Acetyl Oxygen Research Articles

Luminescent Ag i-Cu i heterometallic hexa-, octa-, and hexadecanuclear alkynyl complexes.

A series of Ag(I)-Cu(I) heteronuclear alkynyl complexes were prepared by reaction of polymeric (MCCC(6)H(4)R-4)(n)() (M = Cu(I) or Ag(I); R = H, CH(3), OCH(3), NO(2), COCH(3)) with [M'(2)(mu-Ph(2)PXPPh(2))(2)(MeCN)(2)](ClO(4))(2) (M' = Ag(I) or Cu(I); X = NH or CH(2)). Heterohexanuclear complexes [Ag(4)Cu(2)(mu-Ph(2)PNHPPh(2))(4)(CCC(6)H(4)R-4)(4)](ClO(4))(2) (R = H, 1; CH(3), 2) were afforded when X = NH, and heterooctanuclear complexes [Ag(6)Cu(2)(micro-Ph(2)PCH(2)PPh(2))(3)(CCC(6)H(4)R-4)(6)(MeCN)](ClO(4))(2) (R = H, 3; CH(3), 4; OCH(3), 5; NO(2), 6) were isolated when X = CH(2). Self-assembly reaction between (MCCC(6)H(4)COCH(3)-4)(n) and [M'(2)(mu-Ph(2)PCH(2)PPh(2))(2)(MeCN)(2)](ClO(4))(2), however, gave heterohexadecanuclear complex [Ag(6)Cu(2)(micro-Ph(2)PCH(2)PPh(2))(3)(CCC(6)H(4)COCH(3)-4)(6)](2)(ClO(4))(4) (7). The heterohexanuclear complexes 1 and 2 show a bicapped cubic skeleton (Ag(4)Cu(2)C(4)) consisting of four Ag(I) and two Cu(I) atoms and four acetylide C donors. The heterooctanuclear complexes 3-6 exhibit a waterwheel-like structure that can be regarded as two Ag(3)Cu(CCC(6)H(5))(3) components put together by three bridging Ph(2)PCH(2)PPh(2) ligands. The heterohexadecanuclear complex 7 can be viewed as a dimer of heterooctanuclear complex [Ag(6)Cu(2)(micro-Ph(2)PCH(2)PPh(2))(3)(CCC(6)H(4)COCH(3)-4)(6)](ClO(4))(2) through the silver and acetyl oxygen (Ag-O = 2.534 (4) A) linkage between two waterwheel-like Ag(6)Cu(2) units. All of the complexes show intense luminescence in the solid states and in fluid solutions. The microsecond scale of lifetimes in the solid state at 298 K reveals that the emission is phosphorescent in nature. The emissive state in compounds 1-5 is likely derived from a (3)LMCT (CCC(6)H(4)R-4 --> Ag(4)Cu(2) or Ag(6)Cu(2)) transition, mixed with a metal cluster-centered (d --> s) excited state. The lowest lying excited state in compounds 6 and 7 containing electron-deficient 4-nitrophenylacetylide and 4-acetylphenylacetylide, respectively, however, is likely dominated by an intraligand (3)[pi --> pi] character.

Crystal and Molecular Structure of 5-Acetylamino-3-nitro-1,2,4-triazole

This paper reports on an X-ray diffraction study of 5-acetylamino-3-nitro-1,2,4-triazole. Monoclinic crystals, space group P2_1/c; a=8.2079(7), b=13.096(1), c=13.650(1) A, β=100.64(1)°. Two independent molecules are identical in conformation and both have an N–H···sO intramolecular hydrogen bond; the difference lies in their interactions with neighbors. In molecule A, the acetyl oxygen atom and all nitrogen atoms (except those of the amino group) are involved in the intermolecular hydrogen bonds; in molecule B, these hydrogen bonds are formed by only two NH groups in addition to the above oxygen. The crystal structure of the title compound is compared with that of 5-amino-3-nitro-1,2,4-triazole.

A Curious Case of Partial Spontaneous Resolution on Crystallization of a Racemate. The Crystal and Molecular Structures of N-Acetyl-DL-alanine Methylester and iV-Acetyl-L-alanine Methylester

Abstract N-Acetyl-L-alanine methylester crystallizes in the orthorhombic space group P212121 with one molecule in the asymmetric unit (a = 7.768(1), b = 9.606(1), c = 10.215(2) Å, Z = 4). The individual molecules are linked into infinite strands by intermolecular hydrogen bonds between the amide hydrogen atom as donor and the acetyl oxygen atom as acceptor. The strands run parallel to the crystallographic b axis. The respective racemate, N-acetyl-DL-alanine methylester, crystallizes in the monoclinic space group P 21/n with three molecules in the asymmetric unit (a = 14.442(3), b = 8.467(2), c = 19.336(5) Å, β = 93.68(1)°, Z = 12). Again, the individual molecules are linked into infinite strands by N-H···O= Cacetyl hydrogen bonds which run along the crystallographic a axis. The individual strands are made up of molecules of opposite chirality in a 2:1 ratio. More specifically, one set of strands consists of molecules in the sequence [D, D, L,]∞ while a second set has the sequence [L, L, D]∞ as imposed by the centrosymmetry o f the space group. Thus, although crystals o f N-acetyl-DL-alanine methylester contain equal amounts of the molecules of opposite chirality, the strand formation through intermolecular hydrogen bonds leads to an incom plete resolution of the racemic mixture of molecules within one strand. The reason for the preference of the observed structure o f N-acetyl-DL-alanine methylester over spontaneous resolution is seen in the optimization of hydrogen bonding within one strand versus the overall crystal packing energy. Some principles of the crystallization of achiral molecules, chiral molecules, and racemates are briefly reviewed, as is the phenomenon of spontaneous resolution.

Subtlety in the reactivity of a diketo phosphorus ylide towards mercuric halides: the unprecedented O-coordination of alpha-acetyl-alpha-benzoylmethylenetriphenylphosphorane to Hg(II).

The reactions of the title ylide with HgX2 (X = Cl, Br or I) lead to the regiospecific binding of the acetyl oxygen to soft Hg(II), producing a chloro complex with (2 + 2) coordination and isostructural dimeric bromo and iodo complexes containing halogen bridges with tetrahedral configurations around the metal centres.

Insight to structural subsite recognition in plant thiol protease-inhibitor complexes : Understanding the basis of differential inhibition and the role of water

BackgroundThis work represents an extensive MD simulation / water-dynamics studies on a series of complexes of inhibitors (leupeptin, E-64, E-64-C, ZPACK) and plant cysteine proteases (actinidin, caricain, chymopapain, calotropin DI) of papain family to understand the various interactions, water binding mode, factors influencing it and the structural basis of differential inhibition.ResultsThe tertiary structure of the enzyme-inhibitor complexes were built by visual interactive modeling and energy minimization followed by dynamic simulation of 120 ps in water environment. DASA study with and without the inhibitor revealed the potential subsite residues involved in inhibition. Though the interaction involving main chain atoms are similar, critical inspection of the complexes reveal significant differences in the side chain interactions in S2-P2 and S3-P3 pairs due to sequence differences in the equivalent positions of respective subsites leading to differential inhibition.ConclusionThe key finding of the study is a conserved site of a water molecule near oxyanion hole of the enzyme active site, which is found in all the modeled complexes and in most crystal structures of papain family either native or complexed. Conserved water molecules at the ligand binding sites of these homologous proteins suggest the structural importance of the water, which changes the conventional definition of chemical geometry of inhibitor binding domain, its shape and complimentarity. The water mediated recognition of inhibitor to enzyme subsites (Pn...H2O....Sn) of leupeptin acetyl oxygen to caricain, chymopapain and calotropinDI is an additional information and offer valuable insight to potent inhibitor design.

Structural Basis of the Drastically Increased Initial Electron Transfer Rate in the Reaction Center from a Rhodopseudomonas viridis Mutant Described at 2.00-Å Resolution

It has previously been shown that replacement of the residue His L168 with Phe (HL168F) in the Rhodopseudomonas viridis reaction center (RC) leads to an unprecedented drastic acceleration of the initial electron transfer rate. Here we describe the determination of the x-ray crystal structure at 2.00-A resolution of the HL168F RC. The electron density maps confirm that a hydrogen bond from the protein to the special pair is removed by this mutation. Compared with the wild-type RC, the acceptor of this hydrogen bond, the ring I acetyl group of the "special pair" bacteriochlorophyll, D(L), is rotated, and its acetyl oxygen is found 1.1 A closer to the bacteriochlorophyll-Mg(2+) of the other special pair bacteriochlorophyll, D(M). The rotation of this acetyl group and the increased interaction between the D(L) ring I acetyl oxygen and the D(M)-Mg(2+) provide the structural basis for the previously observed 80-mV decrease in the D(+)/D redox potential and the drastically increased rate of initial electron transfer to the accessory bacteriochlorophyll, B(A). The high quality of the electron density maps also allowed a reliable discussion of the mode of binding of the triazine herbicide terbutryn at the binding site of the secondary quinone, Q(B).

Thermodynamic origin of cis/trans isomers of a proline-containing beta-turn model dipeptide in aqueous solution: a combined variable temperature 1H-NMR, two-dimensional 1H,1H gradient enhanced nuclear Overhauser effect spectroscopy (NOESY), one-dimensional steady-state intermolecular 13C,1H NOE, and molecular dynamics study.

The cis/trans conformational equilibrium of the two Ac-Pro isomers of the beta-turn model dipeptide [13C]-Ac-L-Pro-D-Ala-NHMe, 98% 13C enriched at the acetyl carbonyl atom, was investigated by the use of variable temperature gradient enhanced 1H-nmr, two-dimensional (2D) 1H,1H nuclear Overhauser effect spectroscopy (NOESY), 13C,1H one-dimensional steady-state intermolecular NOE, and molecular dynamics calculations. The temperature dependence of the cis/trans Ala(NH) protons are in the region expected for random-coil peptides in H2O (delta delta/delta T = -9.0 and -8.9 ppb for the cis and trans isomers, respectively). The trans NH(CH3) proton indicates smaller temperature dependence (delta delta/delta T approximately -4.8 ppb) than that of the cis isomer (-7.5 ppb). 2D 1H,1H NOESY experiments at 273 K demonstrate significant NOEs between ProH alpha-AlaNH and AlaNH-NH(R) for the trans isomer. The experimental NOE data, coupled with computational analysis, can be interpreted by assuming that the trans isomer most likely adopts an ensemble of folded conformations. The C-CONH(CH3) fragment exhibits significant conformational flexibility; however, a low-energy conformer resembles closely the beta II-turn folded conformations of the x-ray structure of the related model peptide trans-BuCO-L-Pro-Me-D-Ala-NHMe. On the contrary, the cis isomer adopts open conformations. Steady-state intermolecular solute-solvent (H2O) 13C,1H NOE indicates that the water accessibility of the acetyl carbonyl carbons is nearly the same for both isomers. This is consistent with rapid fluctuations of the conformational ensemble and the absence of a highly shielded acetyl oxygen from the bulk solvent. Variable temperature 1H-nmr studies of the cis/trans conformational equilibrium indicate that the trans form is enthalpically favored (delta H degree = -5.14 kJ mole-1) and entropically (delta S degree = -5.47 J.K-1.mole-1) disfavored relative to the cis form. This demonstrates that, in the absence of strongly stabilizing sequence-specific interresidue interactions involving side chains and/or charged terminal groups, the thermodynamic difference of the cis/trans isomers is due to the combined effect of intramolecular and intermolecular (hydration) induced conformational changes.

Migratory insertion reactions of norbornene, norbornadiene and 7-oxanorbornene derivatives into the palladium–carbon bond of heterodinuclear Fe–Pd–Acyl complexes. Substrate influence on the Pd coordination sphere and crystal structure of [(OC)3Fe{μ-Si(OMe)2(OMe)}(μ-dppa)Pd{C9H9NO3C(O)Me}

[(OC)3Fwe{µ-Si(OMe)2(OMe)}(µ-dppm)Pzd{C(2O)Me}] (1a) (dppm=Ph2PCH2PPh2) to afford the heterodinuclear complexes [(OC)3{(MeO)3Si}Fwe(µ-dppm)Pzdw{C7H10C(2Oz)Me}] (2) and [(OC)3{(MeO)3Si}- Fwe(µ-dppm)Pzdw{C7H8C(2Oz)Me}] (3), respectively. In these insertion products, the alkoxysilyl ligand has become terminally bound whereas a dative bond between the acetyl oxygen and the metal allows the square-planar Pd centre to achieve a 16e configuration, thus resulting in a five-membered Pwd–C–C–C–Oz ring. This chemistry was extended to chiral olefins and when the dppm and dppa complexes 1a,b were reacted with a 9:1 mixture of (±)-(2-cyano-7-oxabicyclo[2.2.1]hept-5-ene-2-endo-yl and -2-exo-yl acetate (4,4′), the stable insertion products [(OC)3Fwe{µ-Si(OMe)2(OMe)}(µ-Ph2PXPPh2)Pzdw{C9H9NO3C(2Oz)Me}] (5a,5a′ X=CH2; 5b,5b′ X=NH) were isolated in high yield. A high stereo- and regioselectivity was found for these metallacarbonylations. Spectroscopic data as well as X-ray diffraction studies show that in contrast to 2 and 3, the µ2-η2-Si–O interaction present in the precursors 1a,b is retained in 5a,b. The Pd atom adopts a pseudo square-pyramidal coordination resulting from an additional weak ketonic interaction with the 5-exo-acetyl group of the 2-endo-acetoxy-2-cyano-7-oxabicyclo[2.2.1]hept-5-exo-yl moiety, which leads to the formation of a five-membered chelate ring. Electronic effects are invoked to account for these different structural features. Under a carbon monoxide atmosphere, CO migratory insertion into the Pd–oxanorbornyl bond of pure 5a,b occurs and leads to the acyl complexes [(OC)3Fwe{µ-Si(OMe)2(OMe)}(µ-Ph2PXPPh2)Pzd{C(2O)C9H9NO3C(2O)Me}] (6a X=CH2; 6b X=NH). These successive insertion reactions constitute the first elementary steps on the way to polyketone chain growth mediated by a bimetallic system.

Crystal Structure and Molecular Conformation of 4-Thiouracil-2′-Trifluorothioacetamide -3′, 5′-Diacetyl-β-D-Riboside

4-thiouracil-2′-trifluorothioacetamide-3′, 5′-diacetyl-β-D-riboside is one of the modified thiouracil analogs synthesized in our institute. The determination of the crystal and molecular structure of this compound was carried out with a view to study the conformation of the molecule in the solid state as well as to investigate the conformations of the trifluoroacetamide and the acetyl substituents of the ribose and their effects on the conformation of the ribose ring. Crystals of 4-thiouracil-2′-trifluorothioacetamide-3′,5′- diacetyl-β-D-riboside are orthorhombic, space group P21 21 21, with cell dimensions a= 15.351 (2), b= 15.535 (1), c= 8.307 (1) Å, V=1981.0 (7) Å3, Z=4, Dm= 1.53, Dc=1.527 g/c.c. and μ=30.1cm -1. The structure was determined using CuKα (λ, =1.5418 Å) at a temperature T of 297K, with 2333 reflections, which were collected on a Enraf-Nonius CAD-4 diffactometer, out of which 2249 (I ≥20) were considered observed. The structure was determined by direct methods using MULTAN and refined by full matrix least squares method to a final reliability factor of 0.054 and a weighted R factor of 0.079. The nucleoside is in the anti conformation [XCN =51.4 (5)°], the ribose has the unusual C (2′) endo -C (1′) exo (2T1), and a g+ conformation [ψ=47.5 (4)] across C(4′)-C(5′) bond. The pseudorotation angle P is 152.8 (4) ° and the amplitude of pucker τm of 42.7 (3)°. The average C-F bond distance is 1.308 Å. There is no base pairing and the typical base-base hydrogen bonded interactions are not present in this structure. On the other hand, a hydrogen bonded dimer is formed involving C(3′) - H(3′)… O (2) and N(3) -H (N3) … O (Al) hydrogen bonds joining the base, ribose ring and the acetyl group. The trend towards longer exocyclic bonds at the acetyl centers in compounds with strongly electronegative aglycones, is also exhibited in this compound, with C(3′)-O(3′) and C(5′)-0(5′) being much longer than C(1′)-O(4′). The acetyl groups also take part in C-H…O hydrogen bonding with the acetyl oxygen atom OA2.

Ab initio calculations on blocked Pro-Ala dipeptides

The effect of the chirality of the amino acid at position i + 2 on a β-turn was investigated by a grid scan ab initio calculation on the Ac- l -Pro- l -Ala-NH 2 and Ac- l -Pro- d -Ala-NH 2 blocked dipeptides. Th6–31G basis set was used to estimate the effect of the alanyl side chain on the conformation of the peptide backbone in a blocked dipeptide as a simple, but complete model for a reverse turn. This study provides a quantum mechanical evaluation of the ability of the NH at the i + 3 residue to form the Hbond that closes the 10 membered ring which stabilizes the turn. The lowest energy of all 64 probed conformations of the D -Ala containing peptide corresponded to a good type II β-turn with a hydrogen bond distance between the acetyl oxygen and the amide terminal hydrogen of 2.21 Å. A comparison with the nonblocked dipeptide ab initio study indicates that the presence of the end blocks enhances the propensity of the d -Ala-containing dipeptide for a type II β-turn, but does not seem to enhance the propensity of the l -Ala-containing dipeptide for a type I β-turn. The energies and geometric parameters for the lowest four optimized conformations identified by the grid scan search for each molecule have been calculated.

Ab initio calculations on blocked Pro-Ala dipeptides

The effect of the chirality of the amino acid at position i + 2 on a β-turn was investigated by a grid scan ab initio calculation on the Ac- l-Pro- l-Ala-NH 2 and Ac- l-Pro- d-Ala-NH 2 blocked dipeptides. Th6–31G basis set was used to estimate the effect of the alanyl side chain on the conformation of the peptide backbone in a blocked dipeptide as a simple, but complete model for a reverse turn. This study provides a quantum mechanical evaluation of the ability of the NH at the i + 3 residue to form the Hbond that closes the 10 membered ring which stabilizes the turn. The lowest energy of all 64 probed conformations of the D-Ala containing peptide corresponded to a good type II β-turn with a hydrogen bond distance between the acetyl oxygen and the amide terminal hydrogen of 2.21 Å. A comparison with the nonblocked dipeptide ab initio study indicates that the presence of the end blocks enhances the propensity of the d-Ala-containing dipeptide for a type II β-turn, but does not seem to enhance the propensity of the l-Ala-containing dipeptide for a type I β-turn. The energies and geometric parameters for the lowest four optimized conformations identified by the grid scan search for each molecule have been calculated.

Binickel(II) complex of a new asymmetrical acyclic ligand: the X-ray crystal structure of [Ni 2L](ClO 4) 2 reveals alternating square-planar and octahedral nickel ions

The binuclear nickel(II) complex of the macrocyclic ligand H 2L which contains one N(CH 2) 2N and one N(CH 2) 2NH– moiety has been studied. In the mixed spin-state complex [Ni 2L][ClO 4] 2 ( 1), one nickel center is square-planar (low spin), accommodated by the smaller N 2O 2 cavity provided by the phenolic oxygens and N(CH 2) 2N nitrogens; the other nickel center is octahedral (high spin), accommodated by the larger cavity provided by the phenolic oxygens and N(CH 2) 2NH– nitrogens. Diethylenetriamine takes the skew configuration, which inhibits the full cyclization. This leads to one primary amino nitrogen and one acetyl oxygen on both sides of the octahedral nickel completing the fifth and sixth coordination sites. The Pauling covalent radius of low-spin Ni 2+ is ca. 0.20 Å shorter than that of high-spin Ni 2+, leading to considerably stronger in-plane ligand-field strength of N 2O 2 donors at the square nickel(II) relative to that of the octahedral nickel(II). Crystallographic data for ( 1): space group P-1, triclinic, a=11.573(2) Å, b=15.608(3) Å, c=10.943(2) Å, α=104.54(2)°, β=114.97(1)°, γ=82.62(1)°, Z=2, R=0.061, R w=0.079.

17O NMR Studies of the Solvation State of cis/trans Isomers of Amides and Model Protected Peptides

17O shielding constants have been utilized to investigate solvation differences of the cis/trans isomers ofN-methylformamide (NMF),N-ethylformamide (NEF), andtert-butylformamide (TBF) in a variety of solvents with particular emphasis on aqueous solution. Comparisons are also made with protected peptides of the formulas CH3CO–YOH, CH3CO–Y–NHR (Y = Pro, Sar), and CH3CO–Y–Z–NHR (Y = Pro; Z =D-Ala) selectively enriched in17O at the acetyl oxygen atom. Hydration at the amide oxygen induces large and specific modifications of the17O shielding constants, which are practically the same for the cis and trans isomers of NMF, NEF, and the protected peptides. Fortert-butylformamide, the strong deshielding of the trans isomer compared to that of the cis isomer may be attributed to an out-of-plane (torsion-angle) deformation of the amide bond and/or a significant reduction of solvation of the trans isomer due to steric inhibition of the bulkytert-butyl group. Good linear correlation between δ(17O) of amides and δ(17O) of acetone was found for different solvents which have varying dielectric constants and solvation abilities. Sum-over-states calculations, within the solvaton model, underestimate effects of the dielectric constant of the medium on17O shielding, while finite-perturbation-theory calculations give good agreement with the experiment.

Free Energy Computations on the Shift of the Special Pair Redox Potential: Mutants of the Reaction Center of Rhodobacter sphaeroides

Shifts of the special pair redox potential of photosynthetic reaction centers of Rhodobacter sphaeroides for five different mutants are considered. The shifts are calculated by the method of free energy perturbation which is based on dynamics simulation data. The influence of the orientation and the torsion potential of the acetyl group on the shift of the redox potential is investigated and compared with recent results obtained by solving Poisson's equation. In the oxidized state of the special pair the acetyl group at DL reorients in the initial part of the dynamics simulation. As a consequence, the hydrogen bond with H(L168) breaks and the oxygen atom binds to the Mg2+ ion of DM. For the mutant FY(M197) there is evidence that the acetyl oxygen atom of DM binds to the Mg2+ ion of DL instead of forming a hydrogen bond with Y(M197).

Synthesis of monomercurated N-protected pyrroles and the use of these compounds as pyrrolyl group transfer reagents to ruthenium and osmium. Crystal structures of (2-C 4H 3NC[O]CH 3)HgCl, (2-C 4H 3NC[O]CH 3) 2 Hg and

N-Acetylpyrrole and N-phenylsulfonylpyrrole are each selectively mercurated in the 2-position on treatment with mercuric chloride. The resulting organomercuric chlorides are symmetrized to the corresponding diorganomercury derivatives (2-C 4H 3NC[O]CH 3) 2Hg ( 2) and (2-C 4H 3NSO[O]Ph) 2Hg ( 4) by the action of sodium iodide. The crystal structures of (2-C 4H 3NC[O]CH 3HgCl ( 1) and (2-C 4H 3NC[O]CH 3) 2Hg ( 2) have been determined. The acetyl oxygen atom interacts weakly with the mercury centre in both cases, but more strongly in 1 than 2. Treatment of the hydrido complexes MHCl(CO)(PPh 3) 3 (M  Ru, Os) with the diorganomercurial 2 or 4 affords in good yield the 2-pyrrolyl complexes ▪ CH 3)Cl(CO)(PPh 3) 2 or ▪, respectively. In each case the pyrrolyl group is coordinated in a bidentate fashion through C-2 and an oxygen atom of the N-substituent. The chloride ion in ▪ ▪( 7) is labile, and upon treatment with AgSbF 6 and CO, the cationic complex ▪ ▪ is formed. Treatment of 7 with AgSbF 6 and sodium dimethyldithiocarbamate gives the neutral monophosphine complex ▪ ( 10), the crystal structure of which has been determined.

Semiempirical and ab initio transition state calculations for the transformation of N-acetyltetrazoles into corresponding 1,3,4-oxadiazoles

The transformation of 2-acetyl-5-substituted-tetrazoles into the corresponding 1,3,4-oxadiazoles was studied with the mopac semiempirical and gaussian ab initio methods. Two mechanisms, one with two transition states and the other with three, were elucidated by mopac. The first mechanism supported by PM3 and MNDO has a two-step, almost concerted, mechanism for the elimination of a nitrogen molecule from the tetrazole ring and formation of the oxadiazole product from an open-chain intermediate through carbon C 5 and acetyl oxygen bond formation. The second mechanism supported by AM1 and MINDO/3 breaks the elimination of the nitrogen molecule into two steps: first breaking the N 4-C 5 and then the N 2-N 3 bonds. Even when the AM1 and MINDO/3 transition state structures were optimized by PM3 and MNDO, the obtained transition states present only one bond breaking. The HF/STO-3G and HF/3-21G ab initio methods agree with the first mechanism where two bonds are breaking almost simultaneously. Despite the disagreement in the mechanism of the nitrogen elimination, the transition state that presents the product formation from open-chain intermediates is quite similar for all methods studied. The semiempirical calculation of this transition state is possible only if it is assumed that it has biradical character. The activation energies calculated by PM3 seem to be insensitive to the nature of the substituents.

Spiropyrazolines from tandem reaction of azides and alkyl vinyl ketones

Spiropyrazolines are obtained from the the tandem reactions of one molecular of azide and three molecules of alkyl vinyl ketone. There are two types of structures in these products: One of them has an intramolecular hydrogen bond the hydroxyl proton and the acetyl oxygen, while the other has none.

Optically active transition-metal compounds. 93. X-ray determination of the structure and absolute configuration of (+) 578-C 5H 5Fe(CO) [P(C 6H 5) 3] COCH 3

The structure and absolute configuration of (+) 578- C 5H 5Fe(CO)[P(C 6H 5) 3]COCH 3 have been determined by single crystal X-ray diffraction methods. The substance crystallizes in the monoclinic space group P2 1 with cell constants of a = 8.084(14), b = 8.527(2), c32.706(21) Å and β = 104.32(10)°; V 2184.18 Å 3 and D(calc: Z 4 mol/unit cell) = 1.381 g cm -3. There are two independent molecules in the asymmetric unit, which allowed us to gauge the effect of packing on the conformation of those groups able, in principle, to be twisted by crystalline forces. Only minor changes in conformations were observed, the largest being at the terminal CH 3 of the acetyl ligand (0.065 Å). All other differences in conformation are less than 0.036 Å. The plane of the acetyl ligand is close to being aligned with the FeC(CO) bond, making the acetyl oxygen point in the direction of the phosphorus atom. It is suggested that in phosphine exchange reactions this conformation persists in solution while the acetyl oxygen, intra- molecularly, attacks the adjacent phosphorus atom to form a dihapto acetyl species as the first intermediate, in which there is retention of configuration at Fe. With the priority of the ligand sequence as C 5H 5 > P(C 6H 5) 3 > CO > COCH 3, the absolute configuration at Fe is (S). So, the formation of (−) 578- C 5H 5Fe(CO)[P(C 6H 5) 3]COCH 3 by reaction of (+) 578- C 5 H 5 Fe(CO)[P(C 6H 5) 3] COOC 10H 19 and LICH 3 requires an inversion to occur at the Fe center.

The Crystal and Molecular Structure of [Pd(tfac)(tfac-O){P(o-tolyl)3}] Containing an O-Unidentate 1,1,1-Trifluoro-2,4-pentanedionate Anion as a Ligand

Abstract The title complex crystallizes in the monoclinic space group C2/c with a=24.900(6) Å, b=8.272(2) Å, c=30.437(9) Å, and β=97.23(2)°. The structure analysis, which converged at R=0.061 and Rw=0.083 with 3017 reflections for which Fo2>3σ(Fo2), showed that the palladium has a complete square-planar coordination geometry. While one of the tfac ligands is chelating, the other is bound to the metal only through the carbonyl oxygen adjacent to CH3. The O-unidentate tfac ligand lies trans to the CF3CO moiety of chelating tfac and has a planar structure in which the dangling CF3CO group is situated trans to the bonding oxygen atom with respect to the C=C bond. The ab initio MO calculation for the Z,Z-form of the tfac anion indicated that the coordination ability of the acetyl oxygen is higher than that of the trifluoroacetyl oxygen in consistence with the observed structure.

Elementary steps in the reaction mechanism of chicken liver fatty acid synthase: acetylation-deacetylation.

The kinetics of the reaction of acetyl coenzyme A (AcCoA) with fatty acid synthase has been studied with a modified quenched-flow technique in 0.1 M potassium phosphate (pH 7.0), 0.5 mM ethylenediaminetetraacetic acid, and 10% glycerol (w/v) at 23 degrees C. The kinetics of the deacetylation of the isolated acetylated enzyme by CoA also was studied. An overall mechanism consistent with the data is (formula; see text) where E represents the enzyme. The equilibrium dissociation constants, K1 and K3, were estimated to be 85 and 70 microM, respectively, and the rate constants k2 and k-2 are 43 and 103 s-1, respectively. The maximum number of acetyl groups bound to the enzyme in terms of this mechanism is 3.8 (mol/mol). This mechanism also is consistent with the amount of acetylated enzyme formed during titrations of the enzyme and radioactive AcCoA with CoA. The spontaneous hydrolysis of the enzyme at 23 degrees C has a rate constant of 4.7 X 10(-4) s-1. The acetyl groups on the native enzyme are rapidly removed by hydroxylamine. However, 0.39 of the acetyl groups remains after treatment with hydroxylamine if the enzyme is first denatured in 4 M urea. This suggests that the acyl binding sites on the native enzyme are an unstable acetyl oxygen ester and an acetyl thio ester. Destruction of the thioesterase activity of the enzyme through chemical modification of the enzyme does not alter the rate of spontaneous hydrolysis of the acetyl-enzyme nor its reactivity toward hydroxylamine.