Nitrate requirement for acetylene inhibition of nitrous oxide reduction in marine sediments

The inhibition of nitrous oxide (N₂O) reduction by acetylene (C₂H₂) in saltmarsh sediment was temporary; we investigated this phenomenon and possible causes. The reduction of N₂O in the presence of C₂H₂ was biological. N₂O consumption in the presence of C₂H₂ began when nitrate concentration became very low. The time course of N₂O consumption after periods of N₂O accumulation was unaffected by initial nitrate concentrations between 16 and 200M, or C₂H₂ concentrations between 10 and 100% of the gas phase. Sulfide had no effect on the kinetics of N₂O reduction in the presence of C₂H₂. In more dilute slurries of saltmarsh sediments and in estuarine sediment, N₂O persisted in the presence of C₂H₂ unless sufficient organic carbon was added to deplete nitrate. In saltmarsh sediments, the rate of N₂O consumption in the presence of C₂H₂ was not changed by preincubation with C₂H₂. Initial positive rates of N₂O production in the presence of C₂H₂ occurred only when the block was apparently effective (i.e., at nitrate concentrations greater than about 5–10M) and appeared to represent a valid estimate of denitrification. Conversely, and in agreement with previous studies, concentrations of NO₃ ^– below these levels resulted in reduced efficiency of C₂H₂ blockage of N₂O reductase.     

Calculations of one-, two- and three-bond nuclear spin-spin couplings in a model peptide and correlations with experimental data

Summary We present ab initio calculations of the Fermi contact term and experimental correlations of six coupling constants, ³J_H ^N _H , ¹J_{C H} , ²J_CH , ¹J_{C N}, ²J_{C N} and ¹J_CN, in a peptide as functions of the backbone dihedral angles, and . Given estimates of experimental uncertainties, we find semiquantitative experimental correlations for ³J_H ^N _H , ¹J_{C N} and ²J_{C N}, qualitative correlations for ¹J_{C H} and ²J_CH , but no experimental correlations of practical utility for ¹J_CN, owing to its complex dependence on at least four dihedral angles. Errors in the estimation of dihedral angles from X-ray crystallographic data for proteins, which result from uncertainties in atom-to-atom distances, place substantial limitations on the quantitative reliability of coupling constant calculations fitted to such data. In the accompanying paper [Edison, A.S. et al., J. Biomol. NMR, 4, 543–551] we apply the results of the coupling constant calculations presented here to the estimation of and angles in staphylococcal nuclease from experimental coupling constants.Abbreviations AO atomic orbital - BPTI basic pancreatic trypsin inhibitor (bovine) - CI-2 chymotrypsin inhibitor 2 - E.COSY exclusive correlation spectroscopy (Griesinger et al., 1986) - ⁿJ_AB single bond (n=1), geminal (n=2), or vicinal (n=3) coupling constant between nuclei A and B - LCAO linear combination of atomic orbitals - NBO natural bond orbital - n lone pair orbitals - bonding orbitals - ^* antibonding orbitals - dihedral angle or molecular orbital wave function; r², correlation coefficient - RHF restricted Hartree-Fock; rmsd, root-mean-square deviation - 3-21G and 6-31G^* molecular orbital basis set designations (Hehre et al., 1986)     

Rhodium and iridium olefin complexes with bulky cyclopentadienyl and hydrotris(pyrazolyl)borate ligands

The synthesis of a series of rhodium and iridium complexes bearing bulky cyclopentadienyl or hydro(trispyrazolyl)borate ligands is described. The rhodium cyclopentadienyl and hydrotris(pyrazolyl)borate diene compounds, [(η⁵-C₅Me₄Bu^t)Rh(η⁴-2,3-MeRC₄H₄] (R = H, 1; Me, 2) and Tp^MsRh(η⁴-2,3-MeRC₄H₄) (R = H, 3; Me, 4; Tp^Ms is hydrotris(3-mesitylpyrazol-1-yl)borate), respectively, have been prepared from the corresponding Rh(I) diene precursors and Zn(C₅Me₄Bu^t)₂ (for 1 and 2), or TlTp^Ms (for 3 and 4), as effective C₅Me₄Bu^t or Tp^Ms transfer reagents. In contrast with these results, attempts to obtain a bis(ethylene) derivative of the Tp^tolIr(I) unit (Tp^tol stands for hydrotris(3-p-tolylpyrazol-1-yl)borate) have provided instead the Ir(III) complex [(κ⁴-N,N′,N″,C-Tp^tol)-Ir(C₂H₅)(C₂H₄)] (5), whose formation requires C-H bond activation of a molecule of ethylene and of one of the Tp^tolp-tolyl substituents. In refluxing toluene 5 experiences metalation of a second p-tolyl substituent to give [(κ⁵-N,N′,N″,C,C′-Tp^tol)-Ir(C₂H₄)] (6), which features unusual κ⁵-Tp^tol coordination. The latter compound reacts with carbon monoxide to yield the corresponding carbonyl, 7.     

Rigid-rod structured palladium complexes

Mononuclear and homobimetallic palladium complexes of structural type [trans-(Me(O)CS-4-C₆H₄)(Ph₃P)₂Pd(N^∩N)]OTf (8a, N^∩NC₄H₄N₂; 8b, N^∩NC₅H₄N-4-CN) and {[trans-(Me(O)CS-4-C₆H₄)(Ph₃P)₂Pd]₂N^∩N}(OTf)₂ (9a, N^∩N = 4,4′-bipyridine (=bipy); 9b, N^∩N = C₆H₄-1,4-(CN)₂; 9c, N^∩N = (C₆H₄-4-CN)₂) are accessible by the reaction of trans-(Ph₃P)₂Pd(C₆H₄-4-SC(O)Me)(OTf) (6) with 1 or 0.5 equivalents of the Lewis-bases N^∩N (7a, N^∩N = C₄H₄N₂; 7b, N^∩N = C₅H₄N-4-CN; 7c, N^∩N = bipy; 7d, N^∩N = C₆H₄-1,4-(CN)₂; 7e, N^∩N = (C₆H₄-4-CN)₂) in high yield. Complex 6 can be prepared in a two-step synthesis procedure. Oxidative addition of I-1-C₆H₄-4-SC(O)Me (2) to Pd(PPh₃)₄ (3) gives trans-(Ph₃P)₂Pd(C₆H₄-4-SC(O)Me)(I) (4), which further reacts with [AgOTf] (5) to afford 6.The formation of 8 and 9 strongly depends on the size of the Lewis-bases N^∩N. It is obvious that the co-ordination of the second N-ligated site of 8a or 8b to a further bulky[(PPh₃)₂Pd(C₆H₄-4-SC(O)Me)]⁺ unit is not possible. In contrast, more extended N^∩N species such as 7c-7e will result in the formation of linear structured homobimetallic 9a-9c.The solid-state structures of 4 and 4 · CH₂Cl₂ are reported. Complex 4 is packed in the orthorhombic space group Pbca. The assembly of dichloromethane into the crystal lattice breaks the symmetry, whereby 4 · CH₂Cl₂ crystallises in the triclinic space group . In both modifications a square-planar palladium(II) ion is present, with the iodo atom and the Me(O)CS-C₆H₄ unit trans-positioned. The different crystal packing has no significant influence onto the geometry around the d⁸-configurated palladium atoms.     

Nitrogen-fixing bacillus species from Egyptian soils: Acetylene reduction and cultural conditions

Summary Among 390 isolates from Egytiian soils initially grown on Brown's N-free agar, 15 facultative Bacillus isolates were able to reduce acetylene in Stanier's N-poor broth under both aerobic and anaerobic (N₂ atmosphere) conditions. Some of these isolates were Gram-positive, with unswollen sporangia and thin-walled endospores. Other strains were with slightly or definitely bulged sporangia. Yeast extract (0.01%) was essential for growth stimulation and N₂[C₂H₂] fixation by these isolates. Replacing yeast extract with 20 g/ml (NH₄)₂SO₄ or biotin, thiamine and amino acids (singly or in combination) resulted in stimulation of growth and N₂[C₂H₂] fixation, though at lower rates than in yeast extract.One isolate was able to grow and reduce C₂H₂ in Stanier's N-free liquid medium. Nitrogenase [C₂H₂] activity of the anaerobically grown and incubated cultures was greater than aerobic cultures. Addition of 0.1% CaCO₃ to the culture media significantly increased and O₂ partially inhibited, N₂[C₂H₂] fixation by these Bacillus isolates.Studies of the characteristics and N₂[C₂H₂] fixing activities of these isolates indicate that at least some of them are new nitrogen-fixingBacillus species.     

Tetrahydrofolate-specific enzymes in <Emphasis Type="Italic">Methanosarcina barkeri</Emphasis> and growth dependence of this methanogenic archaeon on folic acid or <Emphasis Type="Italic">p</Emphasis>-aminobenzoic acid

Methanogenic archaea are generally thought to use tetrahydromethanopterin or tetrahydrosarcinapterin (H₄SPT) rather than tetrahydrofolate (H₄F) as a pterin C₁ carrier. However, the genome sequence of Methanosarcina species recently revealed a cluster of genes, purN, folD, glyA and metF, that are predicted to encode for H₄F-specific enzymes. We show here for folD and glyA from M. barkeri that this prediction is correct: FolD (bifunctional N⁵,N¹⁰-methylene-H₄F dehydrogenase/N⁵,N¹⁰-methenyl-H₄F cyclohydrolase) and GlyA (serine:H₄F hydroxymethyltransferase) were heterologously overproduced in Escherichia coli, purified and found to be specific for methylene-H₄F and H₄F, respectively (apparent K_m below 5 M). Western blot analyses and enzyme activity measurements revealed that both enzymes were synthesized in M. barkeri. The results thus indicate that M. barkeri should contain H₄F, which was supported by the finding that growth of M. barkeri was dependent on folic acid and that the vitamin could be substituted by p-aminobenzoic acid, a biosynthetic precursor of H₄F. From the p-aminobenzoic acid requirement, an intracellular H₄F concentration of approximately 5 M was estimated. Evidence is presented that the p-aminobenzoic acid taken up by the growing cells was not required for the biosynthesis of H₄SPT, which was found to be present in the cells at a concentration above 3 mM. The presence of both H₄SPT and H₄F in M. barkeri is in agreement with earlier isotope labeling studies indicating that there are two separate C₁ pools in these methanogens.     

Regulation of growth in stem sections of deep-water rice

Submergence in water greatly stimulates internodal elongation in excised stem sections of deep-water rice (Oryza sativa L. cv. Habiganj Aman II) and inhibits growth of leaf blades and leaf sheaths. The highest rates of internodal growth have been observed in continuous light. Very little growth occurs in submerged sections kept in darkness or incubated under N₂ in the light. The effect of submergence on the growth of deep-water rice is, at least in part, mediated by C₂H₄, which accumulates in the air spaces of submerged sections. This accumulation results from increased C₂H₄ synthesis in the internodes of submerged sections and reduced diffusion of C₂H₄ from the tissue into the water. Increased C₂H₄ levels accelerate internodal elongation and inhibit the growth of leaves. Compounds capable of changing the rate of C₂H₄ synthesis, namely aminoethoxyvinylglycine, an inhibitor of C₂H₄ synthesis, and 1-aminocyclopropane-1-carboxylic acid, the immediate, precursor of C₂H₄, have opposite effects on growth of internodes and leaves. The enhancement of internodal elongation by C₂H₄ is particularly pronounced in an atmosphere of high CO₂ and low O₂. The increase in C₂H₄ synthesis in internodes of submerged sections is primarily triggered by reduced atmospheric concentrations of O₂. The rate of C₂H₄ evolution by internodes isolated from stem sections and incubated in an atmosphere of low O₂ is up to four times greater than that of isolated internodes incubated in air. In contrast, C₂H₄ evolution from the leaves is reduced under hypoxic conditions. The effect of submergence on growth of stem sections of deep-water rice can be mimicked by exposing non-submerged sections to a gas mixture which is similar to the gaseous atmosphere in the internodal lacunae of submerged sections, namely 3% O₂, 6% CO₂, 91% N₂ (by vol.) and 1 l l^-1 C₂H₄. Our results indicate that growth responses obtained with isolated rice stem sections are similar to those of intact deep-water rice plants.Abbreviations ACC 1-aminocyclopropane-1-carboxylic acid - AVG aminoethoxyvinylglycine     

Intramolecular proton transfer in adenine imino tautomers

The electronic structural impact on intramolecular proton transfer in the cis- and trans-imino N7 and N9 tautomers of adenine (A) has been studied quantum mechanically, using density functional theory (B3LYP/TZVP, SAOP/TZ2P, LB94/TZ2P) and Green function (OVGF/TZVP) models. It is found that proton transfer does not significantly change isotropic properties but has profound impact on electron distributions of the species through anisotropic properties. The relative energies with respect to the canonical A tautomer (amino-9H), ΔE, for imino 7Hcis, imino 7Htrans, imino 9Hcis and imino 9Htrans are calculated as 16.15, 16.43, 18.46 and 13.80 kcal mol^? 1 (B3LYP/TZVP model) and some minor changes in perimeters of the purine ring is also observed. The Hirshfeld atomic charges indicate that whether a proton attached to N₍₇₎ or N₍₉₎ causes a significant local charge redistribution. However, these charges are insensitive to cis–trans proton transfer. Condensed Fukui function reveals N₍₁₀₎ and C₍₈₎ as the most electrophilic reactive site among N- and C-atom sites, respectively. We also found that proton transfer significantly alters in-plane σ orbitals, rather than out of plane π orbitals including the frontier orbital 6a″. Moreover, orbital based responses to various proton transfers are presented: the orbital 29a′ (HOMO-1) is a signature orbital differentiating all the four tautomers. Orbital 27a′ is a site (N₍₇₎ and N₍₉₎) sensitive orbital, whereas orbital 22a′ is only sensitive to proton orientation on the imino group = N–H.     

New open-framework phosphate and phosphite compounds of gallium

Five new open-framework compounds of gallium have been synthesized by hydrothermal methods and their structures determined by single crystal X-ray diffraction studies. The compounds, [C₈N₄H₂₆][Ga₆F₄(PO₄)₆], I, [C₅N₃H₁₁][Ga₃F₂(PO₄)₃]·H₂O, II, [C₆N₃H₁₉][Ga₄(C₂O₄)(PO₄)₄(H₂PO₄)]·2H₂O, III, [Ga₂F₃(HPO₄)(PO₄)]·2H₃O, IV, and [C₃N₂H₅]₂[Ga₄(H₂O)₃(HPO₃)₇], V, possess three-dimensional structures. All the compounds are formed by the connectivity between the Ga polyhedra and phosphite/phosphate units. The observation of SBU-6 (I and II) and spiro-5 (IV) secondary building units (SBUs) are noteworthy. The flexibility of the formation of gallium phosphate frameworks has been established by the isolation of two related structures (I and II) from the same SBU units but different organic amines. Some of the present structures have close resemblance to the gallium phosphate phases known earlier. The compounds have been characterized by CHN analysis, powder XRD, IR, and TGA.     

Synthesis and theoretical analysis of the structures and bonding in five new neutral square planar bis[2,4-di(aryl)-1,3,5-triazapentadienato]nickel(II) complexes

Solvothermal reactions in methanol of nickel acetate tetrahydrate, Ni(OAc)₂ · 4H₂O, with benzonitrile derivatives NC(C₆H₄)X, where X is one of the electron withdrawing substituents -CN, -NO₂, or -CF₃, located at the m- or p-positions relative to -CN, yield complexes of the general formula Ni{HNC(R)-NC(R)-NH}₂. More specifically, 3-nitrobenzonitrile, 4-nitrobenzonitrile, 1,3-dicyanobenzene, 1,4-dicyanobenzene, and ααα-trifluoro-p-toluonitrile are found to react with Ni(OAc)₂ · 4H₂O to yield Ni{HNC(R)-NC(R)-NH}₂, where R = 3-(NO₂)C₆H₄, 4-(NO₂)C₆H₄, 3-(CN)C₆H₄, 4-(CN)C₆H₄, or 4-(CF₃)C₆H₄, respectively. Analogous reactions of nitriles lacking electron withdrawing groups do not occur under similar conditions. Solid-state structures have been determined for the complexes with p-NO₂, p-CN, and p-CF₃ substituents on the phenyl rings. In addition, we describe density functional theory (DFT) and natural bonding orbital theory (NBO) studies on a simplified analog of these compounds, aimed at understanding their molecular bonding. It is shown that the new compounds for which solid-state structures have been determined are model examples of coordination compounds containing robust ω-bonds.     

Supramolecular networks of transition metal sulfates templated by piperazine

Syntheses, structural studies from single-crystal X-ray diffraction and thermal behaviour of (C₄H₁₂N₂)[M^II(H₂O)₆](SO₄)₂ with M^II = Mn, Ni, Fe and Cu are reported. All compounds crystallise in monoclinic system, space group P2₁/n. The two isotypical compounds (C₄H₁₂N₂)[Mn(H₂O)₆](SO₄)₂ (I) and (C₄H₁₂N₂)[Ni(H₂O)₆](SO₄)₂ (II), are isostructural with the related cobalt and zinc phases, while the isotypical sulfates (C₄H₁₂N₂)[Fe(H₂O)₆](SO₄)₂ (III) and (C₄H₁₂N₂)[Cu(H₂O)₆](SO₄)₂ (IV) belong to another structure type. The three-dimensional structure networks for the four compounds consist of isolated [M^II(H₂O)₆]²⁺ and (C₄H₁₂N₂)²⁺ cations and (SO₄)²⁻ anions linked by hydrogen-bonds only. The thermal behaviour of the precursors has been studied by powder thermodiffractometry and thermogravimetric analyses. The first stages of dehydration are discussed with respect to the hydrogen bonds within the compounds.     

Reduction of structural dimensionality through incorporation of auxiliary ligands in lanthanide tetracyanoplatinates

The synthesis of a series of lanthanide tetracyanoplatinates containing the auxiliary ligands 1,10′-phenanthroline (phen) or 2,2′-bipyridine (bpy) have been carried out by reaction of Ln³⁺ nitrate salts with phen or bpy and potassium tetracyanoplatinate in solvent systems containing dimethylsulfoxide and dimethylformamide. The use of these solvents has lead to the isolation of [{Ln(DMSO)₂(C₁₂H₈N₂)(H₂O)₃}₂Pt(CN)₄](Pt(CN)₄)₂·2C₁₂H₈N₂·4H₂O (Ln = Eu (Eu-1), Tb (Tb-1), Yb(Yb-1)), [Ln(DMF)₃(C₁₂H₈N₂)(H₂O)₂NO₃]Pt(CN)₄ (Ln = La (La-2), Eu (Eu-2), Tb (Tb-2)), and [Ln(DMF)₃(C₁₀H₈N₂)(H₂O)₂NO₃]Pt(CN)₄ (Ln = La (La-3), Sm (Sm-3), Eu (Eu-3), Tb (Tb-3)) in the form of single crystals. Single-crystal X-ray diffraction has been used to investigate their structural features. The use of DMSO versus DMF as the solvent results in markedly different structural features. Eu-1 contains [{Eu(DMSO)₂(C₁₂H₈N₂)(H₂O)₃}₂Pt(CN)₄]²⁺ complex cations where the two Eu³⁺ centers are linked by a trans-bridging Pt(CN)₄²⁻ anion to form a dimeric lanthanide complex cation. An additional uncoordinated Pt(CN)₄²⁻ anion balances charge. Eu-2 and Eu-3 consist of zero-dimensional salts with [Eu(DMF)₃(C₁₂H₈N₂)(H₂O)₂(NO₃)]²⁺ or [Eu(DMF)₃(C₁₀H₈N₂)(H₂O)₂(NO₃)]²⁺ complex cations, respectively, and only non-coordinated Pt(CN)₄²⁻ anions. Photoluminescence measurements illustrate that the Eu³⁺ and Tb³⁺ compounds for all three structure types display enhanced emission due to intramolecular energy transfer from the coordinated cyclic amines.     

N2 fixation by red alder (Alnus rubra) and scotch broom (Cytisus scoparius) planted under precommerically thinned Douglas-fir (Pseudotsuga menziesii)

Summary From acetylene reduction assays over a 10-month period starting in April 1979, nodule activities averaged 18.78 (se 4.67) moles C₂H₄ g nodule dw^–1 h^–1 forAlnus rubra and 59.95 (se 12.14) moles C₂H₄ g nodule dw^–1 h^–1 forCytisus scorparius. Plant rates were 1.91 (se. 47) moles C₂H₄ plant^–1 h^–1 forA. rubra and 0.55 (se. 17) moles C₂H₄ plant^–1 h^–1 forC. Scoparius. Plant activity and total leaf N were strongly correlated with the dw of other plant parts, but nodule activity and percent leaf N were not. Plant and nodule activities were not associated with temperature, moisture stress, precipitation events or percent light for either species over the growing season nor for 54A. rubra sampled in mid-season 1979 on one replication. After 5 to 6 growing seasons, 14A. rubra on the same site ranged from 30 to 332 cm in height and showed strong correlation between nodule dw, leaf dw, plant size and total leaf N. Results from this study and others indicate logistic equations may be modified to predict the effect of adding a N₂ fixing plant to a population of non N₂ fixing trees.     

Synthesis and structural investigations of Ni(II)- and Pd(II)-coordinated α-diimines with chlorinated backbones

Novel square planar Pd(II) α-diimines [PdX₂{ArNC(Cl)}₂], where Ar = C₆H₅, (2,6-Me₂C₆H₃), (2,6-ⁱPr₂C₆H₃) and X = Cl or Br, and the octahedral Ni(II) complex [NiBr₂{(C₆H₅)NC(Cl)}₂(THF)₂] have been prepared and characterised by spectroscopic methods. For two of the Pd(II) complexes and the Ni(II) complex the crystal structures were determined by X-ray crystallography. A further insight into the geometry and electronic structure of [PdBr₂{(2,6-Me₂C₆H₃)NC(Cl)}₂] was gained using density functional theoretical calculations (DFT). This compound resembles structurally and electronically typical olefin polymerisation pre-catalysts supported by α-diimines incorporating methyl- and 1,8-naphtalenyl substituents at the ligand backbone. The chlorine-substituted backbone of the free ligand [2,6-Me₂C₆H₃NC(Cl)]₂ can be employed in further alkylation reactions to generate new multifunctional ligand prototypes with potential uses as ansa-metallocene/diimines building blocks for catalytic applications of heterobimetallic complexes.     

s-Block metallabenzene: aromaticity and hydrogen adsorption

Second group metal dimers can replace the carbon atom in benzene to form metallabenzene (C₅H₆M₂) compounds. These complexes possess some aromatic character and promising hydrogen adsorption properties. In this study, we investigated the aromatic character of these compounds using aromaticity indices and molecular orbital analysis. To determine the nature of interactions between hydrogen and the metallic center, variation-perturbational decomposition of interaction energy was applied together with ETS-NOCV analysis. The results obtained suggest that the aromatic character comes from three π orbitals located mainly on the C₅H₅ ^? fragment. The high hydrogen adsorption energy (up to 6.5 kcal mol^?1) results from two types of interaction. In C₅H₆Be₂, adsorption is controlled by interactions between the empty metal orbital and the σ orbital of the hydrogen molecule (Kubas interaction) together with corresponding back-donation interactions. Other C₅H₆M₂ compounds adsorb H₂ due to Kubas interactions enhanced by H₂–π interactions.

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Graphical Abstract First π orbital in C₅H₆Be₂

     

Hierarchy of structures in the family of amine templated open-framework gallium arsenates

Five new gallium arsenate compounds [C₂N₂H₁₀][Ga(H₂AsO₄)(HAsO₄)₂]·H₂O, I; [C₂N₂H₁₀][Ga(OH)(AsO₄)]₂, II; [C₂N₂H₁₀][GaF(AsO₄)]₂, III; [C₃N₂H₁₂][Ga(OH)(AsO₄)]₂, IV; [Ga₂F₃(AsO₄)(HAsO₄)]·2H₃O, V, have been synthesized under hydrothermal conditions and the structures determined employing single crystal X-ray diffraction studies. All the structures consist of octahedral gallium and tetrahedral arsenate units connected together forming a hierarchy of structures. Thus, one- (I), two- (II and IV) and three-dimensionally (III and V) extended structures have been observed. The Ga-O(H)/F-Ga connectivity in some of the structures suggests the coordination requirements posed by the octahedral gallium in these compounds. The observation of only one type of secondary building unit in the structures of III (SBU-4) and V (spiro-5) is unique and noteworthy. All the compounds have been characterized by a variety of techniques that include powder XRD, IR, and TGA.     

Spontaneous resolution of a bis(η-methylcyclopentadienyl)zinc complex

Two crystalline complexes of bis(η¹-methylcyclopentadienyl)zinc, [Zn(C₅H₄Me)₂(py)₂] (1), where py is pyridine, and [Zn(C₅H₄Me)₂(teeda)], 2, where teeda is N,N,N′,N′-tetraethylethylenediamine have been isolated. The crystal structures of 1 and 2 are the first crystal structures for Zn(C₅H₄Me)₂ complexes reported in the literature; both structures display η¹-coordination of the methylcyclopentadienyl ring to zinc, and both compounds display chirogenic α-carbon atoms. While 1 forms racemic crystals, 2 undergoes spontaneous resolution and crystals of 2 are thus enantiomerically pure. ¹H NMR showed that Zn(C₅H₄Me)₂ is stereochemically labile in solution with only one signal for the Cp-protons. This fact opens up the possibility for total spontaneous resolution and absolute asymmetric synthesis.     

Isolation and Characterization of PDE-I and II,the Inhibitors of Cyclic Adenosine-3′,5′-monophosphate Phosphodiesterase

In the screening for inhibitors of cyclic adenosine-3′,5′-monophosphate phosphodiesterase, two compounds, PDE-I (C₁₃H₁₃N₃O₅) and PDE-II (C₁₄H₁₄N₂O₅), were isolated from culture filtrates of a Streptomyces. Concentrations for 50% inhibitions of PDE-I and PDE-II against the high Km enzyme were 15 µm and 13 µm, and those against the low Km enzyme were 65 µm and 130 µm, respectively. Production, isolation and characterization of these compounds are described.     

Electric-field-induced spin switch of endohedral dodecahedrane heterodimers H@C<Subscript>20</Subscript>H<Subscript>n</Subscript>–C<Subscript>20</Subscript>H<Subscript>n</Subscript>@M (M= Cu,Ag and Au,n = 15, 18, and 19): a theoretical study

We designed nine endohedral dodecahedrane heterodimers H@C₂₀H_n-C₂₀H_n@M (M = Cu, Ag, and Au, n = 15, 18, and 19) that may act as single-molecule spin switches, and we predicted theoretically that the ground states of the dimmers shift from low-spin states (S = 0) to the high-spin states (S = 1) under an external electric field applied parallel or perpendicular to the molecular symmetry axes, consisting well with the analyses of Stark effect. Molecular orbitals analyses provide an intuitive insight into the spin crossover behavior. This study expands the application of endohedral chemistry and provides new molecules for designing single-molecule spin switch.     

Estimates of ϕ and ψ torsion angles in proteins from one-, two- and three-bond nuclear spin-spin couplings: Application to staphylococcal nuclease

Summary Calculated coupling constants (³J_H ^N _H , ¹J_{C H} , ²J_CH , ¹J_{C N} and ²J_{C N}) from our accompanying paper [Edison, A.S. et al. (1994) J. Biomol. NMR, 4, 519–542] have been used to generate error surfaces that can provide estimates of the and angles in proteins. We have used experimental coupling data [³J_H ^N _H : Kay, L.E. et al. (1989) J. Am. Chem. Soc., 111, 5488–5490; ¹J_{C H} : Vuister, G.W. et al. (1993) J. Biomol. NMR, 3, 67–80; ²J_CH : Vuister, G.W. and Bax, A. (1992) J. Biomol. NMR, 2, 401–405; ¹J_{C N} and ²J_{C N}: Delaglio, F. et al. (1991) J. Biomol. NMR, 1, 439–446] to create error surfaces for selected residues of the protein staphylococcal nuclease. The residues were chosen to include all those with five experimental couplings, as well as some with four experimental couplings, to demonstrate the relative importance of ³J_H ^N _H and ¹J_{C H} . For most of the cases, we obtained good agreement between the X-ray structure [Loll, P.J. and Lattman, E.E. (1989) Protein Struct. Funct. Genet., 5, 183–201] and the NMR data.Abbreviations CUPID Contin Uous ProbabIlity Distribution analysis of rotamers - ⁿJ_AB single-bond (n=1), geminal (n=2), or vicinal (n=3) coupling constant between nuclei A and B - NOE nuclear Overhauser enhancement - r² correlation coefficient