NMR AND DFT STUDY OF Co-C BOND ACTIVATION MECHANISM IN VITAMIN B_(12) COMPOUNDS

This report focuses on a theoretical study of the cobalt-cabon bond activation mechanism in Vitamin B12, its co enzyme complexes, and model compounds such as cobalamins (Cbl), cobaloximes and cobalt porphyrins. In particular, emphasis is placed on the nucleotide loop and its role in the base on and base off mechanism.     

Methoden zur Identifizierung, Anreicherung und Charakterisierung genetisch bedingter Acylcholin-Acyl-Hydrolase-Varianten (E.C. 3.1.1.8.)

Zusammenfassung Es werden verschiedene Methoden zur Differenzierung genetisch bedingter Enzymvarianten der Pseudocholinesterase (spektrophotometrischer Test, Agar-Diffusionstest, Säulenchromatographie), Versuche zur Charakterisierung der verschiedenen Enzymproteine und der in der Stärkegelelektrophorese auftretenden vier Enzymfraktionen mitgeteilt. Ein Reinigungsverfahren für die Pseudocholinesterase wurde ausgearbeitet; nach der Immunisierung von Kaninchen mit dem angereicherten Enzymprotein wurden mit dem gewonnenen Antiserum immunchemische Versuche durchgeführt.

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Summary Methods are reported for the differentiation and characterisation of genetically determined enzyme variants and the enzyme fractions occuring in starch-gel electrophoresis of pseudocholinesterase (spectrophotometrical assays, agar diffusion test, column chromatography). A method has been developed to purify the pseudocholinesterase. Rabbits were immunized against a purified enzyme protein. Results of these immuno-chemical assays are given.

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Die in der Literatur am häufigsten zu findende Bezeichnung Pseudocholinesterase wird hier verwendet.

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Wir danken der Deutschen Forschungsgemeinschaft, dem Bundesministerium für Wissenschaftliche Forschung und dem Fonds der Chemischen Industrie für ihre Unterstützung.     

Magnetic penetration depth in V3Si and LiTi2O4 measured by μSR

Muon spin relaxation (SR) studies have been performed in the normal spinel LiTi₂O₄ and the A-15 superconductor V₃Si to measure the magnetic penetration depth . The relaxation rate(T) 1/² in field-cooled measurements shows a sharp increase belowT _c followed by saturation at low temperatures in both systems. This feature implies an isotropic energy gap without anomalous zeros, and most likelys-wave pairing. The low temperature penetration depth (T 0) is determined to be 2100Å for LiTi₂O₄ and 1300Å for V₃Si respectively. Assuming a clean limit relation ^–2 n _s /m ^*, we derive the Fermi temperatureT _F n _s/ ^2/3 m ^* from the relaxation rate and the Sommerfeld constant asT _F ^3/4–1/4. Unlike conventional superconductors, both LiTi₂O₄ and V₃Si have a large ratio ofT _c /T _F 0.01, only slightly smaller than those ratios in more exotic superconductors.We thank C. Ballard and K. Hoyle for technical assistance. Work at Columbia University is supported by NSF Grant No. DMR-89-13784 and Packard Foundation (YJU). Ames Laboratory is operated for the U. S. Department of Energy by Iowa State University under Contract No. W-7405-Eng-82. Work at Ames was supported by the Director for Energy Research, Office of Basic Energy Sciences.     

A novel RNA motif based on the structure of unusually stable 2',5'-linked r(UUCG) loops

We have recently shown that hairpins containing 2',5'-linked RNA loops exhibit superior thermodynamic stability compared to native hairpins comprised of 3',5'-RNA loops [Hannoush, R. N.; Damha, M. J. J. Am. Chem. Soc. 2001, 123, 12368-12374]. A remarkable feature of the 2',5'-r(UUCG) tetraloop is that, unlike the corresponding 3',5'-linked tetraloop, its stability is virtually independent of the hairpin stem composition. Here, we determine the solution structure of unusually stable hairpins of the sequence 5'-G(1)G(2)A(3)C(4)-(U(5)U(6)C(7)G(8))-G(9)(U/T(10))C(11)C(12)-3' containing a 2',5'-linked RNA (UUCG) loop and either an RNA or a DNA stem. The 2',5'-linked RNA loop adopts a new fold that is completely different from that previously observed for the native 3',5'-linked RNA loop. The 2',5'-RNA loop is stabilized by (a). U5.G8 wobble base pairing, with both nucleotide residues in the anti-conformation, (b). extensive base stacking, and (c). sugar-base and sugar-sugar contacts, all of which contribute to the extra stability of this hairpin structure. The U5:G8 base pair stacks on top of the C4:G9 loop-closing base pair and thus appears as a continuation of the stem. The loop uracil U6 base stacks above U5 base, while the cytosine C7 base protrudes out into the solvent and does not participate in any of the stabilizing interactions. The different sugar pucker and intrinsic bonding interactions within the 2',5'-linked ribonucleotides help explain the unusual stability and conformational properties displayed by 2',5'-RNA tetraloops. These findings are relevant for the design of more effective RNA-based aptamers, ribozymes, and antisense agents and identify the 2',5'-RNA loop as a novel structural motif.     

Structure and Dynamics of Hydrogen Chelates. Part 1. NMR-spectroscopic studies in the quinazoline-2-acetonitrile series

To get informations on both the structure and dynamics of hydrogen chelates 1 of heteroaromatic systems, a great variety of quinazoline-2-acetonitrile chelates were synthesized (see 2–4 ). Similarly to the situation of the corresponding H-chelates in the pyrimidine-2-acetonitrile series, the investigation of these new derivatives 2–4 by NMR spectroscopic methods (DNMR, COSY, NOESY, ROESY, EXSY, HMQC, HMBC) confirms the presence of an equilibrium of the two possible H-chelate structures (two ‘rotamers’ I and II , i.e., (E)/(Z) isomers; see Scheme). The corresponding equilibria I ? II were determined by complete ¹H-NMR signal assignment at low temperatures (after freezing the rotational processes). In addition, the tautomer equilibria A ? B (relative energies of the two minima of the nonsymmetrical double-well potential) for both ‘rotamers’ are ascertained by H,H and C,H couplings. The results are an important basis for the interpretation of both the UV/VIS absorptions and the dependence of fluorescence and fluorescence quantum yields on temperature.     

Rh多晶表面CO2还原过程的电化学和原位FTIR反射光谱研究

应用程序电位扫描法和电化学原位FTIR反射光谱从定量角度在分子水平 上研究了CO2在Rh电极上的电催化还原性能。红外光谱结果指出CO2还原的吸附产物为线型和桥式吸附态CO物种。在所研究和还原电位范围（－0．15－0．40V）和相同还原时间,CO2还原吸附物种的氧化电量随还原电位的负移而增大,在每个还原电位下,时间超过250s时都可达到一个相应的饱和值。原位红外光谱和电化学研究结果表明,CO2的还原与Rh电极表面氢吸附反应密切相关,同时需要一定数量相邻表面位的参与。因此生成的CO不能在Rh电极表面达到满单层吸附,而是形成均匀的亚单层分布。     

Parallelization of a Two-Phase Metaheuristic for Routing Problems with Time Windows

This paper describes the parallelization of a two-phase metaheuristic for the vehicle routing problem with time windows and a central depot (VRPTW). The underlying objective function combines the minimization of the number of vehicles in the first search phase of the metaheuristic with the minimization of the total travel distance in the second search phase. The parallelization of the metaheuristic follows a type 3 parallelization strategy (cf. Crainic and Toulouse (2001). In F. Glover and G. Kochenberger (eds.). State-of-the-Art Handbook in Metaheuristics. Norwell, MA: Kluwer Academic Publishers), i.e. several concurrent searches of the solution space are carried out with a differently configured metaheuristic. The concurrently executed processes cooperate through the exchange of solutions. The parallelized two-phase metaheuristic was subjected to a comparative test on the basis of 358 problems from the literature with sizes varying from 100 to 1000 customers. The derived results seem to justify the proposed parallelization concept.     

Microcomputer interfaced stopped-flow spectrometer with interactive graphics

One problem in the use of stopped-flow rapid-mixing systems is that data can be collected much more rapidly than they can be digitized and analyzed. Microprocessor based systems can overcome this problem, but until now they have been expensive and have required experts to assemble the appropriate components.