Crystallization of Arthrobacter sp. strain 1C N-(1-D-carboxyethyl)-L-norvaline dehydrogenase and its complex with NAD+

The novel NAD+-linked opine dehydrogenase from a soil isolate Arthrobacter sp. strain 1C belongs to an enzyme superfamily whose members exhibit quite diverse substrate specificites. Crystals of this opine dehydrogenase, obtained in the presence or absence of co-factor and substrates, have been shown to diffract to beyond 1.8 ? resolution. X-ray precession photographs have established that the crystals belong to space group P21212, with cell parameters a = 104.9, b = 80.0, c = 45.5 ? and a single subunit in the asymmetric unit. The elucidation of the three-dimensional structure of this enzyme will provide a structural framework for this novel class of dehydrogenases to enable a comparison to be made with other enzyme families and also as the basis for mutagenesis experiments directed towards the production of natural and synthetic opine-type compounds containing two chiral centres.     

Cooperative two-photon-induced chemical bond formation during a Kr+F2 collision to form KrF

The reactive Kr⁺F₂⁻ potential energy surface is probed by two-photon, laser-induced chemical bond formation during a Kr+F₂ collision. This is compared with the pulsed laser excitation (two-photon) of Kr(2p₉) followed by collision with F₂ leading to the formation of KrF(B, C). In addition to reporting the excitation spectrum for the two-phonon-induced collision process, these techniques were used to determine quenching rate constants of Kr₂F^*. Quenching by Xe gives XeF(B, C) with rate constant (1.5±0.2)×10⁻¹⁰ cm³ s⁻¹; the quenching rate constant for F₂ is (1.5±0.2)×10⁻¹⁰ cm³ s⁻¹, and the radiative lifetime of Kr₂F^* is 240±35 ns. The quenching rate constant for the coupled Kr(2p₈) and Kr(2p₉) levels by F₂ is (13±2)×10⁻¹⁰ cm³ s⁻¹.     

Excitation of XeF* by reactions of XeF2 with Ar(3P0,2),Kr(3P2) and Xe(3P2)

The reactions of the lowest metastable states of Ar, Kr and Xe with XeF₂ were studied in a flowing afterglow apparatus; XeF emission (from D²Π_{$\text{1}\text{2}$} and B ²Π⁺ states) was observed in all cases. The total rate constants (cm³ molecule^?1 s^?1) for XeF^* formation were determined as 75 × 10^?11 ? Xe(³P₂);64 × 10^?11 ? Kr(³P₂) and 20 × 10^?11 ? Ar(³P_0,2). The reactions of Ar(³P_0,2) and Kr(³P₂) with XeF₂ also gave ArF^* and KrF^*, respectively. Analysis of these emissions indicates that at least two different mechanisms are operative: reactive quenching by the ionic—covalent curve-crossing mechanism and excitation transfer. The Ar(³P_0,2 + XeF₂ reaction is a sufficiently strong source of XeF(D—X) emission that the main features of the XeF(D²Π_{$\text{1}\text{2}$} ? X²Σ⁺) system could be photographed and tentative assignments of these vibrational bands are given. The XeF(D → B) emission could not be observed and the ratio of the D—X versus the D—B transition probability must be > 1000 : 1.     

Vibrational energy disposal in reactions of fluorine atoms with hydrides of groups III,IV and V

The HF infrared chemiluminescence from the reactions of F atoms with B₂H₆, CH₄, CH₃F, CH₂F₂, CH₂Cl₂, CH₃ONO. CH₃NO₂, NH₃ (and ND₃). PH₃ and HNCO has been observed from a 300 K flowing-afterglow reactor. Experiments were done for a range of CH₄ and F atom concentrations to identify conditions which were free of vibrational relaxation and secondary reactions, and these conditions were used to assign initial HF(v) vibrational distributions for each reaction. The emission intensity from each reaction also was compared to that from CH₄ in order to obtain the relative HF formation rate constants at 300 K. Since the absolute rate constant for F + CH₄ is well established, the combination of all of these data provides absolute rate constants for HF(v) formation at 300 K. The ND₃ reaction was studied to obtain information on more vibrational levels in order to better estimate the HF(v = 0) and DF(v = 0) components of the ammonia distributions. With NH₃ and ND₃ there is no significant isotope effect on the energy disposal. Except for NHCO, for which an addition-elimination channel is possible, the HF(v) distributions are inverted and <f_v > = 0.60. Differences between the HF(v) distributions reported here and some other reports in the literature are noted: the present data are discussed as representative of direct H atom abstraction for 300 K Boltzmann conditions. The HCl infrared chemiluminescence from the F + CHCl₂ secondary reaction also was observed; the HCl(v) distribution was v₁: v₂: v₃: v₄: v₅ - 0.47: 0.23: 0.18: 0.08: 0.04.     

Laser photoassociation of Xe and Br(2P3/2) atoms and generation of aligned XeBr(B) molecules

The laser-induced photoassociation of colliding pairs of Xe and Br(²P_3/2) atoms has been demonstrated by observing the XeBr(B→A) fluorescence following the XeBr(B→X) laser-induced excitation. Analysis of the B←X excitation spectrum shows that the excitation transition is almost entirely bound — free in nature. The fluore scence and excitation XeBr* spectra are used to discuss the XeBr (X, B and A) potentials. Analysis of the polarization of the XeBr(B-X) fluorescence shows that the XeBr(B) molecules are generated with a high degree of alignment relative to the plane-polarized laser beam. The pressure dependence of the decay rate of the total intensity and of the polarization give radiation lifetimes, quenching rate constants and an estimate for the de-alignment cross section in collisions with Xe.     

Symmetry constraints in energy transfer between state-selected Ar*(3P2, 3P0) metastable atoms and ground state H atoms

The excitation-transfer reaction in thermal energy collisions of state-selected metastable Ar*(³P₂) and Ar*(³P₀) atoms with ground state H atoms, giving excited H*(n = 2) atoms, has been studied with the stationary afterglow technique. The rate constant for the excitation of H atoms by Ar*(³P₂) has been found to be more than one order of magnitude larger than in excitation by Ar*(³P₀). This difference in the reactivity of two metastable species is explained to be a consequence of the attractive nature of the D(²II) and E(²Σ⁺) potentials that develop from the Ar*(³P₂)+H entrance channel and which give curve crossing with the B(²II) and C(²Σ⁺ potentials, respectively, leading to the Ar+H*(n=2) exit channel, whereas only a repulsive ⁴II (Ω=$\text{1}\text{2}$) potential develops from the Ar*(³P₀+H entrance channel.     

Unimolecular elimination of HF and HCl from chemically activated CF3CFClCH2Cl

The unimolecular reactions of CF3CFClCH2Cl molecules formed with 87 kcal mol(-1) of vibrational energy by recombination of CF3CFCl and CH2Cl radicals at room temperature have been characterized by the chemical activation technique. The 2,3-ClH and 2,3-FH elimination reactions, which have rate constants of (2.5 +/- 0.8) x 10(4) and (0.38 +/- 0.11) x 10(4) s(-1), respectively, are the major reactions. The 2,3-FCl interchange reaction was not observed. The trans (or E)-isomers of CF3CFCHCl and CF3CClCHCl are favored over the cis (or Z)-isomers. Density functional theory at the B3PW91/6-31G(d',p') level was used to evaluate thermochemistry and structures of the molecule and transition states. This information was used to calculate statistical rate constants. Matching the calculated to the experimental rate constants for the trans-isomers gave threshold energies of 62 and 63 kcal mol(-1) for HCl and HF elimination, respectively. The threshold energy for FCl interchange must be 3-4 kcal mol(-1) higher than for HF elimination. The results for CF3CFClCH2Cl are compared to those from CF3CFClCH3; the remarkable reduction in rate constants for HCl and HF elimination upon substitution of one Cl atom for one H atom is a consequence of both a lower E and higher threshold energies for CF3CFClCH2Cl.     

Use of a single trajectory to study product energy partitioning in unimolecular dissociation: mass effects for halogenated alkanes

A single trajectory (ST) direct dynamics approach is compared with quasiclassical trajectory (QCT) direct dynamics calculations for determining product energy partitioning in unimolecular dissociation. Three comparisons are made by simulating C(2)H(5)F-->HF + C(2)H(4) product energy partitioning for the MP26-31G(*) and MP26-311 + + G(**) potential energy surfaces (PESs) and using the MP26-31G(*) PES for C(2)H(5)F dissociation as a model to simulate CHCl(2)CCl(3)-->HCl + C(2)Cl(4) dissociation and its product energy partitioning. The trajectories are initiated at the transition state with fixed energy in reaction-coordinate translation E(t) (double dagger). The QCT simulations have zero-point energy (ZPE) in the vibrational modes orthogonal to the reaction coordinate, while there is no ZPE for the STs. A semiquantitative agreement is obtained between the ST and QCT average percent product energy partitionings. The ST approach is used to study mass effects for product energy partitioning in HX(X = F or Cl) elimination from halogenated alkanes by using the MP26-31G(*) PES for C(2)H(5)F dissociation and varying the masses of the C, H, and F atoms. There is, at most, only a small mass effect for partitioning of energy to HX vibration and rotation. In contrast, there are substantial mass effects for partitioning to relative translation and the polyatomic product's vibration and rotation. If the center of mass of the polyatomic product is located away from the C atom from which HX recoils, the polyatomic has substantial rotation energy. Polyatomic products, with heavy atoms such as Cl atoms replacing the H atoms, receive substantial vibration energy that is primarily transferred to the wag-bend motions. For E(t) (double dagger) of 1.0 kcalmol, the ST calculations give average percent partitionings to relative translation, polyatomic vibration, polyatomic rotation, HX vibration, and HX rotation of 74.9%, 6.8%, 1.5%, 14.4%, and 2.4% for C(2)H(5)F dissociation and 39.7%, 38.1%, 0.2%, 16.1%, and 5.9% for a model of CHCl(2)CCl(3) dissociation.     

Evaluation of HX (X = Cl,Br, O) product distributions from chemiluminescence. I. H atom reactions

The HX product state distributions from the H+Cl₂, Br₂, NO₂Cl, PCl₃, and NO₂ reactions have been studied by the infrared chemiluminescence technique in two different laboratories with two types of reactors; a fast-flow system with = 1 Torr of Ar buffer gas and a low-pressure, cold-wall system (usually called the cold-wall arrested-relaxation method). The same Einstein coefficients were used in both laboratories to convert intensities to populations and emphasis is placed upon evaluation of the reliability of the resulting vibrational-rotational HX distributions. Good agreement was found between the HX distributions from the cold-wall reactors from the two laboratories and for both types of reactors for all of the reactions, except PCl₃. For the H+Cl₂, Br₂ and NO₂ reactions, our general results are in good accord with presently accepted data; but, our experiments provide somewhat more detail than in the literature. The NO₂Cl results are new and <f_v(HCl) > = 0.40 and <f_R(HCl) > = 0.01. The H+PCl₃ reaction appears to proceed by two channels and the HCl chemiluminescence cannot be assigned only to HCl formation via direct Cl atom abstraction.     

Energy transfer in collisions of Ar(3P0,2 metastable atoms with H(2S) atoms. I. Total rate constant and mechanism

The Ar(4s,³P₂) + H(1S,²S) reaction, which gives excited H(n=2) atoms, has been studied. The room temperature rate constant for Lyman-α (2p-1s) excitation was measured as 2.4 × 10^?10 cm³ mol^?1 s^?1. The method was based upon comparison of the Lyman-α emission intensity with the Kr resonance emission intensity produced from Ar(³P₂) + Kr, which has a known rate constant. The H atom excitation, which has a large energy defect of 1.3 eV, is discussed in terms of a curve crossing mechanism.