摘要:

A kinetic study has been made of the ribonuclease-catalyzed hydrolyses of three cyclic nucleotides, cytidine-2′,3′-phosphate, uridine-2′,3′-phosphate, andN6,O5′-diacetyl cytidine-2′,3′-phosphate. Rates were measured at pH values ranging from 6 to 8.5. The variation of the kinetic parameters with pH showed that the free enzyme possesses two active groups, having pKvalues of 5.4 and 7.25. When the enzyme–substrate complex is formed, the pKvalues of the groups are increased to 6.6 and 8.4. The pKvalues identify these groups as imidazole groups and show that two histidine residues are present at the active site. Since both increase in pKon complex formation, it is concluded that the acid imidazole group binds the substrate, but that the basic imidazole group cannot be concerned in substrate binding and must function only in the hydrolytic step. The results indicate that the pyrimidine base is concerned in the hydrolytic step and not solely in binding, as had been postulated. It is concluded from all of the evidence that four specific sites are present at the active center of the enzyme; three are involved in binding and one in catalysis. It is proposed that the active site of ribonuclease is composed of: the histidine residue in position 12, which catalyzes the hydrolytic step; the histidine residue in position 119, which binds the 2′-ribose oxygen atom in the substrate; the lysine residue in position 41, which binds the phosphate group or anion; and the aspartic acid residue in position 121, which binds the nitrogen atom at N1in the pyrimidine base. A mechanism for enzyme–substrate complex formation and subsequent hydrolysis is proposed.

ISSN:0008-4042    

DOI:10.1139/v66-392

出版商:NRC Research Press    

年代:1966

数据来源: NRC

摘要:

The interaction of phenol and phenol-dwith a number of donors of varying basicity has been studied in terms of the equilibrium constants, ΔS0, ΔH0, ΔvOH, and ΔvODand various aspects of deuterium isotope effects on hydrogen bonding have been discussed. The deuterium isotope effects are found to vary markedly with the basicity of the donor and the results are discussed in terms of the zero point contributions to theKH/KD.

ISSN:0008-4042    

DOI:10.1139/v66-393

出版商:NRC Research Press    

年代:1966

数据来源: NRC

摘要:

The specific heat and dimensional changes of the porous 96% silica glass – water system were measured simultaneously between −35 °C and +2 °C with coverages of 1.6, 4.0, 5.3, and 7.7 molecular layers. The specific heat of the adsorbate below the transition range was found to be higher than that of ice. No phase change was detected with the lowest concentration. At the higher coverages, transition always occurred near −9.5° and was of the anomalous first order type. The values of the specific latent heat of fusion, calculated by assuming that two monolayers do not freeze, are 49.4, 54.4, and 55.5 calg−1, depending on the concentration. The correlation between the results of calorimetric and dimensional change measurements was considered satisfactory.

ISSN:0008-4042    

DOI:10.1139/v66-394

出版商:NRC Research Press    

年代:1966

数据来源: NRC

摘要:

The hydrodesulfurization of thiophene over five molybdenum disulfide catalysts, an alumina-supported cobalt molybdate, and a chromia catalyst has been investigated. Comparisons on the basis of equal surface areas showed that desulfurization activity did not vary a great deal. It was established mainly by working at very low conversion levels that 1,3-butadiene was a product over all these catalysts, and that a mixture of butadiene, the threen-butenes, andn-butane constituted the actualprimaryreaction product. The reaction rate is strongly dependent on conversion, probably as the result of small quantities of product H2S, the rate decreasing by a factor of 100 in going from 0.01% to 5% conversion in a typical flow experiment. A strong enhancement of the rate over MoS2was noted in the presence of oxygen.

ISSN:0008-4042    

DOI:10.1139/v66-395

出版商:NRC Research Press    

年代:1966

数据来源: NRC

摘要:

The effect of the hyperfine coupling anisotropy on the shape of the electron spin resonance spectra of randomly oriented allyl radicals has been studied by means of electronic computer calculation. The line shape has been shown to be strongly dependent on the relative orientation of the tensor axes, and it can be used, therefore, to characterize the structural isomerism of the radical. The comparison of the computed spectra, taking account of theġtensor anisotropy, with the spectra recorded after γ irradiation of several monoethylenic compounds, allowed the identification of the produced allyl radicals and showed that, in most cases, these radicals keep the geometrical configuration of the corresponding undamaged molecules

ISSN:0008-4042    

DOI:10.1139/v66-396

出版商:NRC Research Press    

年代:1966

数据来源: NRC

摘要:

The overall exchange reactionshave been initiated by radiolysis and measured by a nuclear magnetic resonance spectroscopic method. The reactions occur by a chain mechanism, with a chain length of ~ 104in a 1 Msolution of hydrogen chloride in cyclohexane. The mechanism is complex.The exchange reaction can also be initiated by the photolysis of chlorine or of hydrogen chloride.A chain exchange reaction does not occur betweenc-C6H12and DI, D2S, or ND3. These results are consistent with the suggestion that the exchange reaction in the cyclohexane – hydrogen chloride system occurs by a free radical chain mechanism.This type of exchange reaction can cause difficulties in isotopic tracer studies.

ISSN:0008-4042    

DOI:10.1139/v66-397

出版商:NRC Research Press    

年代:1966

数据来源: NRC

摘要:

The Kihara potential has been applied to the Lennard-Jones and Devonshire cell theory in place of the Lennard-Jones potential. The expressions for the internal energy, heat capacity, and entropy, as well as the compressibility, are given in excess over those of a perfect gas. A comparison between experimental data and the present theory is made.

ISSN:0008-4042    

DOI:10.1139/v66-398

出版商:NRC Research Press    

年代:1966

数据来源: NRC

摘要:

The kinetics and energetics of the interaction of DPPH with a variety of hydroxylic compounds have been investigated by electronic and e.s.r. spectroscopy. Deuterium isotope effects on the kinetics have been examined. A model involving hydrogen bonded complexes has been suggested for the hydrogen abstraction reaction.

ISSN:0008-4042    

DOI:10.1139/v66-399

出版商:NRC Research Press    

年代:1966

数据来源: NRC

摘要:

A study has been made of the pressure and temperature dependence of the rate of combination of propylene with silver tetrafluoroborate to produce AgBF4•2C3H6. Dependent upon the prior history of the salt, the reaction may proceed via predominantly diffusion control, predominantly kinetic control, or a combination of the two mechanisms. With both mechanisms, the adsorption of the olefin on the salt surface appears to exert a rate-controlling influence.

ISSN:0008-4042    

DOI:10.1139/v66-400

出版商:NRC Research Press    

年代:1966

数据来源: NRC

摘要:

The preparation of cyclobutadiene nickel halide complexes (R4C4NiX2)2, where R is phenyl or substituted phenyl, by reaction of the palladium complexes, [R4C4PdX2]2, with atert-phosphine metal complex, e.g. (n-Bu3P)2NiX2, is described. The reaction proceeds according to the equationPossible mechanisms are suggested.

ISSN:0008-4042    

DOI:10.1139/v66-401

出版商:NRC Research Press    

年代:1966

数据来源: NRC