5 and 6 display similar fluorescent emissions at 260 nm in the solid state, which essentially are intraligand transitions.     

Magnetic coupling in trinuclear partial cubane copper(II) complexes with a hydroxo bridging core and peripheral phenoxo bridges from NNO donor Schiff base ligands

Three new trinuclear copper(II) complexes, [(CuL¹)₃(μ₃-OH)](ClO₄)₂·3.75H₂O (1), [(CuL²)₃(μ₃-OH)](ClO₄)₂(2) and [(CuL³)₃(μ₃-OH)](BF₄)₂·0.5CH₃CN (3) have been synthesized from three tridentate Schiff bases HL¹, HL², and HL³ (HL¹ = 2-[(2-amino-ethylimino)-methyl]-phenol, HL² = 2-[(2-methylamino-ethylimino)-methyl]-phenol and HL³ = 2-[1-(2-dimethylamino-ethylimino)-ethyl]-phenol). The complexes are characterized by single-crystal X-ray diffraction analyses, IR, UV-vis and EPR spectroscopy, and variable-temperature magnetic measurements. All the compounds contain a partial cubane [Cu₃O₄] core consisting of the trinuclear unit [(CuL)₃(μ₃-OH)]²⁺ together with perchlorate or fluoroborate anions. In each of the complexes, the three copper atoms are five-coordinated with a distorted square-pyramidal geometry except in complex 1, in which one of the Cu^II ions of the trinuclear unit is six-coordinate being in addition weakly coordinated to one of the perchlorate anions. Variable-temperature magnetic measurements and EPR spectra indicate an antiferromagnetic exchange coupling between the Cu^II ions of complexes 1 and 2, while this turned out to be ferromagnetic for complex 3. Experimental values have been fitted according to an isotropic exchange Hamiltonian. Calculations based on Density Functional Theory have also been performed in order to estimate the exchange coupling constants in these three complexes. Both sets of values indicate similar trends and specially calculated J values establish a magneto-structural correlation between them and the Cu-O-Cu bond angle, in that the coupling is more ferromagnetic for smaller bond angle values.     

A comparative DFT study on aquation and nucleobase binding of ruthenium (II) and osmium (II) arene complexes

The potential energy surfaces of the reactions of organometallic arene complexes of the type [(η ⁶-arene)M^II(pic)Cl] (where pic = 2-picolinic acid, M = Ru or Os) were examined by a DFT computational study. Among the seven density functional methods, hybrid exchange functional B3LYP outperforms the others to explain the aquation of the complexes. The reactions and binding energies of Ru^II and Os^II arene complexes with both 9EtG and 9EtA were studied to gain insight into the reactivity of these types of organometallic complexes with DNA. The obtained data rationalize experimental observation, contributing to partly understanding the potential biological and medical applications of organometallic complexes.

Stepwise syntheses, structure and spectroscopic studies of ruthenium complexes of 2,6-bis(benzimidazol-2-yl)pyridine with o-iminoquinone ancillary ligand

The ruthenium complexes [Ru^II(bbp)(L)(Cl)] (1), [Ru^II(bbp)(L)(H₂O)] (2) and [Ru^II(bbp)(L)(DMSO)] (3) {bbp = 2,6-bis(benzimidazol-2-yl)pyridine, L = o-iminoquinone} have been synthesized in a stepwise manner starting from [Ru^III(bbp)Cl₃]. The single crystal X-ray structures, except for the complex 2, have been determined. All the complexes were characterized by UV-Vis, FT-IR, ¹H NMR, Mass spectroscopic techniques and cyclic voltammetry. The Ru^III/Ru^II couple for complexes 1, 2, and 3 appears at 0.63, 0.49, 0.55 V, respectively versus SCE. It is observed that complex 2, on refluxing in acetonitrile, results into [Ru^II(bbp)(L)(CH₃CN)], 4 which has been prepared earlier in a different method. The structural, spectral and electrochemical properties of complexes 1, 2 and 3 were compared to those of earlier reported complex 4, [Ru^II(bbp)(L)(CH₃CN)].     

Synthesis and structure of iron (III) and iron (II) complexes in S4P2 environment created by diethyldithiocarbamate and 1,2-bis(diphenylphosphino)ethane chelation: Investigation of the electronic structure of the complexes by Mössbauer and magnetic studies

Iron (II) and iron (III) complexes, [Fe^II(DEDTC)₂(dppe)] · CH₂Cl₂ (1), [Fe^II(ETXANT)₂(dppe)] (2) (DEDTC = diethyldithiocarbamate, ETXANT = ethyl xanthate, dppe = 1,2-bis (diphenylphosphino) ethane), and [Fe^III(DEDTC)₂(dppe)] [Fe^IIICl₄] (3) have been synthesized and characterized. Since 3 contains two magnetic centers, an anion metathesis reaction has been conducted to replace the tetrahedral FeCl₄⁻ by a non-magnetic BPh₄⁻ ion producing [Fe^III(DEDTC)₂(dppe)]BPh₄ (4) for the sake of unequivocal understanding of the magnetic behavior of the cation of 3. With the similar end in view, the well-known FeCl₄⁻ ion, the counter anion of 3, is trapped as PPh₄[Fe^IIICl₄] (5) and its magnetic property from 298 to 2 K has been studied. Besides the spectroscopic (IR, UV-Vis, NMR, EPR, Mass and XPS) characterization of the appropriate compounds, especially 2, others viz. 1, 3 and 4 have been structurally characterized by X-ray crystallography. While Fe^II complexes, 1 and 2, are diamagnetic, the Fe^III systems, namely the cations of 3, and 4 behave as low-spin (S = 1/2) paramagnetic species from 298 to 50 K. Below 50 K 3 shows gradual increase of χ_MT up to 2 K suggesting ferromagnetic behavior while 4 exhibits gradual decrease of magnetic moment from 60 to 2 K, indicating the occurrence of weak antiferromagnetic interaction. These conclusions are supported by the Mössbauer studies of 3 and 4. The Mössbauer pattern of 1 exhibits a doublet site for diamagnetic (2-400 K) Fe^II. The compounds 1, 2 and 4 encompass interesting cyclic voltammetric responses involving Fe^II, Fe^III and Fe^IV.     

Nickel(II) and iron(II) mononuclear complexes with 1-methylimidazole-2-aldoximate: New building units for molecule-assembled magnetic materials

A new class of mononuclear metal complexes with 1-methylimidazole-2-aldoximate (miao) has been synthesized and characterized: trans-Ni^II(Cl)₂(Hmiao)₂ (1), trans-Ni^II(miao)₂(py)₂ (2), NO-trans-Ni^II(miao)₂(phen) (3), and NO-trans-Fe^II(miao)₂(phen) (4). The crystal structures of 2, 3, and 4 have been determined by single-crystal X-ray crystallography. Compound 1 having the protonated miao ligand (i.e., Hmiao) is a precursor for synthesizing 2 and 3. Compound 2 is an octahedral Ni^II complex surrounded by two miao bidentate ligands and two monodentate ligands of pyridine in a trans-arrangement. Compound 3 is a cis-type octahedral Ni^II complex with two miao ligands and a bidentate ligand of 1,10-phenanthroline, in which the ligand arrangement around Ni^II center is found in an NO-trans form. Compound 4 is an isostructural Fe^II derivative of 3. Compounds 1, 2, and 3 exhibit paramagnetic nature with an S = 1 spin and a positive zero-field splitting, among which it for 3 is overlapped with intermolecular ferromagnetic interaction (zJ/k_B = +0.16 K). Compound 4 is diamagnetic due to the existence of low-spin Fe^II ion.     

Probing electron transfer reactions between two azurins from Alcaligenes xylosoxidans GIFU 1051 with optically active Ru complexes as molecular recognition probes: Importance of the 43rd residue

Electron transfer reactions between optically-active Ru^II/III complexes incorporating (S)-/(R)-amino acids, and the two azurins, azurin-1 (az-1Cu) and azurin-2 (az-2Cu) isolated from Alcaligenes xylosoxidans GIFU 1051, have been studied to probe molecular recognition sites on the two azurins. The Ru^II/III complexes are K[Ru^II(L)(bpy)] and [Ru^III(L)(bpy)], and have a tripodal ligand (L) derived from the (S)-/(R)-amino acids, which are in turn exchanged for other functional substituent groups, such as (S)-/(R)-phenylalanine, -leucine, -valine, -alanine, and -glutamic acid (L = (S)-/(R)-BCMPA, -BCMLE, -BCMVA, -BCMAL, and -BCMGA). In the oxidation reaction of az-1Cu^I promoted by the Ru^III complexes, the kinetic parameters exhibited enantio- and stereo-selectivities, while the same reaction of az-2Cu^I was less enantio- and stereo-selective. These differences suggest that the processes of formation of the activated states are different for the two azurins. On the other hand, such a difference has not been observed for az-1 and az-2 with respect to the reduction reactions promoted by both azurins Cu^II by the Ru^II complexes within the experimental error. This suggests that the neutrality of the Ru complexes is important for precise molecular recognition of azurins. His117 has been proposed as the electron transfer site. The local structures in the vicinity of the His117 side chain in the two azurins, are essentially identical with the exception of the 43rd residue, Val43 and Ala43 for az-1 and az-2, respectively. Electron transfer reactions between Ru^III complexes and a mutant azurin, V43A-az-1, were also carried out. Interestingly, the activation parameters estimated were very similar to those of az-2, indicating that the 43rd residue acts as the electron transfer site in azurins and provides rationalization for the different mechanisms of az-1 and az-2 in redox reactions.