Spectroscopic studies of metal binding and metal selectivity in Bacillus subtilis BSco, a Homologue of the Yeast Mitochondrial Protein Sco1p

Sco1 is a mitochondrial membrane protein involved in the assembly of the CuA site of cytochrome c oxidase. The Bacillus subtilis genome contains a homologue of yeast Sco1, YpmQ (hereafter termed BSco), deletion of which leads to a phenotype lacking in caa3 (CuA-containing) oxidase activity but expressing normal levels of aa3 (quinol) oxidase activity. Here, we report the characterization of the metal binding site of BSco in its Cu(I)-, Cu(II)-, Zn(II)-, and Ni(II)-bound forms. Apo BSco was found to bind Cu(II), Zn(II), and Ni(II) at a 1:1 protein/metal ratio. The Cu(I) protein could be prepared by either dithionite reduction of the Cu(II) derivative or by reconstitution of the apo protein with Cu(I). X-ray absorption (XAS) spectroscopy showed that Cu(I) was coordinated by two cysteines at 2.22 +/- 0.01 A and by a weakly bound low-Z scatterer at 1.95 +/- 0.03 A. The Cu(II) derivative was reddish-orange and exhibited a strong type-2 thiolate to Cu(II) transition around 350 nm. Multifrequency electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and electron spin-echo envelope modulation (ESEEM) studies on the Cu(II) derivative provided evidence of one strongly coupled histidine residue, at least one strongly coupled cysteine, and coupling to an exchangeable proton. XAS spectroscopy indicated two cysteine ligands at 2.21 A and two O/N donor ligands at 1.95 A, at least one of which is derived from a coordinated histidine. The Zn(II) and Ni(II) derivatives were 4-coordinate with MS2N(His)X coordination. These results provide evidence that a copper chaperone can engage in redox chemistry at the metal center and may suggest interesting redox-based mechanisms for metalation of the mixed-valence CuA center of cytochrome c oxidase.     

Predetermined chirality at metal centers of various coordination geometries: a chiral cleft ligand for tetrahedral (T-4), square-planar (SP-4), trigonal-bipyramidal (TB-5), square-pyramidal (SPY-5), and octahedral (OC-6) complexes

Two tetradentate bispinene-bipyridine type ligands, each with six stereogenic carbon centers, were synthesized from (-)-alpha-pinene. Their ability to predetermine chiral configurations at metal centers was studied. The two diastereoisomers, L1 and L2, differ in their absolute configuration at the bridgehead position. These ligands form metal complexes with Ag(I), Pd(II), Zn(II), Cu(II), and Cd(II), with coordination numbers four, five, and six and with complete control of chirality at the metal centers. Using L1 rather than L2 leads to complexes of inverted absolute configuration at the metal centers. These diastereomeric coordination species can be obtained either as separate compounds or, in some cases, as solids containing them in a 1:1 ratio. Ligands L1 and L2 thus show that the pinene-bipyridines are versatile molecules for the formation of metal complexes with predetermined chirality. In all cases, absolute configurations were determined in the solid state by X-ray diffraction methods and in solution by CD spectroscopy. The sign of exciton couplets from the pi-pi* transitions always agrees with the expectations for a given local configuration at the metal center. The five-coordinate, inherently chiral species of Zn(II) and Cu(II) described in this article are the first examples of trigonal-bipyramidal metal complexes with predetermined absolute configuration containing topologically linear ligands.     

Metal ion detection using a fluorogenic ‘click’ reaction

A fluorogenic Cu(I)-catalyzed azide-alkyne cycloaddition reaction (CuAAC) of 3-azido-7-hydroxycoumarin has been used to detect metal ions in solution. The formation of a highly fluorescent triazole product signals the presence of Cu(I) or Cu(II) ions at micromolar concentrations. CuAAC can be modified by using an exogenous ligand like EDTA to detect and quantify Zn(II), Ca(II), and Cd(II) ions at micromolar concentrations by an allosteric mechanism. The increase in the formation of the triazole product is regulated by the release of Cu(II) from the Cu(II)-EDTA complex by the addition of a second metal ion, the allosteric effector.     

Identification of a novel high affinity copper binding site in the APP(145-155) fragment of amyloid precursor protein

The copper(II) binding features of the APP(145-155) and APP(145-157) fragments of the amyloid precursor protein, Ac-Glu-Thr-His-Leu-His-Trp-His-Thr-Val-Ala-Lys-NH2 and Ac-Glu-Thr-His-Leu-His-Trp-His-Thr-Val-Ala-Lys-Glu-Thr-NH2 were studied by NMR spectroscopy and NMR findings were supported by UV-vis, CD and EPR spectra. Potentiometric measurements were performed only for the more soluble Ac-Glu-Thr-His-Leu-His-Trp-His-Thr-Val-Ala-Lys-Glu-Thr-NH2 peptide fragment. The following was shown: (i) the imidazole rings of all the three His residues are involved in metal coordination; (ii) metal binding induces ionisation of Leu-148 and His-149 amide nitrogens that complete the donor set to copper(II) in the species dominant at neutral pH; (iii) the unusual coordination scheme of the His-Xxx-His-Xxx-His consensus sequence justifies the high specificity for Cu(II) when compared to SOD-like or albumin-like peptides or even in amyloid Abeta fragments. The present findings may represent the key for interpreting the observed requirement of His residues conservation for the redox cycling between Cu(II) and Cu(I) by soluble APP.     

Synthesis and characterization of a new Schiff base derived from 2,3-diaminopyridine and 5-methoxysalicylaldehyde and its Ni(II), Cu(II) and Zn(II) complexes. Electrochemical and electrocatalytical studies

We describe the synthesis and characterization of a new tetradentate Schiff base ligand obtained from 2,3-diaminopyridine and 5-methoxysalicylaldehyde. This ligand (H₂L) reacted with nickel(II), copper(II), and zinc(II) acetates to give complexes. The ligand and its metal complexes were characterized using analytical, spectral data (UV–vis, IR, and mass spectroscopy), and cyclic voltammetry (CV). The crystal structure of the copper complex was elucidated by X-ray diffraction studies. The electrochemical behavior of these compounds, using CV, revealed that metal centers were distinguished by their intrinsic redox systems, e.g. Ni(II)/Ni(I), Cu(II)/Cu(I), and Zn(II)/Zn(I). Moreover, the electrocatalytic reactions of Ni(II) and Cu(II) complexes catalyze the oxidation of methanol and benzylic alcohol.     

Synthesis of [5]rotaxanes containing bi- and tridentate coordination sites in the axis

A new example of a linear [5]rotaxane has been synthesized by using the traditional "gathering-and-threading" approach but based on an unusual axle incorporating a symmetrical bis(bidentate) chelating fragment built on a 4,7-phenanthroline core. The stoppering reaction is particularly noteworthy since, instead of using a trivial bulky stopper as precursor to the blocking group, two semistoppered copper-complexed [2]pseudorotaxanes (namely [2]semirotaxanes) are used, which leads to the desired [5]rotaxane in good yield. The efficiency of the method relies on the use of "click" chemistry, with its very mild conditions, and on the protection by a transition-metal (copper(I)) of the various coordinating groups present in the fragments to be interconnected (terpy and bidentate chelating groups), thus inhibiting potential detrimental side reactions during the copper-catalyzed stoppering reaction. Since the external fragments and the central core of the system contain tri- and bidentate chelating units, respectively, the axle of the final [5]rotaxane incorporates two types of coordinating units: two external terpy groups (terpy: 2,2':6',2'-terpyridine) and two central bidentate ligands. Such a situation enables the system to tidy two different metals centers, and to localize them in a priori well-defined positions. This is what was observed when mixing the free ligand with a mixture of Zn(2+) and Li(+) : the zinc(II) ions were unambiguously shown to occupy the external sites, whereas the Li(+) cations were found in the central part of the [5]rotaxane. An X-ray diffraction study carried out on a [3]pseudorotaxane, the axis of which is similar to the central part of the [5]rotaxane axle, demonstrates that Zn(2+) is clearly five-coordinate, the fifth ligand being a counterion, even when the coordination site of the pseudorotaxane is designed for four-coordinate metals, which is in marked contrast with copper(I) or Li(+) .     

Formation of a copper(I) catenate on an électrode surface via S-Au interactions

A bis-thiol derivative of 2,9-diaryi-1,10-phenanthroline was threaded through a coordinating macrocycle using Cu(I) as a templating agent. This rotaxane adsorbs onto a gold surface, leading to a novel type of catenate, where gold atoms are incorporated within one of the two interlocking rings. The adsorption procedure was monitored by cyclic voltammetry.     

Calix[4]pyrrole Schiff base macrocycles: novel binucleating ligands for Cu(I) and Cu(II)

New bimetallic copper(I) and copper(II) complexes of dipyrromethane-derived Schiff base macrocycles are reported. Two different structural motifs were identified, providing support for the notion that ligands of this type can support a variety of coordination modes. In the case of the Cu(I) complexes, the metal centers were found to have a distorted tetrahedral geometry and be coordinated to two imine nitrogens on each side of the ligand, with the exact structure depending on the choice of Schiff base macrocycle. In contrast to what is seen for Cu(I), with Cu(II) as the coordinated cation the Cu(II) metal centers assumed distorted square planar geometries, and both pyrrole N-Cu and imine N-Cu interactions were confirmed by single-crystal X-ray diffraction analysis. This structural analysis revealed a copper-copper distance of 3.47 A, while SQUID magnetic susceptibility data provided evidence for antiferromagnetic coupling between the two metal centers.     

Tris(triazolyl) calix[6]arene-based zinc and copper funnel complexes: imidazole-like or pyridine-like? A comparative study

Huisgen dipolar cycloaddition leads straightforwardly to new funnel complexes based on the calix[6]arene macrocycle bearing three functionalized triazoles as coordinating units at the small rim. Coordination to Zn(II) and Cu(I) cations was studied using (1)H NMR and IR spectroscopies and cyclic voltammetry. The nature of the substituents on the triazole ring affects the behavior of the ligands and their coordinating ability and controls the host-guest properties of the metal receptors for exogenous substrates. Depending on their substitution pattern but also on the metal ion and the guest ligand, the triazole-based systems behave either imidazole-like or pyridine-like. The ease of preparation and the versatility of 1,4-disubstituted-1,2,3-triazoles with tunable steric and electronic properties make them promising candidates for further applications from biology to materials.     

Crystal chemistry of the 4,4'-dimethyl-2,2'bipyridine/copper bromide system

The reaction of 4,4'-dimethyl-2,2-bipyridine (henceforth dmbp) with copper(I) and/or copper(II) bromide under a wide variety of conditions has led to the isolation of 10 different crystalline materials. These include one Cu(I) salt, [Cu(dmbp)(2)]Br (a distorted tetrahedral Cu species and a lattice Br(-) ion); two mixed valence Cu(I,II) compounds, [Cu(dmbp)(2)Br][CuBr(2)] (discrete 5-coordinated Cu(II) and linear Cu(I) species) and Cu(dmbp)(2)BrCu(2)Br(3) (linked 5-coordinate Cu(II) and trigonal planar Cu(I) species); and seven Cu(II) compounds, (dmbp)CuBr(2) (stacked planar monomers), [(dmbp)CuBr(2)](2)(five coordinate bibridged dimers), (dmbp)Cu(2)Br(4) (stacked planar bibridged dimers), (dmbp)CuBr(2)(DMSO) (five coordinate monomers), [Cu(dmbp)(2)Br]OH.5(1)/(2)H(2)O and [Cu(dmbp)(2)Br](Br/OH).5(1)/(2)H(2)O (five coordinate monomers), and (dmbpH(2))CuBr(4).H(2)O (distorted tetrahedral monomers). The crystal structure determinations of these materials are reported. A common thread in their structural chemistry is the supramolecular architecture developed through interdigitation of the dmbp rings on neighboring molecular species. The interdigitation leads to layer structures in many of the materials. The distances between the interdigitated dmbp rings are in the range 3.4-3.7 A. The Cu(dmbp)(2)Br(+) species exhibits an exceptionally large distortion from tetrahedral geometry due to deviation of the dihedral angle between the mean planes of the Cu(dmbp) fragments from 90 degrees. The Cu(dmbp)(2)Br(+) cations have distorted trigonal bipyramidal geometry, the Br(-) ion occupying an equatorial position. The length of the Cu-Br bond in the Cu(dmbp)(2)Br(+) species is correlated with the change in dihedral angle between the planes of the two dmbp ligands. The mono-dmbp complexes show a greater variation in coordination geometry for the Cu(II) species, including distorted trigonal bipyramidal and augmented square planar 4 + 1 and 4 + 2 coordination.     

Calculation of geometric parameters of macrocyclic metal chelates formed by template synthesis in tertiary systems M(II) ion-ethanedithioamide-formaldehyde-ammonia

The geometric parameters of macrotricyclic Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II) complexes with 2,8-dithio-3,5,7-triazanonanedithioamide-1,9 with the (N,N,S,S) coordination of the chelant donor centers (formed by template synthesis in the M(II)-ethanedithioamide-formaldehyde-ammonia system) have been calculated by the hybrid B3LYP density functional theory method with the use of the 6-31G(d) basis set and the Gaussian 98 program package. The bond lengths and bond angles in the complexes with the MN₂S₂ coordination core have been reported. Calculations demonstrated that in none of the complexes are the five-membered chelate rings planar and that these rings in the Zn(II) complex are significantly different. For all M(II) ions under consideration, an additional six-membered chelate ring resulting from template cross-linking is turned at a rather large angle to the two five-membered rings and this ring itself is nonplanar.     

Chelating Polymers. II. Amino Acetic Acid Chelating Polymers Derived from Hydroxyarylamino Acetic Acids

The prototype dimeric chelating compounds 3,3′-methylenebis-[N(4-hydroxyphenyl) iminodiacetic acid] and 3,3′-methylenebis-[N(4-methoxyphenyl) iminodiacetic acid] were synthesized and characterized by composition analysis, infrared spectroscopy, and potentiometric titration data. Their chelating characteristics with Cu(II), Ni(II), Co(II), and Zn(l1) ions were established by composition analysis and comparative infrared spectroscopy.

The results suggest chelation and structures of the usual iminodiacetatometal complex type for the Ni(II), Co(II), and Zn(I1) compounds of 3,3′methylenebis-[N-(4-hydroxyphenyl) iminodiacetic acid] and for the Cu(I1, Ni(II), and Co(I1) compounds of 3,3′-methylenebis-[N-(4-methoxyphenyl) iminodiacetic acid]. The composition analysis of each of these compounds indicates 1:2:2 mole ratios of ligand to metal ion to water. Square planar structures are proposed wherein the two iminodiacetatometal chelate moieties are essentially independent of each other.

The composition analysis of the Cu(II) compound of 3,3′-methylenebis-[N-(4-hydroxyphenyl) iminodiacetic acid] indicates a 1:2:1 mole ratio of ligand to metal ion to water. Structures are proposed in which some of the carboxylate groups are bridges between two metal coordination centers.

The composition analysis of the Zn(II) compound of 3,3′-methylenebis-[N-(4-methoxyphenyl) iminodiacetic acid] indicates a 1:1 mole ratio of ligand to metal ion. A structure is proposed in which both nitrogen atoms and two of the four carboxylate groups of the dimeric ligand are coordinated to the same metal ion.

A preliminary investigation was made of oligomeric compounds analogous to the prototype dimeric compounds.     

Theoretical study of the two-photon absorption properties of octupolar complexes with Cu(I), Zn(II) and Al(III) as centers and bis-cinnamaldimine as ligands

The equilibrium geometries, electronic structures, one- and two-photon absorption (TPA) properties of a series of octupolar complexes with the Cu(I), Zn(II) and Al(III) as coordinate centers and the bis-cinnamaldimine as ligands have been studied using the B3LYP/6-31G(d) and ZINDO-SOS methods. Compared with the dipolar metal complexes, all the octupolar metal complexes (including tetrahedral and octahedral complexes) have relatively large TPA cross-sections, indicating that building octupolar metal complex is an effective route to design of promising TPA material. Lewis acidity of metal center and molecular symmetry are two important factors for enhancement of TPA cross-section of metal complex. Due to the stronger Lewis acidity of Zn(II) than Cu(I) as well as Al(III) than Zn(II), the tetrahedral Zn(II) complex exhibits a TPA cross-section larger than that of the tetrahedral Cu(I) complex, the maximum TPA position of the octahedral Al(III) complex is red-shifted relative to the octahedral Zn(II) complex, and at the same time, the octahedral Al(III) complex has a large TPA cross-section. Compared with the tetrahedral complexes, the TPA cross-sections of the octahedral complexes are enhanced due to the increased number of ligands.     

Synthesis and characterization of mixed ligand complexes of copper(II), nickel(II), cobalt(II) and zinc(II) with glycine and uracil or 2-thiouracil

Mixed ligand complexes of Cu(II), Ni(II), Co(II) and Zn(II) formed with glycine and uracil or 2-thiouracil have been synthesized and characterized by elemental analysis, conductance, spectral (IR and electronic spectra) and magnetochemical measurements. Results show that glycine is bidentate in all cases; uracil behaves as a bidentate ligand in Cu(II) complex, coordinating through its one carbonyl oxygen and nitrogen, whereas in other cases it is only monodentate, coordinating only through nitrogen. With thiouracil, coordination occurs from carbonyl oxygen and one nitrogen in Cu(II) and Ni(II) complexes, but in the Co(II) complex coordination occurs from thionyl sulphur and nitrogen. In the Zn(II) complex it shows tridentate behaviour, coordinating through oxygen, sulphur and one nitrogen. Mixed Cu(II), Co(II) and Zn(II) complexes of uracil and of Ni(II) and Zn(II) with thiouracil are octahedral, whereas the mixed Ni(II) complex with uracil shows distorted tetrahedral geometry, and the mixed Co(II)-thiouracil complex is square planar. The mixed Cu(II)-thiouracil complex has a binuclear structure, with square planar arrangement around each copper atom.     

Modulating the M-M distance in dinuclear complexes. New ligand with a 2,2'-Biphenol fragment as key unit: synthesis, coordination behavior, and crystal structures of Cu(II) and Zn(II) dinuclear complexes

The synthesis and characterization of the new polyamino-phenolic ligand 3,3'-bis[N,N-bis(2-aminoethyl)aminomethyl]-2,2'-dihydroxybiphenyl (L) are reported. L contains two diethylenetriamine units linked by a 1,1'-bis(2-phenol) group (BPH) on the central nitrogen atom which allows two separate binding amino subunits in a noncyclic ligand. The basicity and binding properties of L toward Cu(II) and Zn(II) were determined by means of potentiometric measurements in aqueous solution (298.1 +/- 0.1 K, I = 0.15 mol dm-3). L behaves as a pentaprotic base and as a monoprotic acid under the experimental conditions used, yielding the H5L5+ or H-1L- species, respectively. L forms both mono- and dinuclear species with both metal ions investigated; the dinuclear species are largely prevalent in aqueous solution with a L/M(II) molar ratio of 1:2 at pH higher than 7. L shows different behavior in Cu(II) and Zn(II) binding, affecting the dinuclear species formed and the distance between the two coordinated metal ions, which is greater in the Zn(II) than in the Cu(II) dinuclear species. This difference can be attributed to the different degree of protonation of BPH which influences the angle between the phenyl rings in the two systems. In this way, it is possible to modulate the M(II)-M(II) distance by the choice M(II) and to space the two M(II) farther away than was possible with the previously synthesized ligands. L does not saturate the coordination sphere of the coordinated M(II) ions in the dinuclear species, and thus, these latter species are prone to add guests. 1H and 13C NMR experiments carried out in aqueous solution, as well as the crystal structures of the dinuclear Cu(II) and Zn(II) species formed in aqueous solution, aided in elucidating the involvement of L and BPH in Zn(II) and Cu(II) stabilization.     

Synthesis of mono- and di-[12]aneN3 ligands and study on the catalytic cleavage of RNA model 2-hydroxypropyl-p-nitrophenyl phosphate with their metal complexes

A series of mono- and di-[12]aneN(3) ligands 1-6, which contain different substituents on the coordinating backbone, different linkers between two [12]aneN(3) units and different N-methylation on the [12]aneN(3) units, have been synthesized and fully characterized. The catalytic activities of their metal complexes on the cleavage of RNA model phosphate 2-hydroxypropyl-p-nitrophenyl phosphate (HPNPP) varied with the structures of the ligands and metal ions. Click reactions afforded an efficient method to prepare a series of [12]aneN(3) ligands, however, the incorporation of triazole moieties reduced the catalytic activities due to their coordination with metal ions and the strong inhibition from the triflate counter ion. Dinuclear zinc(II) complexes containing an m-xylyl bridge showed higher catalytic activities with synergistic effects up to 700-fold. Copper(II) complexes with the ligands without triazole moieties proved to be highly reactive and showed strong cooperativity between the two copper(II) ions. In terms of k(2), dinuclear complexes Zn(2)-3b, Zn(2)-3d, Zn(2)-4b, and Cu(2)-4b afforded activities of 7.9 × 10(5), 3.9 × 10(4), 9.0 × 10(4), and 8.1 × 10(4)-fold higher than that of methoxide. The ortho arrangement of the two [12]aneN(3) units and the presence of 5- or 2-positioned substituents in the benzene ring as well as N-methylation of [12]aneN(3) units greatly reduced the catalytic activities due to the steric effects. These results clearly indicate that the structures of the linker between two [12]aneN(3) units play very important role in their catalytic synergistic effects.     

An unusually flexible expanded hexaamine cage and its Cu(II) complexes: variable coordination modes and incomplete encapsulation

The bicyclic hexaamine "cage" ligand Me(8)tricosaneN(6) (1,5,5,9,13,13,20,20-octamethyl-3,7,11,15,18,22-hexaazabicyclo[7.7.7]tricosane) is capable of encapsulating octahedral metal ions, yet its expanded cavity allows the complexed metal to adopt a variety of geometries comprising either hexadentate or pentadentate coordination of the ligand. When complexed to Cu(II) the lability of the metal results in a dynamic equilibrium in solution between hexadentate- and pentadentate-coordinated complexes of Me(8)tricosaneN(6). Both [Cu(Me(8)tricosaneN(6))](ClO(4))(2) (6-coordinate) and [Cu(Me(8)tricosaneN(6))](S(2)O(6)) (5-coordinate) have been characterized structurally. In weak acid (pH 1) a singly protonated complex [Cu(HMe(8)tricosaneN(6))](3+) has been isolated that finds the ligand binding as a pentadentate with the uncoordinated amine being protonated. vis-NIR and electron paramagnetic resonance (EPR) spectroscopy show that the predominant solution structure of [Cu(Me(8)tricosaneN(6))](2+) at neutral pH comprises a five-coordinate, square pyramidal complex. Cyclic voltammetry of the square pyramidal [Cu(Me(8)tricosaneN(6))](2+) complex reveals a reversible Cu(II/I) couple. All of these structural, spectroscopic, and electrochemical features contrast with the smaller cavity and well studied "sarcophagine" (sar, 3,6,10,13,16,19-hexaazabicyclo[6.6.6]eicosane) Cu(II) complexes which are invariably hexadentate coordinated in neutral solution and cannot stabilize a Cu(I) form.     

Messages in molecules: ligand/cation coding and self-recognition in a constitutionally dynamic system of heterometallic double helicates

Double helicates are known to exhibit self-recognition characteristics determined by the coordination geometry of the metal involved as well as by the topicity of the ligands. Combining tridentate (terpyridine, T) or bidentate (bipyridine, B) subunits in a tritopic strand affords a set of ligands able to assemble by pairs to form double helicates, homo- or heterostranded, homo- or heterotopic, depending on the coordination properties of the metals involved. The four ligand strands, BBB, TTT, BBT, and TBT form constitutionally dynamic sets of double helicates with the metal ions Cu(I), Cu(II), and Zn(II); these helicates correspond to the correct coding of the BB, BT, and TT pairs for tetra-, penta-, and hexacoordinate Cu(I), Cu(II), and Zn(II) cations, respectively.     

A series of tripodal cysteine derivatives as water-soluble chelators that are highly selective for copper(I)

A series of tripodal ligands derived from nitrilotriacetic acid and extended by three converging, metal-binding, cysteine chains was synthesised. Their ability to bind soft metal ions thanks to their three thiolate functions was investigated by means of complementary analytical and spectroscopic methods. Three ligands that differ by the nature of the carbonyl group next to the coordinating thiolate functions were studied: L(1) (ester), L(2) (amide) and L(3) (carboxylate). The negatively charged derivative L(3), which bears three carboxylate functions close to the metal binding site, gives polynuclear copper(I) complexes of low stability. In contrast, the ester and amide derivatives L(1) and L(2) are efficient Cu(I) chelators with very high affinities, close to that reported for the metal-sequestering metallothioneins (log K≈19). Interestingly, these two ligands form mononuclear copper complexes with a unique MS(3) coordination in water solution. An intramolecular hydrogen-bond network involving the amide functions in the upper cavity of the tripodal ligands stabilises these mononuclear complexes and was evidenced by the very low chemical-shift temperature coefficient of the secondary amide protons. Moreover, L(1) and L(2) display large selectivities for the targeted metal ion that is, Cu(I), with respect to bioavailable Zn(II). Therefore the two sulfur-based tripods L(1) and L(2) are of potential interest for intracellular copper detoxication in vivo, without altering the homeostasis of the essential metal ion Zn(II).     

Coordination networks from a bifunctional molecule containing carboxyl and thioether groups

The bifunctional molecule tetrakis(methylthio)-1,4-benzenedicarboxylic acid (TMBD) interacts with the increasingly harder metal ions of Cu (I), Cd (II), and Zn (II) to form the coordination networks of Cu 2TMBD, CdTMBD, and Zn 4O(H 2O) 3(TMBD) 3, where the carboxyl group consistently bonds to metal ions, while the softer methylthio group binds with preference to the softer metal ions (i.e., chelation to Cu (+), single-fold coordination to Cd (2+), and nonbonding to Zn (2+)). Diffuse-reflectance spectra show that the metal-thioether interaction is associated with smaller electronic band gaps of the solid-state networks.