Ligand Coupling Research Articles

My life with affinity

I was born in 1935 in Warsaw, Poland, to a rabbinical family. My early childhood occurred during World War II in German-occupied Poland and then in Russia. My father was killed in the concentration camp of Flossenberg; my mother, my sister and I survived and emigrated from Germany to Israel in 1949. In Israel I finished my primary school and high school. I completed my B.Sc. in chemistry and physics at Bar-Ilan University and my Ph.D. in biochemistry at the Weizmann Institute of Science.

A New Synthesis of Triarylbismuthanes via Directed Ligand Coupling of Oxazoline-Substituted Tetraarylbismuthonium Salts: Synthesis of Polystyrenes Bearing the Diarylbismuthino Group

Triarylbismuthanes bearing a vinyl, formyl, or acetyl group have been prepared in good yield via the directed ligand coupling of oxazoline-substituted tetraarylbismuthonium salts. Radical polymerization of the resulting diaryl(4-vinylphenyl)bismuthanes produced bismuth-containing polystyrenes soluble in organic solvents.

Evidence for a Carbon−Carbon Coupling Reaction To Proceed through a Planar-Tetracoordinate Carbon Intermediate

Treatment of bis(propynyl)zirconocene with B(C6F5)3 results in a linear C−C coupling of the alkynyl ligands to form the zwitterionic complex 3. Its treatment with excess RC⋮N yields an organometallic methylenecyclopropene derivative (6). This reaction topologically requires a very endothermic substituted butenyne to methylenecyclopropene cyclization to become energetically feasible by suitable stabilization effects. A DFT study has revealed that the 3 → 6 conversion is probably triggered by nitrile addition to the metal with formation of a planar-tetracoordinate carbon intermediate, featuring coordination of the three-membered carbocycle through one of its carbon−carbon σ bonds.

Mechanism of Reppe's nickel-catalyzed ethyne tetramerization to cyclooctatetraene: a DFT study.

In this B3 LYP model study, homoleptic nickel(0) ethyne complexes have been predicted as the catalyst resting state for the title reaction. Ethyne ligand coupling of Ni(C(2)H(2))(3) yields monoethyne nickelacyclopentadiene in the rate-determining step. Ethyne coordination is followed by insertion of an ethyne ligand into the Ni--C sigma bond. A highly strained monoethyne trans-nickelacycloheptatriene is formed. This trans intermediate is unable to reductively eliminate benzene without prior isomerization to a cis-structure. Instead, it rapidly collapses to a nickelacyclononatetraene. Ethyne coordination induces reductive elimination to the cyclooctatetraene complex Ni(eta(2)-C(2)H(2))(eta(2)-C(8)H(8)), followed by facile ligand exchange. Other ethyne coupling pathways have been computed to be less favored. The cyclooctatetraene ligand binds significantly weaker to nickel(0) than ethyne, both for mononuclear, and for dinuclear species. For this reason, C--C bond formation steps at Ni(2)(micro-cot) fragments have been predicted to feature prohibitively high overall reaction barriers.

Interligand Electron Transfer Dynamics in [Os(bpy)3]2+: Exploring the Excited State Potential Surfaces with Femtosecond Spectroscopy

Femtosecond transient absorption studies of [Os(bpy)3]2+ have been performed to gain new insights into the excited-state dynamics. Experiments were performed for a series of excitation wavelengths throughout both the singlet and triplet metal-to-ligand charge transfer (MLCT) bands. The dynamics are probed via the π−π* transition on the bpy- anion at 370 nm. Time scales for interligand electron transfer (ILET) and vibrational cooling are extracted from the amplitude and anisotropy of the excited state absorptions. The ILET rate has been measured as a function of temperature and, from these data, estimates for the electron transfer activation energy and ligand−ligand coupling are made. Our data also provide insight into the vibrational cooling dynamics within these large complexes, suggesting that vibrational cooling occurs only when exciting into the singlet MLCT state, after intersystem crossing. It appears that photoexcitation within the triplet MLCT absorption produces molecules that are vibrationally cold, even for excitation at the blue edge of the absorption band.

Methacryloylamidocysteine functionalized poly(2-hydroxyethyl methacrylate) beads and its design as a metal-chelate affinity support for human serum albumin adsorption

Metal chelate affinity chromatography is a widely used technique for separation of proteins. An expensive and time consuming step in this process is coupling of a chelating ligand to the chromatographic support. This chelating ligand coordinates metal ions, which in turn bind proteins. The toxicity of chemicals required for coupling and their slow release during the separation process are of considerable concern. This is an important issue in the context of purification of proteins which are used in pharmaceutical purposes. In this study, a simpler metal-chelate support design was described. It was shown that poly(hydroxyethyl methacrylate-methacryloylamidocysteine) [poly(HEMA–MAC)] beads (formed by chelating MAC with Cu 2+ directly) can be used for metal-chelate affinity separation. MAC was synthesized by using methacryloyl chloride and cysteine. Spherical beads with an average size of 140–180 μm were obtained by the radical suspension polymerization of HEMA and MAC conducted in an aqueous dispersion medium. poly(HEMA–MAC) beads have a specific surface area of 18.9 m 2/g. poly(HEMA–MAC) beads were characterized by swelling studies, FTIR, SEM and elemental analysis. The poly(HEMA–MAC) beads with a swelling ratio of 72%, and containing 3.9 mmol MAC/g were used in the adsorption of human serum albumin (HSA) from aqueous solutions. The adsorption of HSA was 22.8 mg/g. Cu 2+ chelation increased the HSA adsorption (46.6 mg/g). The maximum HSA adsorption was observed at pH 6.0. The adsorption phenomena appeared to follow a typical Langmuir isotherm. The maximum capacity ( q m) of the Cu 2+-chelated beads for HSA adsorption (193 mg/g) was greater than that of the poly(HEMA–MAC) beads (37 mg/g). The dissociation constant ( k d) values was found to be 4.6×10 −5 M with poly(HEMA–MAC) beads, and was found to be 0.22×10 −5 M with Cu 2+-chelated beads. Elution of HSA was obtained using 0.1 M Tris/HCl buffer containing 0.5 M NaSCN. High elution efficiencies (up to 100% of the adsorbed HSA) were observed. It was possible to reuse Cu 2+-chelated beads without significant decreases in the adsorption capacities.

Synthesis and Structure of the First Homoleptic 1-Aza-1,3-diene Titanium Complex: A Tightrope Walk between Ligand Coordination and Ligand Coupling

The reaction of TiCl4(THF)2 or ZrCl4(THF)2 with Mg in the presence of 1-aza-1,3-dienes (1) has been investigated. Reduction of TiCl4(THF)2 with 2 equiv of Mg in the presence of 2 equiv of (R1)NCHC(R2)CH(Ph) yields the first homoleptic 1-aza-1,3-diene complex, [Ti{N(R1)CHC(R2)CH(Ph)}2] (7), but only if the 1-aza-1,3-diene is substituted by a sterically demanding group at the nitrogen atom (1b: R1 = C6H3-2,6-iPr2, R2 = H). X-ray crystallography indicates a σ2,π coordination of the heterodienes, which are reduced to 1-azabut-2-ene-1,4-diyl dianions to form folded 1-aza-2-titanacyclopent-4-ene rings. As a byproduct, the bicyclic complex [{N(R1)CHC(R2)CH(Ph)}Ti{N(R1)CHC(R2)CH(Ph)CH{C(R2)CH(Ph)}N(R1)] (8) was isolated, which is believed to arise from CN insertion of 1b into the Ti−C bond of the intermediate [{N(R1)CHC(R2)CH(Ph)}TiCl2], followed by the coordination of a further 1-aza-1,3-diene ligand. The analogous bicyclic zirconium complex [{N(R1)CHC(R2)CH(Ph)}Zr{N(R1)CHC(R2)CH(Ph)CH{C(R2)CH(Ph)}N(R1)] (6: R1 = cyclo-C6H11, R2 = Me) is the only available product when the reduction of ZrCl4(THF)2 is performed in the presence of the 1-aza-1,3-diene (cyclo-C6H11)NCHC(Me)CHPh (1a), which is substituted by a sterically less demanding group at the terminal nitrogen atom. Addition of 2 equiv of benzophenone to 7 affords the titanium complex [Ti{OCPh2CH(Ph)C(R2)CHN(R1)}2] (9). The solid-state structure of 9 shows the titanium in a distorted-tetrahedral environment, being the center of two folded oxazatitanacycloheptene rings.

Syntheses and Structures of Tungsteno-(Diphenylphosphino)benzaldehyde Complexes Bearing π-Bonded Aldehyde Groups

Reaction of the bidentate ligand Ph2P(o-C6H4)C(O)H (abbreviated as PCHO) with W(CO)3(η3-(MeNCH2)3) at room temperature affords W(CO)3(η1-PCHO)(η3-PCHO) (1), which subsequently loses a carbonyl ligand to give W(CO)2(η3-PCHO)2 (2). Further treatment of 2 with PCHO in refluxing benzene results in carbon−carbon coupling of the PCHO ligands to produce W(CO)(η3-PCHO)(η3-(PCHO)2) (3). The structures of 1−3 have been determined by an X-ray diffraction study. The PCHO ligands in these compounds act as chelating phosphine−aldehydes with the aldehyde groups coordinating in a π fashion.

Mechanism of the ruthenium-catalyzed formation of pyrane-2-one and their sulfur and selen analogs from acetylene and CX2(X = O, S, Se): a theoretical study

Complete catalytic cycles for the (actual or potential) cycloaddition of acetylene with CX2 (X = O, S, Se), mediated by CpRu(COD)Cl as the precatalyst, are proposed on the basis of DFT/B3LYP calculations. The common initial step is the replacement of labile COD by two molecules of acetylene to afford a bisacetylene complex. Oxidative coupling of the acetylene ligands results in the formation a coordinatively saturated metallacyclopentatriene complex. This metallacycle adds the CX bond of CX2 directly to the RuC bond in a concerted fashion, forming an unusual bicyclic carbene intermediate. The activation energies for this addition are 14.2, 15.3 and 30.0 kcal mol−1 for X = O, S and Se, respectively, thus rendering the CSe2 addition the most difficult. In a subsequent reductive elimination a coordinatively unsaturated metallaheteronorbornene intermediate is formed, which eventually rearranges into RuCpCl complexes bearing pyrane-2-one, thiopyrane-2-thione and selenopyrane-2-selenone ligands coordinated in an η4 fashion. The energy barrier for the reductive elimination step decreases in the order O > S > Se, being 28.1, 17.7 and 14.6 kcal mol−1, respectively. Overall, the reaction with CS2 is thus the most favorable. Completion of the cycles is achieved by an exothermic displacement of the respective heterocyclic product by two acetylene molecules, regenerating the bisacetylene complex.

Selective Synthetic Transformations Based on Vanadium-Mediated Redox Reactions

Low-valent vanadium-, or titanium-catalyzed reduction reactions including dehalogenation, pinacol coupling, and the related radical reaction have been developed by constructing a multi-component redox system in combination with a co-reductant and an additive, which serve for the recycling and liberation of the catalyst. High stereoselectivity is attained in these catalytic transformations. Oxovanadium (V) compounds, which are evaluated as Lewis acids with oxidation capability, induce one-electron oxidative desilylation of organosilicon compounds such as silyl enol ethers, allylic silanes, and benzylic silanes, allowing chemoselective-catalyzed with the less oxidizable organosilicon compounds. The oxovanadium (V) -induced or catalyzed oxidation of benzylsilane and benzyltin compounds is carried out under oxygen atmosphere to afford aromatic aldehydes (ketones) and/or carboxylic acids. Oxidative ligand coupling of dialkylarylaluminums is achieved by treatment with oxovanadium (V) compounds, leading to the formation of the alkylarenes. The ate complexes derived from arylaluminums or 1-alkenylaluminums and 1-alkynyllithiums undergo facile oxidative ligand coupling. Similar oxidation is observed with organoborons and organozincs or their ate complexes. This method is applied to vicinal dialkylation with the diorganozincs or ate complexes at both the α and β positions of 2-cycloalkenones.

Evaluation of different linker regions for multimerization and coupling chemistry for immobilization of a proteinaceous affinity ligand.

Alkaline conditions are generally preferred for sanitization of chromatography media by cleaning-in-place (CIP) protocols in industrial biopharmaceutical processes. The use of such rigorous conditions places stringent demands on the stability of ligands intended for use in affinity chromatography. Here, we describe efforts to meet these requirements for a divalent proteinaceous human serum albumin (HSA) binding ligand, denoted ABD*dimer. The ABD*dimer ligand was constructed by genetic head-to-tail linkage of two copies of the ABD* moiety, which is a monovalent and alkali-stabilized variant of one of the serum albumin-binding motifs of streptococcal protein G. Dimerization was performed to investigate whether a higher HSA-binding capacity could be obtained by ligand multimerization. We also investigated the influence on alkaline stability and HSA-binding capacity of three variants (VDANS, VDADS and GGGSG) of the inter-domain linker. Biosensor binding studies showed that divalent ligands coupled using non-directed chemistry demonstrate an increased molar HSA-binding capacity compared with monovalent ligands. In contrast, equal molar binding capacities were observed for both types of ligands when using directed ligand coupling chemistry involving the introduction and recruitment of a unique C-terminal cysteine residue. Significantly higher molar binding capacities were also detected when using the directed coupling chemistry. These results were confirmed in affinity chromatography binding capacity experiments, using resins containing thiol-coupled ligands. Interestingly, column sanitization studies involving exposure to 0.1 M NaOH solution (pH 13) showed that of all the tested constructs, including the monovalent ligand, the divalent ligand construct containing the VDADS linker sequence was the most stable, retaining 95% of its binding capacity after 7 h of alkaline treatment.

Isolation from phage display libraries of lysine-deficient human epidermal growth factor variants for directional conjugation as targeting ligands.

Ligand-targeted anticancer therapeutics represent an opportunity for the selective and efficient delivery of drugs to tumours. The chemical coupling of ligands to drugs or drug carrier systems is, however, often hampered by the presence of multiple reactive groups within the ligand, for example, epsilon-NH(2) groups in lysine side chains. In this paper, we describe the isolation by phage display of human epidermal growth factor (EGF) variants without lysine and a reduced number of arginine residues. The selection on A431 carcinoma cells also revealed that R41 is indispensable for EGF binding activity as all EGF variants contained an arginine residue at this position. One EGF variant (EGFm1) with K28Q, R45S, K48S and R53S mutations was expressed in bacteria and showed an identical binding activity as wild-type EGF. EGFm1 could be labelled with fluorescein isothiocyanate demonstrating the accessibility of the N-terminal amino group for coupling reagents. Furthermore, coupling of EGFm1 to PEGylated liposomes resulted in target cell-specific binding and internalization of the liposomes. These human EGF variants should be advantageous for the generation of anticancer therapeutics targeting the EGF receptor, which is overexpressed by a wide variety of different tumours.

Synthesis and biological evaluation of 4-arylcoumarin analogues of combretastatins.

A series of A-ring polymethoxylated neoflavonoids was prepared by ligand coupling reactions involving either Suzuki or Stille reactions. Cytotoxicity studies indicated a potent activity against a CEM leukemia cell line for the compounds presenting a substitution pattern related to that of combretastatin A-4. The two compounds having a 3'-OH and a 4'-OCH(3) substituents on the 4-phenyl B-ring have no effect on human topoisomerases I and II but potently inhibit, in vitro, microtubule assembly. At the cell level, the active compounds were characterized as proapoptotic agents, but they can also trigger cell death via a nonapoptotic pathway.

The activation and transformations of acenaphthylene by osmium carbonyl cluster complexes

The reaction of Os 3(CO) 10(NCMe) 2 ( 1) with an excess of acenaphthylene at room temperature provided the complex Os 3(CO) 10(μ-H)(μ–η 2-C 12H 7) ( 2). Compound 2 contains a σ–π coordinated acenaphthyl ligand bridging an edge of the cluster. Compound 2 was converted to the complex Os 3(CO) 9(μ-H) 2(μ 3–η 2-C 12H 6) ( 3) when heated to reflux in a cyclohexane solution. Compound 3 contains a triply bridging acenaphthyne ligand. Compound 3 reacts with acenaphthylene again at 160 °C to yield four new cluster complexes: Os 4(CO) 12(μ 4–η 2:η 2-C 12H 6) ( 4); Os 2(CO) 6(μ–η 4-C 24H 12) ( 5); Os 3(CO) 9(μ-H)(μ 3–η 4-C 24H 13) ( 6); and Os 2(CO) 5(μ–η 4-C 24H 12)(η 2-C 12H 8) ( 7). All compounds were characterized crystallographically. Compound 4 is a butterfly cluster of four osmium atoms bridged by a single acenaphthyne ligand. Compounds 5 and 7 are dinuclear osmium clusters containing metallacycles formed by the coupling of two equivalents of acenaphthyne. Compound 6 is a triosmium cluster formed by the coupling of an acenaphthyne ligand to an acenapthyl group that is coordinated to the cluster through a combination of σ and π-bonding.

Reactivities, Properties and Structures

AbstractFor Abstract see ChemInform Abstract in Full Text.

Fragmentations of (E)- and (Z)- isomers of 2-methylbuten-1-yl(aryl)iodonium triflates: competing mechanisms for enol triflate formation

We examined fragmentation reactions of (E)- and (Z)-2-methylbuten-1-yl(aryl)iodonium triflates (aryl = C6H5-, 4-(CF3)C6H4, 3,5-(CF3)2C6H4-) to afford aryl idodides and six enol triflates. Four of these vinyl triflates involve alkyl migrations followed by triflate trapping of secondary vinyl cations whereas two do not involve migrations. Fragmentation rates in dry, neutral CDCl3 were determined as were the distributions of enol triflate products. The ratios of rate constants for the (E)-/(Z)- isomers ranged between 5.0 and 8.5 and, in all salts, the rearranged enol triflate derived from migration of the alkyl moiety trans- to the aryliodonio- nucleofuge was observed in the greatest quantities. These data indicate that the fragmentation rates are significantly determined by the migratory aptitude of the trans-β-alkyl substituent and departure of the aryliodonionucleofuge occurs by anchimeric assistance. The ratios of inverted “unrearranged” enol triflate products were greater for the (Z)-isomers of the iodonium salt precursors indicating that steric effects play a role and implies that these inverted, unrearranged products are derived from inplane (σ*) SN2 reaction. The presence of the remaining, retained, unrearranged enol triflate can be explained by a ligand coupling mechanism (π* SN2) and the fragmentation mechanism(s) do not require the intermediacy of a primary vinyl cation.

Surface-engineered nanoparticles for multiple ligand coupling

The design of surface-engineered nanoparticles for targeting to specific sites is a major challenge. To our knowledge, no study in the literature deals with ligand functionalization of biodegradable nanoparticles through biotin–avidin interactions. With the aim of conceiving small-sized nanoparticles which can be easily functionalized with a variety of ligands or mixtures thereof, biotinylated and PEGylated biotin-poly(ethylene glycol)-poly(ε-caprolactone) (B-PEG-PCL) copolymers were synthesized and used to prepare nanoparticles of around 100 nm. Avidin, followed by biotinylated wheat germ agglutinin as a model lectin, were coupled to their surface by taking advantage of the strong biotin–avidin complex formation. The cytotoxicity of the nanospheres towards Caco-2 cells in culture was negligible (more than 82% cell survival for nanoparticle concentrations up to 300 μg/well). The amount of radiolabeled poly(lactic acid) (PLA) or PEG-PLA nanoparticles associated with Caco-2 cells was only 0.7% and 1.5% of the amount added, respectively. This value was increased to 8.5% when a sufficient amount of lectin was bound to the PEG-PLA copolymer. After further studies, the biotin-PEG-coated nanoparticles could be helpful tools for studying the interaction between cells and functionalized nanoparticles with various surface characteristics (PEG layer density and thickness, ligand type and density).

Medicinal chemistry of plasmid DNA with peptide nucleic acids: A new strategy for gene therapy

In this chapter, we describe an approach using a peptide nucleic acid (PNA) clamp to directly and irreversibly modify plasmid DNA, without affecting either its supercoiled conformation or its ability to be efficiently transcribed. This strategy enables investigators to `functionalize' their gene of interest by direct coupling of ligands (fluorophores, peptide, proteins, sugars or oligonucleotides) to plasmid DNA. This approach provides versatile tools to study the mechanisms of gene delivery and to circumvent some of the main obstacles of synthetic gene delivery systems, such as specific targeting and efficient delivery. The proof-of-principal of PNA-dependent gene chemistry (PDGC) was demonstrated with a fluorescently labeled PNA that allowed generation of a highly fluorescent preparation of plasmid DNA that was functionally and conformationally intact. Fluorescent-PNA/DNA was used to identify critical parameters involved in naked DNA and non-viral gene delivery technology. The greatest potential of PDGC lies in the ability to attach specific ligands (e.g., peptides, proteins) to the plasmid DNA in order to overcome cellular barriers of non-viral gene delivery systems. In this regard, specific examples of ligands coupled to DNA are described and their effect on increasing the efficacy of gene therapy is presented.

Caspase-8 expression and proteolysis in human brain after severe head injury.

Programmed cell death involves a complex and interrelated cascade of cysteine proteases termed caspases that are synthesized as inactive zymogens, which are proteolytically processed to active enzymes. Caspase-8 is an initiator caspase that becomes activated when Fas death receptor-Fas ligand (FasL) coupling on the cell surface leads to coalescence of a "death complex" perpetuating the programmed cell death cascade. In this study, brain tissue samples removed from adult patients during the surgical management of severe intracranial hypertension after traumatic brain injury (TBI; n=17) were compared with postmortem control brain tissue samples (n=6). Caspase-8 mRNA was measured by semiquantitative reverse transcription and polymerase chain reaction, and caspase-8 protein was examined by Western blot and immunocytochemistry. Fas and FasL were also examined using Western blot. Caspase-8 mRNA and protein were increased in TBI patients vs. controls, and caspase-8 protein was predominately expressed in neurons. Proteolysis of caspase-8 to 20-kDa fragments was seen only in TBI patients. Fas was also increased after TBI vs. control and was associated with relative levels of caspase-8, supporting formation of a death complex. These data identify additional steps in the programmed cell death cascade involving Fas death receptors and caspase-8 after TBI in humans.

Theoretical and experimental studies of IR spectra of 4-aminopyridine metal(II) complexes

In this study FT-IR spectra of M(L) 2Ni(CN) 4 {where M=Fe or Zn, L=4-aminopyridine} complexes are reported for the first time in the 400–4000 cm −1 range. The spectral features suggest that the compounds are similar in structure to the Hofmann-type complexes with infinite polymeric layers formed with Ni(CN) 4 −2 ions bridged by M(L) 2 +2 cations. IR frequency shifts, upon formation of coordination compound are reliable indicators of the coordination mode of 4-aminopyridine. It is concluded that the ring nitrogen and not the amino nitrogen is involved in complex formation. In order to investigate metal–ligand coupling peculiarities, the vibrational wavenumbers of free and coordinated 4-aminopyridine have been calculated by a force field refinement method. The results indicated that the force field of free 4-aminopyridine should be altered in complex formation in order to represent the experimental data.