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PubMed1975

Immune complexes in the spleen. The difference between competitive inhibition of immune complex trapping in spleen follicles and inhibition by paratyphoid vaccine.

Paratyphoid vaccine injected between 4 days and 3 hours before injection of labelled immune complexes (125-I-labelled BGG-anti-BGG), inhibits follicular trapping of these complexes in the mouse spleen. Inhibition is maximal when paratyphoid vaccine is given 1 day before, almost no label being found in the spleen follicles. No inhibition of follicular trapping of the complexes occurred when paratyphoid vaccine was injected simultaneously with the labelled immune complexes. Competitive inhibition was found when unlabelled immune complexes were given together with labelled immune complexes. Simultaneous injection of mice with paratyphoid vaccine and labelled immune complexes resulted in an additonal form of localization of the labelled immune complexes in the white pulp, heavily labelled clumps also appearing in the periarteriolar lymphocyte sheaths and follicles. The results are discussed in relation to the mechanism of immune complex trapping in spleen follicles.

Animals↗

PubMed1989

Capillary-localized low-affinity antibody-antigen complexes act as a focus for the deposition of high-affinity complexes.

The hypothesis that low-affinity antibody-antigen complexes localized in the glomerular capillary wall can act as a focus for the subsequent deposition of complexes containing high-affinity antibody was tested with three experimental systems: (1) Experimental zinc deficiency was used to modulate antibody affinity and to determine its effect on the development of glomerulonephritis. Low-affinity (LA) mice fed a zinc-containing diet (Zn+) produce low-affinity antibody and develop glomerulonephritis when injected daily with antigen. However, LA mice fed a zinc deficient diet (Zn-) produce high-affinity antibody and do not develop chronic glomerulonephritis. Furthermore, when LA mice fed on a Zn+ diet and given daily antigen injections for 25 days were then given a Zn- diet and 25 further daily antigen injections, they developed glomerulonephritis more severely than did control LA mice given Zn+ diet throughout the whole experiment; (2) Immune complex localization was induced in LA mice by daily injections of ovalbumin and then i.v. injection of preformed high affinity anti-DNP-DNP-HSA complexes. These localized in the glomerular capillary wall in ovalbumin-injected animals in contrast to their mesangial localization in controls; and (3) High-affinity mice (HA) were given injections of preformed high- or low-affinity anti-DNP-DNP-HSA complexes and then 50 daily injections of DNP-HSA. The localization of complexes in HA mice following daily antigen injection was markedly influenced by the immunochemical characteristics of the complexes initially injected. These results suggest that the capillary localization of small, low affinity antibody-containing antibody-antigen complexes acts as a focus for the subsequent localization of larger, high-affinity antibody-containing complexes.

Animals↗

PubMed1979

Soluble oligovalent antigen-antibody complexes. I. The effect of antigen valence and combining ratio on the composition of fluorescein-carrier anti-fluorescein complexes.

Soluble oligovalent antigen--antibody complexes were prepared and analysed by ultracentrifugation in order to study the effect of the combining ratio, antigen valence and concentration upon the size and molecular composition of the composition of the complexes. Fluorescein (F) conjugates of rabbit serum albumin (RSA) and thyroglobulin (RTg) were combined with high affinity rabbit anti-F antibodies to form soluble complexes. The effect of the combining ratio paralleled findings in precipitating systems in that the largest soluble complexes were found at equimolarity and mild molar antibody excess. Tetravalent antigen formed precipitates at combining ratios near equimolarity, whereas trivalent antigens failed to precipitate at similar concentrations. Complexes prepared near equimolarity were most sensitive to changes in concentration, higher concentrations leading to larger complexes. The Ab/Ag ratios of different-size complexes in the same preparation were remarkably similar. This ratio was dependent on the antibody--antigen combining ratio, was limited by antigen valence and was not affected by concentration differences. The data support the hypothesis that soluble complexes are formed in two steps. First, antigen and antibody combine to form subunits whose Ab/Ag ratio is determined by the combining ratio and antigen valence. These subunits then combine to form larger complexes in a manner analogous to polymerization.

Antibodies↗

PubMed1984

Influence of immune-complex size and antigen-antibody ratio on immune complex detection with monoclonal rheumatoid factor and C1q.

Stabilized aggregates of human IgG were prepared over a wide range of molecular weights. These fractions with increasing molecular weight were adjusted to the same molarity or the same protein concentration and were tested in the solid phase C1q and monoclonal rheumatoid factor immune complex assays. At constant molarity results were linearly correlated with the size of the aggregates. At constant protein concentration, results were also linearly correlated with the size of the aggregates in the lower molecular weight fractions, although the number of aggregates in the fractions decreased with increasing molecular weight. It is concluded that results in these assays can only be compared with respect to concentration if the immune complexes have identical sizes. Consequently, we studied the relative affinity of C1q and monoclonal rheumatoid factor for antigen/antibody immune complexes of different sizes or different antigen/antibody ratios, using model immune complexes composed of tetanus toxoid/anti-toxoid and streptolysine O/anti-streptolysine O. Compared to C1q, monoclonal rheumatoid factor was found to have higher affinity for smaller complexes, and for complexes with higher antigen/antibody ratio. Immune complex containing sera of 5 patients with connective tissue diseases, and one normal serum, were fractionated on a Sepharose 4B column. The binding patterns of the different fractions to solid phase C1q and monoclonal rheumatoid factor were quite variable and in only one of these sera monoclonal rheumatoid factor had the expected preference for small complexes and C1q for large complexes. Moreover a remarkably high binding to C1q and/or monoclonal rheumatoid factor of the monomeric IgG fraction was found in 4 out of 5 patient sera.

Antibodies, Monoclonal↗

PubMed1996

Intra- and inter-complex cross-linking of subunits in the quinol oxidase super-complex from thermophilic Bacillus PS3.

Gram-positive thermophilic Bacilli contain quinol-cytochrome c reductase and cytochrome c oxidase as two major respiratory complexes of the electron transfer chain, and these enzymes can be extracted with mild detergents as an associated quinol oxidase super-complex. The reductase is composed of three subunits; cytochrome b6, cytochrome c1, and FeS protein, whereas cytochrome c oxidase consists of four subunits numbered 1 through 4. In order to clarify the interactions between the subunits, the super-complex isolated from Bacillus PS3 was cross-linked with three bifunctional cross-linkers; disuccinimidyl tartrate, 3,3'-dithiobis(succinimidylpropionate), and ethylene glycolbis(sulfosuccinimidylsuccinate). The most prominent cross-linking was observed for the combination of subunit 1 plus 2 in cytochrome c oxidase, and for that of cytochrome b6 plus cytochrome c1 in the reductase. In addition to these intra-complex cross-linkings, inter-complex linking was observed for the combination of cytochrome b6 plus subunit 1 with ethylene glycolbis(sulfosuccinimidylsuccinate), and for the combinations of cytochrome b6 plus subunit 1 and cytochrome b6 plus subunit 2 with 3,3'-dithiobis(succinimidylpropionate). Incubation in the presence of Triton X-100, which was confirmed to cleave the two enzyme complexes, selectively reduced the inter-complex cross-linking, suggesting that the chemical cross-linking reflect the spatial arrangement of subunits in the super-complex.

Bacillus↗

PubMed1996

Subunit 8 of the Saccharomyces cerevisiae cytochrome bc1 complex interacts with succinate-ubiquinone reductase complex.

We have investigated the function of subunit 8 of the cytochrome bc1 complex by generating six site-directed mutants, F46C, R51S, P62V, G64A, R91N, and W69-stop, in the cloned QCR8 gene and expressing the mutated genes in a Saccharomyces cerevisiae strain in which the chromosomal copy of QCR8 is deleted. The W69-stop mutation impairs assembly of the bc1 complex and growth of yeast on nonfermentable carbon sources as does deletion of QCR8 [Maarse, A. C., De Haan, M., Schoppink, P. J., Berden J. A., and Grivell, L. A. (1988) Eur. J. Biochem. 172, 179-184], implying that the C-terminus of subunit 8 is important for assembly and/or the stability of the bc1 complex. The F46C, R51S, P62V, G64A, and R91N mutations do not affect the growth of yeast on nonfermentable carbon sources, not do they lower the activity or alter the inhibitor sensitivity of the bc1 complex. Rather, some of the mutations increase the cytochrome C reductase activity of the bc1 complex by as much as 40%. However, succinate-ubiquinone reductase activity was consistently reduced 40-60% in mitochondrial membranes from these mutants, while NADH-ubiquinone reductase activity was not affected. In addition, the activation of succinate-ubiquinone reductase activity by succinate was diminished by the F46C, R51S, P62V, and G64A mutations. These results indicate that the cytochrome bc1 complex participates in electron transfer from succinate to ubiquinone in situ and also suggest an interaction between succinate-ubiquinone reductase and cytochrome bc1 complex which involves subunit 8 of the bc1 complex.

Amino Acid Sequence↗

PubMed2000

Fischer Carbene Complexes as Chemical Multitalents: The Incredible Range of Products from Carbenepentacarbonylmetal alpha,beta-Unsaturated Complexes.

The metal carbene complexes, discovered by E. O. Fischer at the start of the 1960s and carrying his name, have since proved themselves to be irreplaceable building blocks for organic synthesis. In particular, since the discovery of the Dötz reaction, a formal cycloaddition of Fischer alpha,beta-unsaturated carbene complexes to alkynes with CO insertion, this area of chemistry has become increasingly interesting to organic chemists. In spite of the considerable diversity of reactions performed with these complexes, proper selection of substrates and careful adjustment of the reaction conditions have allowed, in most cases the perfectly selective preparation of individual compounds of this enormous range of products. The spectrum of new successes begins with the conventional Diels-Alder reaction of alkynylcarbene complexes and the formal regioselective [3+2] cycloaddition of alkenylcarbene complexes to alkynes. It extends much further, however, from cascade reactions with the formation of oligofunctional and oligocyclic products of impressive molecular complexity to complex, formal [3+6] cocyclizations in which six bonds are formed in a single operational step. Beyond doubt, the methodological arsenal of preparative organic chemistry cannot be imagined any more without the valuable transformations of the Fischer carbene complexes; it only remains to be seen whether one or other of the numerous new types of cocyclization products of these complexes can establish itself as a lead structure in the search for biologically active compounds.

Journal Article↗

PubMed1982

Spectroscopic studies on bleomycin-iron complexes with carbon monoxide, nitric oxide, isocyanide, azide, and cyanide and comparison with iron-porphyrin complexes.

The bleomycin-iron complexes with CO, NO, C2H5NC, OH-, N-3, CN-, and CH3NH2 were characterized by electronic, ESR, 1H-NMR, and Mössbauer spectroscopies and the findings were compared with the corresponding hemoprotein complexes. The 1H-NMR and Mössbauer features for the CO and C2H5NC adducts of the bleomycin-Fe(II) complex are consistent with an S = 0 ferrous assignment. The OH-, CH3NH2, and N-3 adducts of the bleomycin-Fe(III) complex show the ESR, 1H-NMR, and Mössbauer spectra typical of a low-spin Fe(III). The unique Mössbauer parameters of the bleomycin-Fe(II)-NO complex demonstrate mixing between the NO pi- and the Fe 3d-orbitals. The magnitude of the proton chemical shifts over +/- 50 ppm indicates a high-spin ferric type for the bleomycin-Fe(III)-CN complex. The Mössbauer parameters (delta EQ = 0.89 and delta = 0.48 mm/s) of the CN- adduct differ substantially from those of typical low-spin hemoprotein-cyanide complexes. Except for the CN- adduct, the Mössbauer and crystal field parameters of these bleomycin-iron complexes are similar to those of the corresponding hemoprotein complexes.

Azides↗

PubMed2002

Polymer complexes XXXVII novel models and structural of symmetrical poly-Schiff base on heterobinuclear complexes of dioxouranium(VI).

Some binary and ternary novel complexes of dioxouranium(VI) with 5-vinylsalicylaldehyde (VSH) have been prepared and characterized by various physico-chemical techniques. The amine exchange reactions of coordinated poly-Schiff bases in these complexes have been also carried out which give symmetrical tetradentate poly-Schiff base complexes. Metal exchange reaction of these dioxouranium(VI) complexes with copper(II) gives the corresponding Cu(II) complexes. Reaction of tetradentate poly-Schiff base complexes of Cu(II) so obtained with ZrCl4 gives heterobinuclear polymer complexes. Magnetic, electronic and IR spectral information commensurate that configurations of square planar copper(II) polymer complexes. All the polymer complexes are coloured and appear to be nonelectrolytes in DMF. The ligands behave as bi-(O, O) and tetradentate (N2, O2) donors. El-Sonbati equation was used to evaluate the symmetric stretching frequency from which the fU-O and fUO, UO- were calculated.

Aldehydes↗

PubMed1995

Protein-protein interactions in colicin E9 DNase-immunity protein complexes. 1. Diffusion-controlled association and femtomolar binding for the cognate complex.

The cytotoxic activity of the secreted bacterial toxin colicin E9 is due to a nonspecific DNase housed in the C-terminus of the protein. A kinetic and thermodynamic analysis of complex formation for both the holotoxin and the isolated DNase domain with the cytoplasmic inhibitor of this enzyme, the immunity protein Im9, is presented. The dissociation constant for each complex was calculated from the ratio of the association and dissociation rate constants. Association was monitored by stopped-flow fluorescence and comprises at least two steps for both complexes, an initial fluorescence enhancement followed by a fluorescence quench. The data are consistent with a two-step binding mechanism in which the rate of formation of an encounter complex (k1) is rate determining and essentially diffusion controlled (4.0 x 10(9) M-1 s-1 for colicin E9) in buffer of low ionic strength. This encounter complex then rearranges to the final stable complex. Sequential stopped-flow experiments using 5-hydroxy-L-tryptophan labeled DNase domain support the two-step mechanism and further show that the rate of encounter complex rearrangement is significantly faster than its dissociation. The overall rate of dissociation of the colicin E9-Im9 complex (k(off)) was determined by radioactive subunit exchange to be 3.7 x 10(-7) s-1. Thus, the Kd for the complex (k(off)/k1) is 9.3 x 10(-17) M, which corresponds to a change in free energy on binding of -21.9 kcal mol-1 at 25 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Bacterial Proteins↗

PubMed1985

Structures of manganese(II) complexes with ATP, ADP, and phosphocreatine in the reactive central complexes with creatine kinase: electron paramagnetic resonance studies with oxygen-17-labeled ligands.

Coordination of Mn(II) to the phosphate groups of the substrates and products in the central complexes of the creatine kinase reaction mixture has been investigated by electron paramagnetic resonance (EPR) spectroscopy with regiospecifically 17O-labeled substrates. The EPR pattern for the equilibrium mixture is a superposition of spectra for the two central complexes, and this pattern differs from those observed for the ternary enzyme-Mn(II)-nucleotide complexes and from that for the dead-end complex enzyme-Mn(II)ADP-creatine. In order to identify those signals that are associated with each of the central complexes of the equilibrium mixture, spectra were obtained for a complex of enzyme, Mn(II)ATP, and a nonreactive analogue of creatine, 1-(carboxymethyl)-2-iminoimidazolidin-4-one, which is a newly synthesized competitive inhibitor. This inhibitor permits an unobstructed view of the EPR spectrum for Mn(II)ATP in the closed conformation of the active site. The EPR spectrum for this nonreactive complex with Mn(II)ATP matches one subset of signals in the spectrum for the equilibrium mixture, i.e., those due to the enzyme-Mn(II)-ATP-creatine complex. Chemical quenching of the samples followed by chromatographic assays for both ATP and ADP indicates that the enzyme-Mn(II)ADP-phosphocreatine and the enzyme-Mn(II)ATP-creatine complexes are present in a ratio of approximately 0.7 to 1. A similar value for the equilibrium constant for enzyme-bound substrates is obtained directly from the EPR spectrum for the equilibrium mixture.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Diphosphate↗

PubMed1979

CO and O2 complexes of soybean leghemoglobins: pH effects upon infrared and visible spectra. Comparisons with CO and O2 complexes of myoglobin and hemoglobin.

The effects of pH upon infrared spectra [CO stretching frequency (vco) region] and visible spectra of the CO complexes of soybean leghemoglobins a, c1, and c2, sperm whale myoglobin, and human hemoglobin A are reported. The vco for leghemoglobin--CO complexes was 1947.5 cm-1 at neutral pH. At acid pH myoglobin-- and hemoglobin--CO complexes developed vco bands at 1966--1968 cm-1, whereas leghemoglobin--CO complexes developed vco bands at approximately 1957 cm-1. All pKapp co values determined by pH-dependent variation of vco fell in the range 4.0--4.6. The pKapp co values determined from visible spectra were consistent with vco-determined values except for that of myoglobin--CO (visible pKapp co = 5.8). The pKapp co values in the 4.0--4.6 range appear to be pK values of the distal histidines, while the visible pKapp co of myoglobin--CO appears to be the pK of a group other than the distal and proximal histidines. The data are consistent with a model in which protonation of the distal histidine permits protein-free heme FeCO geometry in leghemoglobin--CO complexes but not in myoglobin-- or hemoglobin--CO complexes. Thus the heme pockets of leghemoglobins appear to be more flexible than the heme pockets of myoglobin and hemoglobin. The effects of pH upon visible spectra of the O2 complexes of soybean leghemoglobins a, c1, and c2, sperm whale myoglobin, and human hemoglobin A also are reported. pKapp o2 values of approximately 5.5 (leghemoglobins) and 4.4 (hemoglobin) are probably the pK values of the distal histidines. Comparisons of pKapp o2 values with pKapp co values indicate a more flexible heme pocket in leghemoglobins than in hemoglobin. The O2 complex of leghemoglobin c2 differed significantly from the O2 complexes of leghemoglobins a and c1 in visible spectra and titration behavior. These differences might be associated with the small structural differences in the region between the E and F helixes of leghemoglobins.

Animals↗

PubMed2002

New monomeric cobalt(II) and zinc(II) complexes of a mixed N,S(alkylthiolate) ligand: model complexes of (His)(His)(Cys) metalloprotein active sites.

The new N(2)S(alkylthiolate) ligand 2-methyl-1-[methyl-(2-pyridin-2-ylethyl)amino]propane-2-thiolate, PATH (1), has been prepared and reacted with zinc(II) and cobalt(II) to give the monomeric complexes [(PATH)ZnBr] (2), [(PATH)ZnNCS] (3), [(PATH)CoBr] (4), and [(PATH)CoNCS] (5). The molecular structures of 4 and 5 have been determined by X-ray diffraction. Each complex displays a distorted tetrahedral geometry at the metal center, with the PATH ligand providing the N(2)S(alkylthiolate) donors. These complexes are close structural mimics of the active site of metalloproteins with a His(2)Cys-M(II) site such as that found in peptide deformylase. Complexes 4 and 5 are the first examples of crystallographically characterized Co(II) complexes with an N(2)SL (L not equal N,S) donor set. Only one diastereomer for 2-5 is observed in the solid state, and simple molecular mechanics (Chem3D) calculations suggest this isomer is stable because of a favorable ligand conformation. NMR studies in the case of Zn(II) and UV-vis studies in the case of Co(II) provide strong evidence that their solid-state structures are retained in solution. Cyclic voltammetry reveals processes for both the Co(II/I) (4, - 1.51 V; 5, - 1.49 V) and Co(III/II) (4, + 0.9 V; 5, + 0.9 V) couples. The UV-vis data for the cobalt complexes are consistent with a monomeric, four-coordinate geometry regardless of the nature of the solvent (i.e., donating (MeOH, CH(3)CN) vs nondonating (CH(2)Cl(2))) and are compared with other cobalt complexes as well as cobalt-substituted His(2)Cys metalloproteins (peptide deformylase and blue-copper proteins). In addition, reaction of the bromide complexes 2 and 4 with hydroxide anion leads to the formation of 1:1 hydroxide:M(II) complexes which have been characterized in situ by (1)H NMR and UV-vis spectroscopy, respectively.

Amidohydrolases↗

PubMed2002

Novel copper(II) induced formation of a porphyrinogen derivative: X-ray structural, spectroscopic, and electrochemical studies of porphyrinogen complexes of Cu(II) and Co(III) complex of a trispyrazolyl tripodal ligand.

Copper(II) complexes of a novel pyrazole containing porphyrinogen and cobalt(III) and zinc(II) complexes of a pyrazole containing tripodal ligand having N-donor atoms have been investigated. 5-Methyl-3-formylpyrazole (MPA) on reaction with copper(II) nitrate or perchlorate in the presence of tris(2-aminoethyl)amine (tren) forms novel pyrazole-based porphyrinogen complexes [Cu(T(3)-porphyrinogen)(H(2)O)](NO(3))(2) (1a) and [Cu(T(3)-porphyrinogen)(H(2)O)](ClO(4))(2) (1b) where T(3)-porphyrinogen is 1,6,11,16-tetraaza-5,10,15,20-tetrahydroxy-2,7,12,17-tetramethylporphyrinogen. The same products are also obtained when tren is replaced by triethylamine. By contrast, the reaction between MPA, tren, and cobalt(II) perchlorate produces the cobalt(III) complex [Co(HMPz(3)tren)]ClO(4) (2) derived from the tripodal Schiff base tris[4-(3-(5-methyl-pyrazolyl)-3-aza-3-butenyl]amine (H(3)MPz(3)tren). The X-ray crystal structures of the copper(II) complexes (1a and 1b) and the cobalt(III) complex (2) have been determined. The structures show distorted square pyramidal coordination environments for 1a and 1b with the water molecule occupying the apical site, while for complex 2 a distorted octahedral geometry is obtained. Data for 1a follow: a = 19.476(3) A, b = 9.4116(8) A, c = 14.204(3) A; alpha = 90 degrees = gamma, beta = 107.58(2) degrees; V = 2482.0(7) A(3), Z = 4. Data for 1b follow: a = 20.967(3) A, b = 9.1563(18) A, c = 14.858(4) A; alpha = 90 degrees = gamma, beta = 108.44(3) degrees; V = 2706.0(10) A(3), Z = 4. Data for 2 follow: a = 21.293(3) A, b = 12.724(2) A, c = 19.777(4) A; alpha = 90 degrees = gamma, beta = 93.03(2) degrees; V = 5350.6(15) A(3), Z = 8. All three complexes crystallize in the monoclinic crystal system with the C2/c space group. The complexes are further characterized by UV-vis, IR, EPR, and electrochemical studies.

Journal Article↗

PubMed1998

Copper(II) Complexes with Unusual Axial Phenolate Coordination as Structural Models for the Active Site in Galactose Oxidase: X-ray Crystal Structures and Spectral and Redox Properties of [Cu(bpnp)X] Complexes.

The crystal structures of [Cu(bpnp)(SCN)].NH(4)SCN (1), [Cu(bpnp)(CH(3)COO)].CH(3)OH.C(8)H(10) (2), and [Cu(bpnp)ClO(4)] (3) [Hbpnp = 2-(bis(pyrid-2-ylmethyl)aminomethyl)-4-nitrophenol] reveal a distorted square pyramidal geometry around Cu(II) with an unusual axial coordination of phenolate. The mononuclear complex [Cu(bpnp)(SCN)].NH(4)SCN crystallizes in the triclinic space group P&onemacr; with a = 10.796(2) Å, b = 10.804(2) Å, c = 12.559(2) Å, alpha = 71.38(1) degrees, beta = 72.68(1) degrees, gamma = 61.69(1) degrees, and Z = 2. The mononuclear acetate [Cu(bpnp)(CH(3)COO)].CH(3)OH.C(8)H(10) crystallizes in the triclinic space group P&onemacr; with a = 10.480(6) Å, b = 12.116(4) Å, c = 12.547(3) Å, alpha = 98.77(3) degrees, beta = 113.37(3) degrees, gamma = 100.78(3) degrees, and Z = 2. The binuclear perchlorate complex crystallizes in the monoclinic space group C2/c with a = 13.417(3) Å, b = 20.095(2) Å, c= 16.401(2) Å, alpha = 102.21(2) degrees, and Z = 8. The coordination plane in all these complexes is comprised of the tertiary amine and two pyridine nitrogens. The fourth equatorial position is occupied by SCN(-)/CH(3)COO(-) in the mononuclear complexes but by the coordinated phenolate ion from the adjacent molecule in the perchlorate complex, resulting in its dimerization. The unusual occupation of phenolate ion in the axial site is possibly due to the steric constraint at copper imposed by the 5,5,6-chelate ring sequence. The thiocyanate/acetate coordination geometry is reminiscent of the active site of the radical copper enzyme galactose oxidase (GOase) with an axial phenolate and equatorial SCN(-)/CH(3)COO(-) ligands. Further, the present complexes exhibit several spectral features also similar to this enzyme. The addition of chloride or thiocyanate or acetate ions dissociates the dimeric structure of the perchlorate complex to produce the corresponding monomeric derivatives. The study of the interaction of the acetate complex with N(3)(-) and CN(-) ions provide insight into the anion binding properties of the enzyme. The sensitivity of the acetate complex to protons suggests the facile dissociation of the axial phenolate which then acts as a base to bind to protons. The implication of this reaction to the GOase mechanism is discussed.

Journal Article↗

PubMed2002

Formation of cis-enediyne complexes from rhenium alkynylcarbene complexes.

Dimerization of the alkynylcarbene complex Cp(CO)(2)Re=C(Tol)C(triple bond)CCH(3) (8) occurs at 100 degrees C to give a 1.2:1 mixture of enediyne complexes [Cp(CO)(2)Re](2)[eta(2),eta(2)-TolC(triple bond)CC(CH(3))=C(CH(3))C(triple bond)CTol] (10-Eand 10-Z), showing no intrinsic bias toward trans-enediyne complexes. The cyclopropyl-substituted alkynylcarbene complex Cp(CO)(2)Re=C(Tol)C(triple bond)CC(3)H(5) (11) dimerizes at 120 degrees C to give a 5:1 ratio of enediyne complexes [Cp(CO)(2)Re](2)[eta(2),eta(2)-TolC(triple bond)C(C(3)H(5))C=C(C(3)H(5))C(triple bond)CTol] (12-E and 12-Z); no ring expansion product was observed. This suggests that if intermediate A formed by a [1,1.5] Re shift and having carbene character at the remote alkynyl carbon is involved, then interaction of the neighboring Re with the carbene center greatly diminishes the carbene character as compared with that of free cyclopropyl carbenes. The tethered bis-(alkynylcarbene) complex Cp(CO)(2)Re=C(Tol)C(triple bond)CCH(2)CH(2)CH(2)C(triple bond)CC(Tol)= Re(CO)(2)Cp (13) dimerizes rapidly at 12 degrees C to give the cyclic cis-enediyne complex [Cp(CO)(2)Re](2)[eta(2),eta(2)-TolC(triple bond)CC(CH(2)CH(2)CH(2))=CC(triple bond)CTol] (15). Attempted synthesis of the 1,8-disubstituted naphthalene derivative 1,8-[Cp(CO)(2)Re=C(Tol)C(triple bond)C](2)C(10)H(6) (16), in which the alkynylcarbene units are constrained to a parallel geometry, leads to dimerization to [Cp(CO)(2)Re](2)(eta(2),eta(2)-1,2-(tolylethynyl)acenaphthylene] (17). The very rapid dimerizations of both 13 and 16 provide compelling evidence against mechanisms involving cyclopropene intermediates. A mechanism is proposed which involves rate-determining addition of the carbene center of A to the remote alkynyl carbon of a second alkynylcarbene complex to generate vinyl carbene intermediate C, and rearrangement of C to the enediyne complex by a [1,1.5] Re shift.

Journal Article↗

PubMed2002

Preparation of benzyne complexes of group 10 metals by intramolecular suzuki coupling of ortho-metalated phenylboronic esters: molecular structure of the first benzyne-palladium(0) complex.

A series of nickel(II) and palladium(II) aryl complexes substituted in the ortho position of the aromatic ring by a (pinacolato)boronic ester group, [MBr[o-C(6)H(4)B(pin)]L(2)] (M = Ni, L(2) = 2PPh(3) (2a), 2PCy(3) (2b), 2PEt(3) (2c), dcpe (2d), dppe (2e), and dppb (2f); M = Pd, L(2) = 2PPh(3) (3a), 2PCy(3) (3b), and dcpe (3d)), has been prepared. Many of these complexes react readily with KO(t)Bu to form the corresponding benzyne complexes [M(eta(2)-C(6)H(4))L(2)] (M = Ni, L(2) = 2PPh(3) (4a), 2PCy(3) (4b), 2PEt(3) (4c), dcpe (4d); M = Pd, L(2) = 2PCy(3) (5b)). This reaction can be regarded as an intramolecular version of a Suzuki cross-coupling reaction, the driving force for which may be the steric interaction between the boronic ester group and the phosphine ligands present in the precursors 2 and 3. Complex 3d also reacts with KO(t)Bu, but in this case disproportionation of the initially formed eta(2)-C(6)H(4) complex (5d) leads to a 1:1 mixture of a novel dinuclear palladium(I) complex, [(dcpe)Pd(mu(2)-C(6)H(4))Pd(dcpe)] (6), and a 2,2'-biphenyldiyl complex, [Pd(2,2'-C(6)H(4)C(6)H(4))(dcpe)] (7d). Complexes 2a, 3b, 3d, 4b, 5b, 6, and 7d have been structurally characterized by X-ray diffraction; complex 5b is the first example of an isolated benzyne-palladium(0) species.

Journal Article↗

PubMed2002

Chiral discrimination in cyclodextrin complexes of amino acid derivatives: beta-cyclodextrin/N-acetyl-L-phenylalanine and N-acetyl-D-phenylalanine complexes.

In a systematic study of molecular recognition of amino acid derivatives in solid-state beta-cyclodextrin (beta-CD) complexes, we have determined crystal structures for complexes of beta-cyclodextrin/N-acetyl-L-phenylalanine at 298 and 20 K and for N-acetyl-D-phenylalanine at 298 K. The crystal structures for the N-acetyl-L-phenylalanine complex present disordered inclusion complexes for which the distribution of guest molecules at room temperature is not resolvable; however, they can be located with considerable confidence at low temperature. In contrast, the complex with N-acetyl-D-phenylalanine is well ordered at room temperature. The latter complex presents an example of a complex in this series in which a water molecule is included deeply in the hydrophobic torus of the extended dimer host. In an effort to understand the mechanisms of molecular recognition giving rise to the dramatic differences in crystallographic order in these crystal structures, we have examined the intermolecular interactions in detail and have examined insertion of the enantiomer of the D-complex into the chiral beta-CD complex crystal lattice.

Amino Acids↗