Axial Ligand Dissociation Research Articles

Penta -hexa coordination behaviour of ABA-P(V) corrole

• Phosphorous(V) corroles exist in penta-hexa coordination in non-coordinating solvents. • Hexa-coordinated geometries were obtained in coordinating solvents. • Solid state geometries prefer to be in hexa-coordinated systems. • 31 P, 1 H and 19 F NMR provide proof for penta-hexa behavior. ABA (where A = nitrophenyl, and B = pentafluorophenyl group) corrole was isolated as hexacoordinate P(V) complexes that contained two axial hydroxyl groups. NMR spectroscopy ( 1 H, 19 F and 31 P) in CDCl 3 indicated that the hexacoordinated P(V) complexes were prone to axial-ligand dissociation to form pentacoordinated P(V) complexes. Although, in the presence of strong coordinating solvents, such as CH 3 OH and CD 3 OD, the P(V) corrole complexes were observed to exist in a hexacoordinated geometry in which the strong solvent molecules acted as the ligands at the axial position. Also, we report here the existence of the hexacoordinated P(V) complexes in non-coordinating solvents such as CHCl 3, CH 2 Cl 2 by using fluoride and methyl substituted silyloxy groups acting as the ligands at the axial position. Moderately bulky nature and electron-donation effect of silyloxy groups proved to be of great aid to prevent the axial ligand dissociation of silyloxy substituted hexacoordinated P(V) complex and strong P-F bond in non-coordinating solvents. The structure of hexacoordinate P(V) complex containing two axial -OH groups and hexacoordinate P(V) complex containing two axial -OCH 3 groups were confirmed by X-ray crystallography.

Abstract 480: Molecular mechanism of action of photoimmunotherapy with antibody-IR700 dye conjugates

Abstract INTRODUCTION: Cancer cell-targeted photoimmunotherapy (PIT) is a platform technology under development for the treatment of various cancers. PIT is a drug and device combination that utilizes an antibody conjugated to the IR700 (IRDye 700DX®) light-excitable dye, which upon antigen binding and illumination with non-thermal red light, leads to necrotic cell death. The objective of these studies was to characterize the molecular pharmacology and the underlying mechanism of action of PIT. METHODS: In vitro PIT studies using human and mouse cancer cells were performed to evaluate conjugate (antibody + dye) binding, dose response and light dosimetry response. Mouse xenograft experiments assessed necrotic markers in tumor tissue and surrounding normal tissue post-PIT treatment. Oxidative damage to lipids was detected with a reporter dye. Mass spectrometry analysis was performed to assess protein oxidation, crosslinking and axial ligand dissociation from the dye. RESULTS: Binding of the conjugate to the target cancer cells was required for PIT treatment to induce cell death as measured by antibody competition studies, however, internalization of the conjugate was not required for cytotoxic effects. Administration of non-thermal red light to the target cells bound with the conjugate led to morphologic changes and membrane disruption within minutes, and necrotic cell death was observed in a conjugate- and light-dose dependent manner. Requirement of conjugate binding to the cancer antigen provided high level of tumor specificity as demonstrated by lack of damage to normal surrounding tissue after PIT treatment. Light excitation of the conjugate generated 1O2 with high quantum yield. In vitro PIT treatment led to cell membrane lipid peroxidation, mediated by 1O2 as shown by abrogation with sodium azide, a selective 1O2 quencher. In addition, protein oxidation, cross-linking and aggregation of the conjugate was observed. In low oxygen concentrations and in the presence of ascorbate, photo-induced dissociation of axial ligands from the dye was detected. CONCLUSIONS: These studies suggest the molecular mechanism of action of PIT includes generation of 1O2 that leads to lipid peroxidation and cell membrane disruption. Additional mechanisms may include protein oxidation and non-oxygen dependent mechanisms such as axial ligand loss of the IR700 dye. These results describe a biophysical process that damages and disrupts the cell membrane integrity resulting in necrotic cell death and immunogenic cell death. Citation Format: Roger Heim, Lew Makings, Torsten Wiemann, Anthony Manibusan, Miguel Garcia-Guzman. Molecular mechanism of action of photoimmunotherapy with antibody-IR700 dye conjugates [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 480.

A Molecular Silane-Derivatized Ru(II) Catalyst for Photoelectrochemical Water Oxidation.

Photoanodes in dye-sensitized photoelectrosynthesis cells integrate molecular chromophore/catalyst assemblies on mesoporous n-type metal oxide electrodes for light-driven water oxidation. One limitation for sustainable photoanodes is the stability of chromophore/catalyst assembly on electrode surfaces for long periods. Progress has been made in stabilizing chromophores based on atomic layer deposition, polymer dip coating, C-C cross-coupling by electropolymerization, and silane surface binding, but little progress has been made on catalyst stabilization. We report here the silane-derivatized catalyst, Ru(bda)(L)2 (bda = 2,2'-bipyridine-6,6'-dicarboxylate, L = 4-(6-(triethoxysilyl)hexyl)pyridine), catalyst 1, which is stabilized on metal oxide electrode surfaces over an extended pH range. A surface stabilization study shows that it maintains its reactivity on the electrode surface toward electrochemical oxidation over a wide range of conditions. Its electrochemical stability on electrode surfaces has been systematically evaluated, and its role as a catalyst for water oxidation has been explored. On surfaces of mesoporous nanostructured core/shell SnO2/TiO2, with a TiO2 stabilized inner layer of the Ru(II) polypyridyl chromophore, [Ru(4,4'-(PO3H2)2bpy)(bpy)2]2+ (RuP2+; bpy = 2,2'-bipyridine), highly efficient photoelectrochemical water oxidation catalysis occurs to produce O2 with a maximum efficiency of ∼1.25 mA/cm2. Long-term loss of catalytic activity occurs with time owing to catalyst loss from the electrode surface by axial ligand dissociation in the high oxidation states of the catalyst.

Probing Electronic Communications in Heterotrinuclear Fe-Ru-Fe Molecular Wires Formed by Ruthenium(II) Tetraphenylporphyrin and Isocyanoferrocene or 1,1'-Diisocyanoferrocene Ligands.

Two new heterotrinuclear Fe-Ru-Fe complexes of ruthenium(II) tetraphenylporphyrin axially coordinated with a pair of isocyanoferrocene ((FcNC)2RuTPP, 1) or 1,1'-diisocyanoferrocene (([C5H4NC]2Fe)2RuTPP, 2) ligands [Fc = ferrocenyl, TPP = 5,10,15,20-tetraphenylporphyrinato(2-) anion] were synthesized and characterized by UV-vis, magnetic circular dichroism, NMR, and FTIR spectroscopies as well as by electrospray ionization mass spectrometry and single-crystal X-ray diffraction. Isolation of insoluble polymeric {([C5H4NC]2Fe)RuTPP}n molecular wires (3) was also achieved for the first time. The redox properties of the new trinuclear complexes 1 and 2 were probed using electrochemical (cyclic voltammetry and differential pulse voltammetry), spectroelectrochemical, and chemical oxidation methods and correlated to those of the bis(tert-butylisocyano)ruthenium(II) tetraphenylporphyrin reference compound, (t-BuNC)2RuTPP (4). In all cases, the first oxidation process was attributed to the reversible oxidation of the Ru(II) center. The second and third reversible oxidation processes in 1 are separated by ∼100 mV and were assigned to two single-electron Fe(II)/Fe(III) couples, suggesting a weak long-range iron-iron coupling in this complex. Electrochemical data acquired for 2 are complicated by the interaction between the axial η(1)-1,1'-diisocyanoferrocene ligand and the electrode surface as well as by axial ligand dissociation in solution. Spectroelectrochemical and chemical oxidation methods were used to elucidate the spectroscopic signatures of the [1](n+), [2](n+), and [4](n+) species in solution. DFT and time-dependent DFT calculations aided in correlating the spectroscopic and redox properties of complexes 1, 2, and 4 with their electronic structures.

Unprecedented Mechanism Employed by the Salmonella enterica EutT ATP:CoIrrinoid Adenosyltransferase Precludes Adenosylation of Incomplete CoIIrrinoids

AbstractThree distinct families of ATP:corrinoid adenosyltransferases (ACATs) exist that are capable of converting vitamin B12 derivatives into coenzyme B12 by catalyzing the thermodynamically challenging reduction of CoIIrrinoids to form “supernucleophilic” CoI intermediates. While the structures and mechanisms of two of the ACAT families have been studied extensively, little is known about the EutT enzymes beyond the fact that they exhibit a unique requirement for a divalent metal cofactor for enzymatic activity. In this study we have obtained compelling evidence that EutT converts cob(II)alamin into an effectively four‐coordinate CoII species so as to facilitate CoII→CoI reduction. Intriguingly, EutT fails to promote axial ligand dissociation from the substrate analogue cob(II)inamide, a natural precursor of cob(II)alamin. This unique substrate specificity of EutT has important physiological implications.

Unprecedented Mechanism Employed by the Salmonella enterica EutT ATP:Co(I)rrinoid Adenosyltransferase Precludes Adenosylation of Incomplete Co(II)rrinoids.

Three distinct families of ATP:corrinoid adenosyltransferases (ACATs) exist that are capable of converting vitamin B12 derivatives into coenzyme B12 by catalyzing the thermodynamically challenging reduction of Co(II) rrinoids to form "supernucleophilic" Co(I) intermediates. While the structures and mechanisms of two of the ACAT families have been studied extensively, little is known about the EutT enzymes beyond the fact that they exhibit a unique requirement for a divalent metal cofactor for enzymatic activity. In this study we have obtained compelling evidence that EutT converts cob(II)alamin into an effectively four-coordinate Co(II) species so as to facilitate Co(II)→Co(I) reduction. Intriguingly, EutT fails to promote axial ligand dissociation from the substrate analogue cob(II)inamide, a natural precursor of cob(II)alamin. This unique substrate specificity of EutT has important physiological implications.

Synthesis, Structure, Spectroscopic, and Electrochemical Properties of Highly Fluorescent Phosphorus(V)–meso‐Triarylcorroles

The synthesis, spectroscopic, and electrochemical properties of seven new P(V)-meso-triarylcorroles (1-7) are reported. Compounds 1-7 were prepared by heating the corresponding free-base corroles with POCl(3) at reflux in pyridine. Hexacoordinate P(V) complexes of meso-triarylcorroles were isolated that contained two axial hydroxy groups, unlike the P(V) complex of 8,12-diethyl-2,3,7,13,17,18-hexamethylcorrole, which was pentacoordinate, or the P(V) complex of meso-tetraphenylporphyrin, which was hexacoordinate with two axial chloro groups. (1)H and (31)P NMR spectroscopy in CDCl(3) indicated that the hexacoordinated P(V)-meso-triarylcorroles were prone to axial-ligand dissociation to form pentacoordinated P(V)-meso-triarylcorroles. However, in the presence of strongly coordinating solvents, such as CH(3)OH, THF, and DMSO, the P(V)-meso-triarylcorroles preferred to exist in a hexacoordinated geometry in which the corresponding solvent molecules acted as axial ligands. X-ray diffraction of two complexes confirmed the hexacoordination environment for P(V)-meso-triarylcorroles. Their absorption spectra in two coordinating solvents revealed that P(V)-meso-triarylcorroles showed a strong band at about 600 nm together with other bands, in contrast to P(V)-porphyrins, which showed weak bands in the visible region. These compounds were easier to oxidize and more difficult to reduce compared to P(V)-porphyrins. These compounds were brightly fluorescent, unlike the weakly fluorescent P(V)-porphyrins, and the quantum yields for selected P(V)-corroles were as high as Al(III) and Ga(III) corroles, which are the best known fluorescent compounds among oligopyrrolic macrocycles.

Design of Amphiphilic Protein Maquettes: Enhancing Maquette Functionality through Binding of Extremely Hydrophobic Cofactors to Lipophilic Domains

We demonstrate coordination of the extremely hydrophobic 13(2)-OH-Ni-bacteriochlorophyll (Ni-BChl) to the lipophilic domain of a novel, designed amphiphilic protein maquette (AP3) dispersed in detergent micelles [Discher et al. (2005) Biochemistry 44, 12329-12343]. Sedimentation velocity and equilibrium experiments and steady-state absorption spectra indicate that Ni-BChl-AP3 is a four-helix bundle containing one Ni-BChl axially ligated by one or two histidines. The nature of the ligation was pursued with ultrafast visible spectroscopy. While it is well established that light excitation of axially ligated mono- and bisimidazole Ni-BChl in solution leads to rapid imidazole dissociation and nanosecond recombination, there is no evidence of axial ligand dissociation in the light-excited Ni-BChl-AP3. This indicates that Ni-BChl is confined within the AP3 protein, ligated to histidines with severely restricted mobility. Dissociation constants show that Ni-BChl binding to AP3 is considerably weaker than the nanomolar range usual for heme and hydrophilic (HP) maquettes; moreover, there is a tendency for the Ni-BChl-AP3 four-helix bundles to dimerize into eight-helix bundles. Nevertheless, the preparation of the Ni-BChl-AP3 four-alpha-helix maquettes, supported by time-resolved spectroscopic analysis of the nature of the ligation, provides a viable new approach to AP maquette designs that address the challenges involved in binding extremely hydrophobic cofactors.

Chiral recognition of amino esters by a ruthenium porphyrin complex: kinetics of the exchange process determined by 1H NMR

The synthesis of a (carbonyl) (valine methyl ester) ruthenium(II) picket-fence complex bearing optically active α-methoxy-α(trifluoromethyl)phenylacetyl residues on both sides of the porphyrin plane (α,β,α,β-isomer) is described. For various amino esters, chiral recognition was observed for the complexation of the ligand with up to 52% enantiomeric excess for tert-leucine methyl ester. The dissociation rate constants of the two enantiomers of valine methyl ester were determined by 1H NMR using magnetization transfer experiments, showing that the origin of the enantioselectivity in favour of the ( l)-valine (ca. 2.6:1) resides in the difference between the kinetics of the axial ligand dissociation of the two enantiomers.

Interaction of Escherichia coli cobalamin-dependent methionine synthase and its physiological partner flavodoxin: binding of flavodoxin leads to axial ligand dissociation from the cobalamin cofactor.

Cobalamin-dependent methionine synthase from Escherichia coli catalyzes the last step in de novo methionine biosynthesis. Conversion of the inactive cob(II)alamin form of the enzyme, formed by the occasional oxidation of cob(I)alamin during turnover, to an active methylcobalamin-containing form requires a reductive methylation of the cofactor in which an electron is supplied by reduced flavodoxin and the methyl group is derived from S-adenosyl-L-methionine. E. coli flavodoxin acts specifically in this activation reaction, and neither E. coli ferredoxin nor flavodoxin from the cyanobacterium Synechococcus will substitute, despite their highly similar midpoint potentials for one-electron transfer. As assessed by EPR spectroscopy, the binding of flavodoxin to cob(II)alamin methionine synthase results in a change in the coordination geometry of the cobalt from five-coordinate to four-coordinate. Histidine 759 of methionine synthase, which replaces the normal lower ligand dimethylbenzimidazole on binding of methylcobalamin to methionine synthase, is dissociated from the cobalt of the cobalamin by the binding of flavodoxin. The association of flavodoxin and methionine synthase depends on ionic strength and pH; the pH dependence corresponds to the uptake of one proton on association. The formation of a complex between flavodoxin and methionine synthase perturbs the midpoint potentials of the flavin and cobalamin cofactors only marginally and without any significant thermodynamic advantage for electron transfer to the cobalamin of methionine synthase. No significant binding was seen between oxidized flavodoxin and methylcobalamin methionine synthase. A model for the interaction of methionine synthase with flavodoxin is proposed in which flavodoxin binding leads to changes in the distribution of methionine synthase conformations.

ChemInform Abstract: Comparative Spectra and Kinetics of Ferrous Bis(dioxime) Complexes of π‐Acid Ligands.

AbstractSyntheses (by known methods), visible spectra, and kinetic data for axial ligand dissociation for the low‐spin Complexes (Ia) and (Ib) are reported and compared with corresponding dimethylglyoxime analogues (Ic).

Axial ligand dissociation of phthalocyaninatoiron(II) adducts; further evidence for a dissociative mechanism of substitution

The kinetics of reaction of [Fe(pc)L2](pc = phthalocyaninate, L = imidazole or 4-aminopyridine) with dimethyl sulphoxide have been studied over the range 20–45 °C. The rate-determining step is loss of the first ligand. Dilution with acetone or benzene enhances the observed first-order rate constant such that it is proportional to the reciprocal of the concentration of dmso over the range 1–14 mol dm–3. This is shown to be consistent with a D mechanism and some rate constants for the individual steps are derived. A comparison of trans effects at the phthalocyaninatoiron(II) centre is made.