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Positive muon spectroscopy (μSR) and 170Yb Mossbauer absorption measurements have been performed in the cubic Kondo insulator YbB12 down to the 50 mK temperature range. We observe a paramagnetic fluctuation mode at low temperature, with a weak moment amplitude ( ≈ 10−2 μB) and a slow fluctuation frequency ( ≈ 60 MHz) which remains constant between 0.04 K and 4.2 K. No indication of a magnetic phase transition is found down to 0.04 K.

A pathological variant of human phosphoglycerate kinase, phosphoglycerate kinase-Uppsala, associated with chronic nonspherocytic hemolytic anemia has been found to differ from the normal enzyme by substitution of an arginine at position 206 (corresponding to position 203 in yeast) by a proline. In order to understand the structural and functional consequences of this mutation, the corresponding mutant in yeast phosphoglycerate kinase was constructed. The three-dimensional structure of this mutant was resolved at 2.9 A. Although the overall structure is not modified, small local changes were observed. The kinetic parameters of the mutant were not found to be greatly affected, the catalytic constant being lowered by only 10-20%. The most significant difference when compared with the wild-type enzyme is a decrease in stability by about 3 kcal/mol. The physiological implications of this instability are discussed.

Nucleoside diphosphate kinase (NDP kinase) has a ping-pong mechanism with a phosphohistidine intermediate. Crystals of the enzymes from Dictyostelium discoideum and from Drosophila melanogaster were treated with phosphoramidate, and their X-ray structures were determined at 2.1 and 2.2 A resolution, respectively. The atomic models, refined to R factors below 20%, show no conformation change relative to the free proteins. In both enzymes, the active site histidine was phosphorylated on N delta, and it was the only site of phosphorylation. The phosphate group interacts with the hydroxyl group of Tyr56 and with protein-bound water molecules. Its environment is compared with that of phosphohistidines in succinyl-CoA synthetase and in phosphocarrier proteins. The X-ray structures of phosphorylated NDP kinase and of previously determined complexes with nucleoside diphosphates provide a basis for modeling the Michaelis complex with a nucleoside triphosphate, that of the phosphorylated protein with a nucleoside diphosphate, and the transition state of the phosphate transfer reaction in which the gamma-phosphate is pentacoordinated.

A microscopic investigation of the cubic ternary alloys YbPdBi and YbNiSb, using 170Yb Mossbauer spectroscopy, reveals that the point symmetry at the site of the Yb3+ is far from cubic in both compounds. An analysis of the thermal variation of the quadrupolar 4f moment shows that the energy of the first crystal electric field transition is close to 20 K in YbPdBi and to 10 K in YbNiSb. Specific heat measurements are also presented in YbPdBi, and magnetic ordering is detected below 1 K in YbPdBi, with saturated Yb3+ moments of 1.25 mu B, and below 0.85 K in YbNiSb, with saturated moments of 1 mu B. A tentative explanation of the Yb site symmetry breaking is made in terms of a Jahn-Teller distortion within the Gamma 8 cubic quartet.

The reaction of NO 3 and dimethyl sulfide (DMS) has been studied at 298 K and 4 Torr pressure using the discharge flow mass spectometric technique. Chemical titration of the primary radical produced by Br 2 and Cl 2 has shown that the reaction proceeds predominantly through NO 3 +CH 3 SCH 3 →CH 3 SCH 2 +HNO 3 (1), the alternative channel yielding CH 3 and CH 3 SONO 2 being less than 2%

Rate constants for the reactions of OH radicals and chlorine atoms with halogenated aldehydes of the type CX 3 CHO (X=H, Cl, or F) have been determined at 298±2 K. The OH radical rate data were determined using a pulsed laser photolysis resonance fluorescence technique at total pressures in the region 15-100 Torr and a conventional photolytic relative rate technique at atmospheric pressure. The chlorine atom rate data were determined using the relative rate method only. The rate data for reaction with both OH radicals and chlorine atoms indicate that increasing halogen atom substitution decreases the rate constant for reaction. Evidence is presented which suggests that the deactivation of these compounds is due mainly to polar effects in the transition states of the reactions induced by the halomethyl substituents, rather than decreassed overall reaction enthalpy

We report 170 Yb Mossbauer spectroscopy and magnetisation measurements in the magnetically ordered Kondo lattices YbP, YbN, YbAuCu 4 and YbNiSn, which reveal hybridisation-induced reductions of the Yb spontaneous moment and of the magnetic ordering temperature. Using the self-consistent NCA method with a RKKY derived molecular field, we calculate these reductions and compare them with experiment.

Rate constants for the reactions of OH radicals and chlorine atoms with halogenated aldehydes of the type CX 3 CHO (X=H, Cl, or F) have been determined at 298±2 K. The OH radical rate data were determined using a pulsed laser photolysis resonance fluorescence technique at total pressures in the region 15-100 Torr and a conventional photolytic relative rate technique at atmospheric pressure. The chlorine atom rate data were determined using the relative rate method only. The rate data for reaction with both OH radicals and chlorine atoms indicate that increasing halogen atom substitution decreases the rate constant for reaction. Evidence is presented which suggests that the deactivation of these compounds is due mainly to polar effects in the transition states of the reactions induced by the halomethyl substituents, rather than decreassed overall reaction enthalpy

The magnetic ordering of the hexagonal multiferroic compound ${\text{YbMnO}}_{3}$ has been studied between 100 and 1.5 K by combining neutron powder diffraction, $^{170}\text{Y}\text{b}$ M\"ossbauer spectroscopy, and magnetization measurements. The Yb moments of the two crystallographic sites order at two different temperatures: the $4b$ site together with the Mn moments (at ${T}_{N}\ensuremath{\simeq}85\text{ }\text{K}$) and the $2a$ site well below (at 3.5 K). The temperature dependences of the Mn and Yb moments are explained within a molecular-field model, showing that the $4b$ and $2a$ sites order via Yb-Mn and Yb-Yb interactions, respectively. A simple picture taking into account the local Mn environment of the rare-earth $R$ $(4b)$ ion is proposed to couple $R$ and Mn orders in hexagonal $R{\text{MnO}}_{3}$ manganites. The nature and symmetry of the $R\text{-Mn}$ interactions yielding the $R$ order are discussed.

Low temperatyre μSR, specific heat and magnetic susceptibility measurements are presented in a non-magnetically ordered sample of the cubic intermetallic YbPdSb. They reveal the presence of rather slow Yb moment fluctuations down to 0.05 K in a heavy fermion state characterised by a very high Sommerfeld ratio of 3 J K−2 mol−1. The relevance of these observations with respect to the formation of a spin-liquid phase, expected in materials close to Doniach's magnetic instability for Kondo lattices, is discussed.

The hydroxyl radical initiated oxidation of diisopropyl ether has been studied in the large-volume outdoor European Photoreactor (EUPHORE) and in a small, laboratory-based reactor system. The product distributions determined from the experiments were found to be significantly dependent on the reaction conditions and provide strong evidence for the existence of three distinct regimes within the reaction system. In the presence of NO x , the peroxy radicals react with NO to produce chemically activated (CH 3 ) 2 CHOC(O)(CH 3 ) 2 alkoxy radicals which undergo decomposition by C C bond scission to yield isopropyl acetate and formaldehyde as the major products. Under conditions where the self-reaction of peroxy radicals dominates, thermoneutral (CH 3 ) 2 CHOC(O)(CH 3 ) 2 radicals are produced, which appear to undergo two reaction pathways; C C bond scission to yield isopropyl acetate and formaldehyde and isomerisation to form acetone, acetic acid and formaldehyde. Under conditions where the reaction between peroxy and hydroperoxy radicals dominates, unstable hydroperoxides are produced which decompose to yield acetone as the only major reaction product. The results of our study are used to construct chemical mechanisms for the gas-phase photooxidation of diisopropyl ether under various tropospheric conditions.

REACTIONS of free radicals are the driving force for most chemical processes in the atmosphere. Although hydroxyl (OH) and nitrate (NO3) radicals are the most reactive, peroxy radicals such as HO2 and CH3O2 have an important role as intermediates in tropospheric ozone generation and as precursors to OH radicals1. Moreover, self-reaction of peroxy radicals is the only known gas-phase source of peroxides to the atmosphere. It has long been known that there exist pathways to the formation of radicals—in particular NO3 (refs 2–5) —that do not require sun-light, yet radical chemistry is usually discussed in terms of photochemistry occurring during the day1. The possibility of night-time production of peroxy radicals has received little attention, except perhaps in highly polluted air6–8. Here we show that recent kinetic investigations9,10 combined with field observations of NO3, organic and other relevant species11,12 suggest that tropospheric high night-time concentrations of peroxy radicals—possibly even exceeding daytime values—may occur under certain atmospheric conditions, for instance in coastal areas or forests and in air masses affected by urban emission or biomass burning. The prerequisite for this occurrence is the simultaneous presence in the same air mass of reactive organic species, oxides of nitrogen and ozone.

Background: Nucleoside diphosphate (NDP) kinases provide precursors for DNA and RNA synthesis. In mammals, these enzymes are also involved in cell regulations. Human NDP kinase B, product of the human nm23-H2 gene, is both an enzyme and a transcription factor. It activates transcription of the c-myc oncogene independently of its catalytic function, by binding to its promoter DNA. How do the two functions coexist? Results Recombinant human NDP kinase B was co-crystallized with GDP. The X-ray structure was solved at 2.0 a resolution by molecular replacement from the homologous Drosophila Awd protein. Both enzymes are homo-hexamers with a characteristic β α β β α β fold. GDP binds near the active site His118. The guanine base is in a surface cleft and interacts with the C terminus of another subunit. Conclusion The β α β β α β fold, also present in the ‘palm' domain of Escherichia coli DNA polymerase I and HIV reverse transcriptase, is both a mononucleotide- and a polynucleotide-binding fold. If NDP kinase B binds DNA in the same way as the polymerases, the enzyme must undergo a conformation change in order to carry out gene activation.

The X-ray structure of the nucleoside diphosphate kinase (NDP kinase) from Dictyostelium discoideum has been refined at 1.8 A resolution from a hexagonal crystal form with a 17 kDa monomer in its asymmetric unit. The atomic model was derived from the previously determined structure of a point mutant of the protein. It contains 150 amino acid residues out of 155, and 95 solvent molecules. The R-factor is 0.196 and the estimated accuracy of the average atomic position, 0.25 A. The Dictyostelium structure is described in detail and compared to those of Drosophila and Myxococcus xanthus NDP kinases. The protein is a hexamer with D3 symmetry. Residues 8 to 138 of each subunit form a globular alpha/beta domain. The four-stranded beta-sheet is antiparallel; its topology is different from other phosphate transfer enzymes, and also from the HPr protein which, like NDP kinase, carries a phosphorylated histidine. The same topology is nevertheless found in several other proteins that bind mononucleotides, RNA or DNA. Strand connections in NDP kinase involve alpha-helices and a 20-residue segment called the Kpn loop. The beta-sheet is regular except for a beta-bulge in edge strand beta 2 and a gamma-turn at residue Ile120 just preceding strand beta 4. The latter may induce strain in the main chain near the active site His122. The alpha 1 beta 2 motif participates in forming dimers within the hexamer, helices alpha 1 and alpha 3, the Kpn loop and C terminus, in forming trimers. The subunit fold and dimer interactions found in Dictyostelium are conserved in other NDP kinases. Trimer interactions probably occur in all eukaryotic enzymes. They are absent in the bacterial Myxococcus xanthus enzyme which is a tetramer, even though the subunit structure is very similar. In Dictyostelium, contacts between Kpn loops near the 3-fold axis block access to a central cavity lined with polar residues and filled with well-defined solvent molecules. Biochemical data on point mutants highlight the contribution of the Kpn loop to protein stability. In Myxococcus, the Kpn loops are on the tetramer surface and their sequence is poorly conserved. Yet, their conformation is maintained and they make a similar contribution to the substrate binding site.

The X-ray structure of nucleoside diphosphate kinase (NDP kinase) from the slime mold Dictyostelium discoideum has been determined to 2.2-A resolution and refined to an R-factor of 0.19 with and without bound ADP-Mg2+. The nucleotide binds near His 122, a residue which becomes phosphorylated during the catalytic cycle. The mode of binding is different from that observed in other phosphokinases, and it involves no glycine-rich sequence. The adenine base makes only nonpolar contacts with the protein. It points outside, explaining the lack of specificity of NDP kinase toward the base. The ribose 2'- and 3'-hydroxyls and the pyrophosphate moiety are H-bonded to polar side chains. A Mg2+ ion bridges the alpha- to the beta-phosphate which approaches the imidazole group of His 122 from the N delta side. The geometry at the active site in the ADP-Mg2+ complex suggests a mechanism for catalysis whereby the gamma-phosphate of a nucleoside triphosphate can be transferred onto His 122 with a minimum of atomic motion.

Nucleoside diphosphate kinase from the slime mold Dictyostelium discoideum is highly homologous to gene products that are involved in development in Drosophila and in oncogenesis in human cells. The cloned protein expressed in Escherichia coli has been purified and crystallized in a hexagonal space group with a = b = 74.9 A, c = 211.4 A. The asymmetric unit contains either one or two 17,000 Mr subunits of the hexamer.

The molecular modulation spectroscopic technique was employed to study the kinetics of NO3 radicals produced in the 253.7 nm photolysis of flowing gas mixtures of HNO3/CH4/O2 at room temperature. By computer fitting of the NO3 temporal behaviour, a rate coefficient of (2.30 ± 0.64)xl0-12 cm3 molecule-1 s-1 was obtained for the reaction between CH3O2 and NO3.

The molecular modulation spectroscopic technique was employed to study the kinetics of NO3 radicals produced in the 253.7 nm photolysis of flowing gas mixtures of HNO3/CH4/O2 at room temperature. By computer fitting of the NO3 temporal behaviour, a rate coefficient of (2.30 ± 0.64)xl0-12 cm3 molecule-1 s-1 was obtained for the reaction between CH3O2 and NO3.

The reaction of atomic fluorine with dichloromethane has been studied by the diffusion cloud in a flow technique. Fluorine atoms were generated through F 2 dissociation in a high-frequency discharge. The reaction products were detected mass spectrometrically, applying the technique of focusing the paramagnetic component of the molecular beam in an inhomogeneous magnetic field to detect radical species. Cl atoms and CHCl 2 and CF 3 free radicals have been identified among the reaction products. The initial step was shown to be hydrogen atom abstraction. The room temperature rate constant of this reaction was found to be k 0 = (1.51 ± 0.28) X 10 −11 cm 3 /s. The rate constant of the secondary reaction of fluorine atoms with dichloromethyl radicals, which is suggested to produce mainly HCl, was evaluated as 3 X 10 −10 cm 3 /s.

The reactions of OH radicals with dimethylsulfide (DMS) diethylsulfide (DES), tetrahydrothiophene (THT), and thiophene have been studied at room temperature for DES and THT, at 273, 293, and 318 K for DMS, and at 293 and 318 K for thiophene by the discharge flow EPR technique in a halocarbon wax coated reactor. The following rate constants were obtained at room temperature. For the reaction OH + DMS (1), a very low temperature dependence was noticed. Some additional results concerning the mass spectrometric analysis of the products are also reported.

The reactions of OH radicals with dimethylsulfide (DMS) diethylsulfide (DES), tetrahydrothiophene (THT), and thiophene have been studied at room temperature for DES and THT, at 273, 293, and 318 K for DMS, and at 293 and 318 K for thiophene by the discharge flow EPR technique in a halocarbon wax coated reactor. The following rate constants were obtained at room temperature. For the reaction OH + DMS (1), a very low temperature dependence was noticed. Some additional results concerning the mass spectrometric analysis of the products are also reported.