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PubMed2007

Matrix isolation infrared spectroscopic and theoretical study of noble gas coordinated rhodium-dioxygen complexes.

Reactions of rhodium atoms with dioxygen molecules in solid argon have been investigated using matrix isolation infrared absorption spectroscopy. The rhodium-dioxygen complexes, Rh(eta2-O2), Rh(eta2-O2)2, and Rh(eta2-O2)2(eta1-OO), are produced spontaneously on annealing. The Rh(eta2-O2) complex rearranges to the inserted RhO2 molecule under visible light irradiation. Experiments doped with xenon in argon show that the rhodium-dioxygen complexes are coordinated by one or two noble gas atoms in solid noble gas matrixes. Hence, the Rh(eta2-O2), Rh(eta2-O2)2, and Rh(eta2-O2)2(eta1-OO) molecules trapped in solid noble gas matrixes should be regarded as the Rh(eta2-O2)(Ng)2, Rh(eta2-O2)2(Ng)2, and Rh(eta2-O2)2(eta1-OO)(Ng) (Ng = Ar or Xe) complexes. The product absorptions are identified on the basis of isotopic substitution and density functional theory calculations.

Journal Article↗

PubMed2005

Corroborative models of the cobalt(II) inhibited Fe/Mn superoxide dismutases.

Attempting to model superoxide dismutase (SOD) enzymes, we designed two new N3O-donor ligands to provide the same set of donor atoms observed in the active site of these enzymes: K(i)Pr2TCMA (potassium 1,4-diisopropyl-1,4,7-triazacyclononane-N-acetate) and KBPZG (potassium N,N-bis(3,5-dimethylpyrazolylmethyl) glycinate). Five new Co(II) complexes (1-5) were obtained and characterized by X-ray crystallography, mass spectrometry, electrochemistry, magnetochemistry, UV-vis, and electron paramagnetic resonance (EPR) spectroscopies. The crystal structures of 1 and 3-5 revealed five-coordinate complexes, whereas complex 2 is six-coordinate. The EPR data of complexes 3 and 4 agree with those of the Co(II)-substituted SOD, which strongly support the proposition that the active site of the enzyme structurally resembles these models. The redox behavior of complexes 1-5 clearly demonstrates the stabilization of the Co(II) state in the ligand field provided by these ligands. The irreversibility displayed by all of the complexes is probably related to an electron-transfer process followed by a rearrangement of the geometry around the metal center for complexes 1 and 3-5 that probably changes from a trigonal bipyramidal (high spin, d7) to octahedral (low spin, d6) as Co(II) is oxidized to Co(III), which is also expected to be accompanied by a spin-state conversion. As the redox potentials to convert the Co(II) to Co(III) are high, it can be inferred that the redox potential of the Co(II)-substituted SOD may be outside the range required to convert the superoxide radical (O2*-) to hydrogen peroxide, and this is sufficient to explain the inactivity of the enzyme. Finally, the complexes reported here are the first corroborative structural models of the Co(II)-substituted SOD.

Binding Sites↗

PubMedDate not supplied

Photooxidation of oligodeoxynucleotides by UO2(2+).

Photooxidation and consequent cleavage of oligodeoxynucleotides by the uranyl ion is greatly enhanced in the presence of a terminal 5'- or 3'-phosphate group. This enhanced cleavage is confined to nearby phosphodiester bonds in the case of a 5'-phosphate, but is less localized in the case of a 3'-phosphate. Several attempts to use a complementary oligodeoxynucleotide to direct a uranyl "warhead" against an oligodeoxynucleotide target were unsuccessful. Our results are most easily explained if we suppose that uranyl ions form coordination complexes with terminal phosphates and that, on photoexcitation, coordinated uranyl ions extract a hydrogen atom from the CH bond of a nearby deoxyribose residue.

Base Sequence↗

PubMed2006

Cell growth control: little eukaryotes make big contributions.

The story of rapamycin is a pharmaceutical fairytale. Discovered as an antifungal activity in a soil sample collected on Easter Island, this macrocyclic lactone and its derivatives are now billion dollar drugs, used in, and being evaluated for, a number of clinical applications. Taking advantage of its antifungal property, the molecular Target Of Rapamycin, TOR, was first described in the budding yeast Saccharomyces cerevisiae. TORs encode large, Ser/Thr protein kinases that reside in two distinct, structurally and functionally conserved, multi-protein complexes. In yeast, these complexes coordinate many different aspects of cell growth. TOR complex 1, TORC1, promotes protein synthesis and other anabolic processes, while inhibiting macroautophagy and other catabolic and stress-response processes. TORC2 primarily regulates cell polarity, although additional readouts of this complex are beginning to be characterized. TORC1 appears to be activated by nutrient cues and inhibited by stresses and rapamycin; however, detailed mechanisms are not known. In contrast, TORC2 is insensitive to rapamycin and physiological regulators of this complex have yet to be defined. Given the unsurpassed resources available to yeast researchers, this simple eukaryote continues to contribute to our understanding of eukaryotic cell growth in general and TOR function in particular.

Animals↗

PubMed2004

The ternary complex of Pseudomonas aeruginosa alcohol dehydrogenase with NADH and ethylene glycol.

Pseudomonas aeruginosa alcohol dehydrogenase (PaADH; ADH, EC 1.1.1.1) catalyzes the reversible oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones, using NAD as coenzyme. We crystallized the ternary complex of PaADH with its coenzyme and a substrate molecule and determined its structure at a resolution of 2.3 A, using the molecular replacement method. The PaADH tetramer comprises four identical chains of 342 amino acid residues each and obeys ~222-point symmetry. The PaADH monomer is structurally similar to alcohol dehydrogenase monomers from vertebrates, archaea, and bacteria. The stabilization of the ternary complex of PaADH, the coenzyme, and the poor substrate ethylene glycol (k(cat) = 4.5 sec(-1); Km > 200 mM) was due to the blocked exit of the coenzyme in the crystalline state, combined with a high (2.5 M) concentration of the substrate. The structure of the ternary complex presents the precise geometry of the Zn coordination complex, the proton-shuttling system, and the hydride transfer path. The ternary complex structure also suggests that the low efficiency of ethylene glycol as a substrate results from the presence of a second hydroxyl group in this molecule.

Alcohol Dehydrogenase↗

PubMedDate not supplied

1H NMR studies of heme pocket conformation in zinc-substituted leghemoglobin, a diamagnetic analog of deoxyleghemoglobin.

Reconstitution of apoleghemoglobin with zinc protoporphyrin IX is reported. NMR spectra show that the reconstitution is orientation specific and that there is no detectable heme isomerism or conformational heterogeneity. Resonances of heme substituents and distal and proximal amino acid protons have been assigned. Only minor differences in porphyrin-protein packing occur between zinc leghemoglobin and the CO complex of ferrous leghemoglobin. The zinc is five-coordinate and is ligated by the proximal histidine. Comparisons with diamagnetic six-coordinate complexes show that the distal His-61 and Leu-65 side chains move away from the binding site upon coordination of exogenous ligands. Conformational changes are minimal when the ligand is O2.

Heme↗

PubMed2005

Synthesis and characterization of seven-coordinate tripodal imidazole complexes of iron(II) and manganese(II).

The iron(II) and manganese(II) complexes of the N(7) Schiff-base condensate of tris(3-aminopropyl)amine with 1-methyl-2-imidazolecarbaldehyde and the manganese(II) complex of the N(7) Schiff-base condensate of tris(3-aminopropyl)amine with 4-imidazolecarbaldehyde are high-spin mono capped octahedral seven-coordinate complexes with a short, approximately 2.44 è, metal to apical nitrogen bond.

Journal Article↗

PubMed1995

The 1,10-phenanthroline micelles-copper(I) complex catalyzes protein degradation.

The 1,10-phenanthroline micelles-copper(I) coordinated complex, not the mono-dispersed bis 1,10-phenanthroline-copper(I) one, rapidly degraded proteins in the presence of beta-mercaptoethanol in the neutral-to-mild acidic region under aerobic conditions. The degradation products derived from alpha-casein were peptides and amino acids.

Aerobiosis↗

PubMed2000

Structure of triamidoaluminum complexes: a theoretical ab initio/IMOMM study.

The electronic structure of three-coordinated complexes of aluminum Al(NR2)3 has been studied through theoretical calculations. In the unsubstituted system Al(NH2)3, ab initio calculations (MP2/6-31G(d,p) level) show that the optimal geometry results from a conrotatory motion of the three amido substituents starting from the fully conjugated planar species. The energy difference between these two structures is found to be small (less than 0.5 kcal/mol). In methylated species Al(NMe2)3, steric effects become important and the planar geometry is destabilized. The conrotatory geometry corresponds to the absolute minimum and is located 14.2 kcal/mol below the planar structure (MP2/6-31G(d,p) level). Several coupled motions of the amido substituents have been computed and have been found to stabilize the system with respect to the fully conjugated structure. A rough estimate of the steric repulsion is calculated by comparison between the unsubstituted and methylated species. Finally, the real molecules Al[N(SiMe3)2]3 and Al(NiPr2)3 have been studied through a coupled quantum mechanics/molecular mechanics method. In accordance with the experimental data, it is found that the conrotatory minimum is the absolute minimum in the R = SiMe3 case whereas a less symmetrical minimum is found in the R = iPr case. In this last minimum, an amido group is almost deconjugated and the two other groups move in a conrotatory manner. The different behavior of these two systems may originate from the quasi-spherical shape of the SiMe3 group, which leads to unavoidable steric repulsion.

Journal Article↗

PubMed1995

Synthesis, antitumor activity, and chemical properties of silaplatin and related platinum (II) and platinum (IV) complexes derived from beta-silyl amines.

Platinum (II) and platinum (IV) coordination complexes derived from beta-silyl-substituted amines were prepared. The solubility of selected complexes in water and physiological saline was measured, and the effect of the beta-silicon on the reactivity of the complex in aqueous solution was determined by HPLC. The stabilities of selected silyl complexes were compared to the carbon analogues. The cyclic complexes 2a ("silaplatin") and its Pt(IV) analogue, 2b, were very active against L1210 leukemia in vivo. Both the platinum (II) complex 2a and the platinum (IV) complex 2b produced a significant number of cures over the dose range 10-40 mg/kg. The platinum (II) complex 2a, silaplatin, was very active in vivo against an L1210 leukemia subline that was resistant to cisplatin; 2a was also active, when given ip, against ic implanted L1210. The cyclobutanedicarboxylic acid complex 3c was synthesized; this complex was active against both cisplatin sensitive and resistant L1210 leukemia but was less potent than the analogous dichloro compound 2a. The acyclic platinum (II) and platinum (IV) complexes 1a,b were synthesized and unexpectedly found to be inactive in vivo against L1210 leukemia. More lipophilic silaplatin analogues were prepared--Pt(II) complex 2c and Pt(IV) complex 2d have one additional methylene carbon compared to 2a,b, whereas Pt(II) complex 2e and Pt(IV) complex 2f have two additional methylene carbons. Cyclization of the alkyl groups attached to the silicon gave the spiro bicyclic Pt(II) complexes 10a and 11a and the Pt(IV) complexes 10b and 11b.

Animals↗

PubMed2002

Solid-state (67)zn NMR spectroscopy in bioinorganic chemistry. Spectra of four- and six-coordinate zinc pyrazolylborate complexes obtained by management of proton relaxation rates with a paramagnetic dopant.

Solid-state (67)Zn NMR spectra of model compounds for metalloproteins, such as [H(2)B(3,5-Me(2)pz)(2)](2)Zn (pz denotes pyrazolyl ring), have been obtained using low temperatures (10 K) to enhance the Boltzmann factor in combination with cross polarization (CP) from (1)H to (67)Zn. Attempts to observe spectra of other model compounds, such as [H(2)B(pz)(2)](2)Zn, were hindered by long relaxation times of the protons. To decrease the proton relaxation times, the high-spin six-coordinate complex [HB(3,4,5-Me(3)pz)(3)](2)Fe has been investigated as a dopant. NMR and EPR measurements have shown that this Fe(II) dopant effectively reduces the (1)H spin lattice relaxation time, T(1), of the zinc samples in the temperature range 5-10 K with minimal perturbations of the (1)H spin lattice relaxation time in the rotating frame, T(1)(rho). Using this methodology, we have determined the (67)Zn NMR parameters of four- and six-coordinate zinc(II) poly(pyrazolyl)borate complexes that are useful models for systems of biological importance. The (67)Zn NMR parameters are contrasted to the corresponding changes in the (113)Cd NMR parameters for the analogous compounds. Further, these investigations have demonstrated that a temperature-dependent phase transition occurs in the neighborhood of 185 K for [HB(3,5-Me(2)pz)(3)](2)Zn; the other poly(pyrazolyl)borate complexes we investigated did not show this temperature-dependent behavior. This conclusion is confirmed by a combination of room-temperature high-field (18.8 T) solid-state (67)Zn NMR spectroscopy and low-temperature X-ray methods. The utilization of paramagnetic dopants should enable low-temperature cross polarization experiments to be performed on a wide variety of nuclides that are important in bioinorganic chemistry, for example, (25)Mg, (43)Ca, and (67)Zn.

Borates↗

PubMed2006

Removing the sting from the tail: reversible protonation of scorpionate ligands in cobalt(II) tris(carbene)borate complexes.

Low-temperature deprotonation of the phenylborane dications, PhB(RIm)3OTf2 (R = tBu, Mes), followed by in situ reaction with CoCl2(thf)1.5, results in the formation of the four-coordinate complexes, kappa3-PhB(RIm)3CoCl, in which the metal is supported by tripodal N-heterocyclic carbene-based ligands. The chloride complexes are exceptionally sensitive to acid and can be reversibly protonated to form the zwitterions kappa2-{PhB(RIm)2(RIm.H)}CoCl2. This unexpected reactivity is attributed to the highly basic nature of the tris(carbene)borate ligands. Reaction of the chloride complexes with methylating reagents results in products that depend on the N-heterocyclic carbene substituent. For R = tBu, the four-coordinate high-spin complex, kappa3-PhB(tBuIm)3CoMe, is formed, while for R = Mes, reduction to a multitude of complexes occurs.

Boronic Acids↗

PubMed2005

Methyl transfer reactivity of five-coordinate CH3CoIIIPc.

CH3CoIIIPc (Pc = dianion of phthalocyanine) has been characterized by equilibrium studies of its trans axial ligation and cyclic voltammetry as a relatively "electron poor" model of methylcobalamin, which in noncoordinating solvents persists as a five-coordinate complex. Axial base (N-donors, PBu3, SCN-, weakly binding O-donors) inhibition of methyl transfer from CH3CoIIIPc shows that the reaction proceeds via the reactive five-coordinate species, even in coordinating solvents. The virtual inactivity of six-coordinate CH3CoIIIPc(L) complexes provides a reference point for important biological processes.

Indoles↗

PubMed1997

Synthesis and Structural Characterization of Five-, Six-, and Seven-Coordinate Manganese(II) Complexes of the Tripodal Ligand Tris(2-benzimidazolylmethyl)amine.

A series of manganese(II) tris(2-benzimidazolylmethyl)amine (ntb) complexes were synthesized and characterized. In these complexes, ntb functions as a tripodal tetradentate ligand, resulting in trigonal pyramidal geometry. The complexes have five- to seven-coordinate manganese(II) ions depending on the additional ligand used. In each complex, an additional ligand, chloride anion, acetate anion, or nitrite anion, is coordinated at the "opened" site trans to the apical tertiary nitrogen atom of the ntb ligand. Complex 1, Mn(II)(ntb)Cl]Cl, has trigonal bipyramidal geometry. This geometry, corresponding to the active-site geometry of native manganese superoxide dismutase, was easily constructed using ntb as a tetradentate ligand and chloride as a monodentate ligand. Complex 2, [Mn(II)(ntb)(OAc)]Cl, was synthesized using ntb as a tetradentate ligand and acetate as a chelating bidentate ligand. The seven-coordinate complex [Mn(II)(ntb)(NO(2))(2)], 3, was also synthesized using ntb as a tetradentate ligand and nitrites as both monodentate and bidentate ligands. In each case, up to three donor atoms could be accommodated by the "opened" site without significant disturbance to the trigonal pyramidal portion of the complex. Crystal data are as follows. [Mn(II)(ntb)Cl]Cl.2.5MeOH, 1: triclinic space group P&onemacr;; a = 13.388(2) Å, b = 14.427(2) Å, c = 17.595(3) Å; alpha = 79.13(1) degrees, beta = 83.28(2) degrees, gamma = 66.02(2) degrees; V = 3046.3(8) Å(3); Z = 4; R1 = 0.0786, wR2 = 0.2063 for reflections of I > 2sigma(I). [Mn(II)(ntb)(OAc)]Cl.0.5MeOH.3.25H(2)O, 2: triclinic space group P&onemacr;; a = 13.308(1) Å, b = 14.689(2) Å, c = 18.335(2) Å; alpha = 75.48(1) degrees, beta = 79.21(1) degrees, gamma = 62.99(1) degrees; V = 3195.7(6) Å(3); Z = 4; R1 = 0.0899, wR2 = 0.2594 for reflections of I > 2sigma(I). [Mn(II)(ntb)(NO(2))(2)].4MeOH, 3: monoclinic space group P2(1)/n; a = 14.484(14) Å, b = 15.454(14) Å, c = 14.925(11) Å; beta = 114.86(6) degrees; V = 3031(5) Å(3); Z = 4; R1 = 0.0748, wR2 = 0.1614 for reflections of I > 2sigma(I).

Journal Article↗

PubMed2003

Non-covalent and coordination interactions in Cu(II) systems with uridine, uridine 5'-monophosphate and triamine or tetramine as biogenic amine analogues in aqueous solutions.

Reactions of metallation and non-covalent interactions have been studied in ternary systems of Cu(II) ions with uridine, uridine 5'-monophosphate and diamines or triamines. It has been found that in metal-free systems the reaction centres of the nucleoside with the polyamine are the donor nitrogen atoms N(3) and protonated -NH(x) groups of the amines. In comparison to systems with adenosine or cytidine, the pH range of complex formation is shifted towards higher values. It is a consequence of significantly higher basicity of uridine and in agreement with the ion-ion, ion-dipole interaction model assumed. Formation of molecular complexes of uridine 5'-monophosphate with polyamines at a low pH is the result of activity of the phosphate group which plays the role of a negatively charged reaction site. Non-covalent interactions interfere in processes of bioligand metallation. Centres of weak interactions are simultaneously binding sites of metal ions. In protonated Cu(Urd)(PA)H(x) complexes, coordination has been found to involve the N(3) atom from the nucleoside and two donor nitrogen atoms from the polyamine (PA). In the heteroligand species Cu(Urd)(PA), despite deprotonation of all amine groups, one of these groups is located outside the inner coordination sphere. In complexes with uridine-5'-monophosphate, the phosphate group is active in metallation. Moreover, in certain coordination compounds this group is engaged in non-covalent interactions with PA molecules, despite binding Cu ions, as has been shown on the basis of equilibrium and spectral studies.

Biogenic Polyamines↗

PubMed2003

Molecular motions in the solid state: the thermochromic nitro-nitrito Interconversion in nickel(II) bis(diamine) complexes.

The NO(2)(-) ion, in the trans-octahedral [Ni(II)(N,N'-dimethylethylenediamine)(2)(NO(2))(2)](H(2)O) complex, coordinates the metal through the nitrogen atom (nitro form). On heating the solid complex, the anion rotates to give nitrito coordination (oxygen bound), according to a reversible process. The coordination mode of NO(2)(-) to Ni(II) is related to the steric interplay between the anion and the alkyl substituents on the diamine.

Journal Article↗

PubMed2003

Manganese(II) and copper(II) hexafluoroacetylacetonate 1:1 complexes with 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)pyrimidine: regiochemical parity analysis for exchange behavior of complexes between radicals and paramagnetic cations.

Mn(hfac)(2) and Cu(hfac)(2) form coordination complexes with 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)pyrimidine, PyrimPh-NIT. (Mn[PyrimPh-NIT](hfac)(2))(2) and (Cu[PyrimPh-NIT](hfac)(2))(2), 1 and 2, respectively, are cyclic M(2)L(2) dimers that exhibit strong exchange coupling between the coordinated paramagnetic dication (M) and nitroxide (NIT) unit. The M-NIT exchange is strongly antiferromagnetic (AFM) in 1 and strongly ferromagnetic (FM) in 2. Magnetic susceptibility measurements for 1 were fitted to an AFM spin pairing model with J/k = -0.25 K between Mn-NIT spin sites units. Complex 2 also exhibits AFM spin pairing between S = 1 Cu-NIT spin units that is somewhat field dependent at low temperature. The fit of corrected paramagnetic susceptibility chi(T) to an AFM spin pairing model at 200 Oe yields J/k = (-)3.8 K, quite similar to earlier measurements at 1000 Oe yielding J/k = (-)5.0 K. At 1.40 K, the magnetization of 2 does not approach saturation until somewhat above 170 kOe, giving an S-shaped curve; at 0.55 K, the magnetization curve shows steps characteristic of field-induced crossover between the S = 0 ground state and excited spin states. From the steps in the 0.55 K data, we estimate J/k = (-)3.8-4.0 K for 2, in good agreement with the analysis of chi(T).

Journal Article↗

PubMed2000

Conformation of heterocycles controlled by the existence of unusual C-H...X hydrogen bonds: syntheses and structure determination of aluminum aryloxides.

The reactions of AlMe3 in diethyl ether with 1 molar equiv of 2,2'-methylenebis(4-methyl-6-tert-butylphenol) (MMBP-H2), 2,2'-methylenebis(4,6-di-tert-butylphenol) (MDBP-H2), and 2,2'-ethylidenebis(4,6-di-tert-butylphenol) (EDBP-H2) afford series of four-coordinate monomeric aluminum aryloxides, MeAl(O-O)(OEt2), 1-3 (1, (O-O) = MMBP; 2, (O-O) = MDBP; 3, (O-O) = EDBP). In THF, 1 molar equiv of EDBP-H2 reacts with AlMe3 to provide the THF-coordinated complex MeAl(EDBP)(THF) 4. However, in the absence of a coordinating solvent, the reaction of EDBP-H2 with AlMe3 yields the dimeric complex [MeAl(mu-EDBP)]2 (5). Complex 5 further reacts with Et4NCl, Et4NBr, and Ph3PO to afford the corresponding monomeric ionic complex [Et4N][MeAl(EDBP)(X)] (6, X = Cl; 7, X = Br) and the neutral complex [MeAl(EDBP)(O=PPh3)] (8), respectively. Complexes 1, 2, 4 and 6-8 are subjected to X-ray structure analyses, and the solid state structures reveal that the conformations of the eight-membered heterocycles are governed by the formation of the unusual C-H...X hydrogen bonds.

Journal Article↗