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.     

Kinetics and mechanisms of S(IV) reductions of bromite and chlorite ions

The reaction of bromite with aqueous S(IV) is first order in both reactants and is general-acid catalyzed. The reaction half-lives vary from 5 ms (p[H+] 5.9) to 210 s (p[H+] 13.1) for 0.7 mM excess S(IV) at 25 degrees C. The proposed mechanism includes a rapid reaction (k(1) = 3.0 x 10(7) M(-1) s(-1)) between BrO(2)(-) and SO(3)(2-) to form a steady-state intermediate, (O(2)BrSO(3))(3-). General acids assist the removal of an oxide ion from (O(2)BrSO(3))(3-) to form OBrSO(3)(-), which hydrolyzes rapidly to give OBr(-) and SO(4)(2-). Subsequent fast reactions between HOBr/OBr(-) and SO(3)(2-) give Br(-) and SO(4)(2-) as final products. In contrast, the chlorite reactions with S(IV) are 5-6 orders of magnitude slower. These reactions are specific-acid, not general-acid, catalyzed. In the proposed mechanism, ClO(2)(-) and SO(3)H(-)/SO(2) react to form (OClOSO(3)H)(2)(-) and (OClOSO(2))(-) intermediates which decompose to form OCl(-) and SO(4)(2-). Subsequent fast reactions between HOCl/OCl(-) and S(IV) give Cl- and SO(4)(2-) as final products. SO(2) is 6 orders of magnitude more reactive than SO(3)H-, where k(5)(SO(2)/ClO(2)(-)) = 6.26 x 10(6) M(-1) s(-1) and k(6)(SO(3)H(-)/ClO(2)(-)) = 5.5 M(-1) s(-1). Direct reaction between ClO(2)(-) and SO(3)(2-) is not observed. The presence or absence of general-acid catalysis leads to the proposal of different connectivities for the initial reactive intermediates, where a Br-S bond forms with BrO(2)(-) and SO(3)(2-), while an O-S bond forms with ClO(2)(-) and SO(3)H-.     

Microtubule particle dispersion in liquid crystal hosts

Abstract

Microtubule particles and metal-coated microtubules were dispersed in various host liquid crystal mixtures. Dispersion effects were evaluated as a function of liquid crystal type, viscosity, dielectric anisotropy and surface interaction. Experimental results indicated that all the types of liquid crystals studied were aligned perpendicular to the microtubule surfaces, regardless of liquid crystal composition or various surface coatings used on the metal-coated microtubules. Low concentrations of the metal-coated microtubules in nematic liquid crystal hosts were aligned by flow or cell surface alignment conditions, and could be modulated by electric or magnetic fields. We observed better microtubules dispersion uniformity in high viscosity liquid crystal host mixtures and in liquid crystal-monomers than in isotropic fluids. Microtubules particles dispersed in ROTN-404 liquid crystal mixture had a much higher birefringence in the microwave region than dispersion in a paraffin oil.     

Synthesis, structure, redox properties and azide binding for a series of biphenyl-based Cu(II) complexes

A series of biphenyl-based N(3)O ligands, 2, 4, 6, and 8 were synthesized and their Cu(II) complexes prepared. These complexes were characterized by a combination of elemental analysis, FAB-MS, UV-vis spectroscopy and electrochemistry. The structure of [Cu(N(3)O-mpy-NO2)Cl2], 12 [N(3)O-mpy = 2-(3-pyridylmethylimino)-2'-(2-methylaminophenol)biphenyl], was solved and showed that the ligand coordinates through the three nitrogens with the phenol oxygen uncoordinated. Titration of azide anion into solutions of the complexes in methanol resulted in the appearance of a new band between 485-495 nm at the expense of the starting peak at 380 nm. Cyclic voltammetry studies indicated that the complexes undergo quasi-reversible one-electron reductions in acetonitrile at potentials between 0.13-0.58 V vs. Ag/AgCl. The complexes were found to be weakly active for the oxidation of di-tert-butyl catechol (DTBC).     

A rapid-scanning electron paramagnetic resonance spectrometer with stopped-flow mixing

Modifications to a commercial electron paramagnetic resonance (e.p.r.) spectrometer are described which allow data to be obtained from 64 e.p.r, spectra (0.1–100 gauss per scan) with scan times of 0.010–0.900 s and with 0.020–900 s delay times between scans. Reagents are delivered into an e.p.r, cell from a stopped-flow mixer that triggers the generation of a microprocessor-controlled waveform to drive a rapid-scan unit. This digitally synthesized waveform is designed to correct inherent imperfections in the Helmholtz-type sweep coil circuit in the rapid-scan unit. The spectra are digitized (250 points per scan) and can be processed to determine rates of reaction. Performance of the system is demonstrated by the determination of the kinetics of rearrangement of a bis(di- peptide)nickelate(III) complex.     

Chlorine dioxide reduction by aqueous iron(II) through outer-sphere and inner-sphere electron-transfer pathways

The reduction of ClO(2) to ClO(2)(-) by aqueous iron(II) in 0.5 M HClO(4) proceeds by both outer-sphere (86%) and inner-sphere (14%) electron-transfer pathways. The second-order rate constant for the outer-sphere reaction is 1.3 x 10(6) M(-1) s(-1). The inner-sphere electron-transfer reaction takes place via the formation of FeClO(2)(2+) that is observed as an intermediate. The rate constant for the inner-sphere path (2.0 x 10(5) M(-1) s(-1)) is controlled by ClO(2) substitution of a coordinated water to give an inner-sphere complex between ClO(2) and Fe(II) that very rapidly transfers an electron to give (Fe(III)(ClO(2)(-))(H(2)O)(5)(2+))(IS). The composite activation parameters for the ClO(2)/Fe(aq)(2+) reaction (inner-sphere + outer-sphere) are the following: DeltaH(r)++ = 40 kJ mol(-1); DeltaS(r)++ = 1.7 J mol(-1) K(-1). The Fe(III)ClO(2)(2+) inner-sphere complex dissociates to give Fe(aq)(3+) and ClO(2)(-) (39.3 s(-1)). The activation parameters for the dissociation of this complex are the following: DeltaH(d)++= 76 kJ mol(-1); DeltaS(d)++= 32 J K(-1) mol(-1). The reaction of Fe(aq)(2+) with ClO(2)(-) is first order in each species with a second-order rate constant of k(ClO2)- = 2.0 x 10(3) M(-1) s(-1) that is five times larger than the rate constant for the Fe(aq)(2+) reaction with HClO(2) in H(2)SO(4) medium ([H(+)] = 0.01-0.13 M). The composite activation parameters for the Fe(aq)(2+)/Cl(III) reaction in H(2)SO(4) are DeltaH(Cl(III))++ = 41 kJ mol(-1) and DeltaS(Cl(III))++ = 48 J mol(-1) K(-1).     

Equilibrium and Kinetics of Bromine Hydrolysis

Equilibrium constants for bromine hydrolysis, K(1) = [HOBr][H(+)][Br(-)]/[Br(2)(aq)], are determined as a function of ionic strength (&mgr;) at 25.0 degrees C and as a function of temperature at &mgr; approximately 0 M. At &mgr; approximately 0 M and 25.0 degrees C, K(1) = (3.5 +/- 0.1) x 10(-)(9) M(2) and DeltaH degrees = 62 +/- 1 kJ mol(-)(1). At &mgr; = 0.50 M and 25.0 degrees C, K(1) = (6.1 +/- 0.1) x 10(-)(9) M(2) and the rate constant (k(-)(1)) for the reverse reaction of HOBr + H(+) + Br(-) equals (1.6 +/- 0.2) x 10(10) M(-)(2) s(-)(1). This reaction is general-acid-assisted with a Br?nsted alpha value of 0.2. The corresponding Br(2)(aq) hydrolysis rate constant, k(1), equals 97 s(-)(1), and the reaction is general-base-assisted (beta = 0.8).     

Copolymerizable initiator for improved polymer dispersed liquid crystal films

The problems of photoinitiator contamination are addressed for the liquid crystal phase in polymer dispersed liquid crystal films formed by photopolymerization induced phase separation of liquid crystal from monomer solutions. Initiator contamination lowers the clearing point of the liquid crystal phase, and decreases the photostability and resistivity of the polymer dispersed liquid crystal. These problems are minimized by replacing the conventional photoinitiators with copolymerizable initiators which become incorporated in the polymer phase as it separates. Copolymerizable photoinitiators are studied and used to form polymer dispersed liquid crystals with higher clearing point liquid crystal phases, higher resistivity, and better photostability than polymer dispersed liquid crystals formed with conventional photoinitiators. These improvements provide very significant advantages for many polymer dispersed liquid crystal applications.     

Analysis of uranium-zinc alloys

Zinc can be determined in zinc-uranium alloys by titration with Versene (ethylcnediaminetetra-acetic acid), using Murexide as the indicator. Carbonate is added before the titration to mask the uranium.