Some biological transformations involving unsaturated linkages: the importance of charge separation and charge neutralization in enzyme catalysis

Current status of knowledge on the biological reduction of CC and CO is briefly reviewed. It is Argued that the crucial event in the reduction of CC is the addition of a proton to the more electron-rich terminal of the double bond to produce an electron-deficient species which is then neutralized through hydride transfer from NADPH. The activation for the reduction of a CO group may also be achieved by a related process in which the carbonyl oxygen is polarized by H-bonding to an acidic group on the enzyme, prior to hydride transfer from NAD(P)H.Thus with both these systems an early event in catalysis is the protonation of the substrate for which, normally, strong adds win be required. Since the groups available at the active-met of enzymes are weak acids, a mechanism through which powerful proton donating species could be transiently generated from them is proposed. The salient features of this mechanism may be enuinciatcd as follows: Let us consider the enzyme-substrate complex (A) in which an imidazolium group is about to play a role as a proton donating species. It is argued that rearrngement of initial complex(A)to the catalytic-complex(B), in which the negatively chared counter ion is removed away from the imidazolium cation, would transiently convert the latter group into a powerful proton danating species. the rearrangement (A)→(B) could occur through a protein conformational change or via a charge-relay system or a combination of both processes.     

Quantum mechanical scattering calculations on a spline-fitted ab initio surface: the He + H+2 (ν = 0, 1, 2) → HeH+ + H reaction

All-channel time-dependent quantum mechanical reaction probabilities are reported for the collinear He + H⁺₂(ν = 0, 1, 2) → HeH⁺ + H reaction at a total energy of 1.2 eV on previously reported diatomics-in-molecule (DIM) and spline fitted ab initio (SAI) surfaces. These results are in agreement with the previous quasiclassical trajectory results in that there is vibrational enhancement of the reaction probability on the SAI surface but not on the DIM surface. This agreement lends support to our previously drawn conclusion that small differences in the potential-energy surface can lead to substantially different dynamic results.     

Chemistry of the phenoxathiins IV. Synthesis of 9-substituted 1 -azaphenoxathiins

In an extension of recently reported syntheses of the I-azaphenoxathiin nucleus as well as several 7-substituted analogs, the synthesis of several 9-substituted members of this series is now reported. In addition, the first ¹³C-nmr spectral evidence of an interaction between a sulfur atom and the oxygen of an ortho-nilro group which has been previously observed only in X-ray erystallographic studies is also described. The possible consequences of this interaction on the reaction pathway leading to the cyclization of the 9-substituted J-azaphenoxathiin nucleus is also presented.     

Synthèse stéréospécifique récurrente de structures apparentées à celle de l'hormone juvénile de Hyalophora cecropia

A stereospecific synthesis of Hyalophora cecropia juvenile hormone analogs is proposed. The three double bonds are obtained successively by addition of an alkyl copper reagent to the apporpriate 1-alkyne.     

Polychloroprene rubbers used for adhesives: a small angle X-ray scattering preliminary investigation under stretching

Summary Three types of polychloroprene rubbers (Du Pont de Nemours Co. Neoprenes AD, WHV and W) used for adhesives and having different crystallization rates have been investigated.The mechanical properties of the Neoprenes samples were determined and the films were examined under stretching by SAXS for a preliminary investigation on their different superreticular order.

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Zusammenfassung Drei Typen von Polychloropren-Kautschuk mit verschiedener Kristallisationsgeschwindigkeit (Neoprene AD, WHV und W von Du Pont de Nemours), die als Klebemittel dienen, wurden als gedehnte Filme mechanisch und röntgenographisch (SAXS) untersucht. Sie zeigen Unterschiede in der Überstruktur.

     

Thermochemical properties of benzoic acid derivatives VIII. Enthalpies of combustion and formation of o-, m-, and p- t-butylbenzoic acids

The enthalpies of formation in the crystalline state at 298.15 K of o-, m-, and p-t-butyl-benzoic acids have been determined by static-bomb calorimetry. Vapour-pressure determinations were made by the Knudsen-effusion technique and the sublimation enthalpies at the mean temperatures of the measurement ranges have been derived for the three acids. The values obtained are:

     

947.

Massenspektren neuer Phloroglucide,insbesondere solcher mit Valerylseitenketten     

M. Lounasmaa  C.-J. Widn  T. Reichstein 《Helvetica chimica acta》1973,56(3):1133-1144

Mass spectra of phloroglucinol derivatives recently isolated from African Dryopteris species are presented. Phloroglucides with n-valeryl side chain were found in these ferns for the first time. In the mass spectra they show strong peaks corresponding to loss of propene (C₃H₆) produced by McLafferty rearrangement, as shown in model compounds. Aside of the rottleron change (discussed formerly) this must be considered in order to avoid wrong interpretation, particularly when mass spectra are used to analyse mixtures of homologues, as usually present in the plants. If valeryl side chains are present other methods must be used therefore to measure the amount or show the absence of lower homologues with acetyl side chains.     

948.

A kinetic approach to homogeneous Ziegler type polymerization. Steady state     

Maria Merces Marques  C. Costa  F. Lemos  F. Ram?a Ribeiro  A. R. Dias 《Reaction Kinetics and Catalysis Letters》1997,62(1):9-15

In this paper we present a kinetic approach to the analysis of steady-state homogeneous Ziegler-Natta polymerization activity data. The influence of the number of monomeric species that are coordinated to the active site on the apparent rate law is discussed and the equations are fitted to the experimental results.     

949.

Identification of soy protein in meat by pyrolysis-high-resolution gas chromatography     

S K Raghavan  C T Ho  H Daun 《Journal of chromatography. A》1986,351(2):195-202

Pyrolysis-gas chromatography was applied to the characterization of ground beef and soy protein isolate and its mixtures at the 50, 30 and 10% levels. A cryogenic trap pyrolysis mode was employed and the volatile pyrolysates were analysed by a high-resolution capillary gas chromatograph. Two phenolic derivatives, o-methoxy- and dimethoxyphenol, as well as 2,3-dithiabutane are uniquely observed at a large concentration in the soy pyrolysate. Quantification of these characteristic compounds allow the estimation of the level of soy inclusion in the mixture. This approach effectively denotes soy addition up to the 10% level in comminuted beef.     

950.

Molecular structures of two unexpected metal complexes obtained from additions of coordinated phosphines to acetylenes     

P.M. Treichel  W.K. Wong  J.C. Calabrese 《Journal of organometallic chemistry》1978,159(4):C20-C24

The syntheses and structures of two new compounds are reported. The first compound, [Cr(CO)₄]₂[C₄F₂(PPh₂)₄], obtained from Cr(CO)₄(PPh₂H)₂ and CF₃CCF₃ in the presence of one equivalent of BuLi has a structure with the ligand 1,2,3,4-tetrakis(diphenylphosphino)-1,4-difluorobutadiene chelating to two Cr(CO)₄ groups via the 1,4 and 2,3 phosphine groups. A mechanism for the formation of this compound is suggested which involves sequential deprotonation of a phosphine, nucleophilic attack on the fluorocarbon, and fluoride ion elimination. The second compound, CrC₃₆H₂₈P₂O₇, arises from a similar base promoted reaction of Cr(CO)₄-(PPhH₂)₂ and PhCCCOOEt. Here the expected initial product from cyclization of these reactants acts as a nucleophile to attack a second equivalent of the acetylene. The intermediate carbanion from this reaction can undergo a ring closure by displacement of OEt^-, giving the observed product.     

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	$\text{ΔH}{}_{\mathrm{f}}{}^{\mathrm{o}}\text{(c, 298.15 K)/kJ mol}{}^{\mathrm{?1}}$	$\text{ΔH}{}_{\mathrm{s}}\text{/kJ mol}{}^{\mathrm{?1}}$
o-t-butylbenzoic acid	476.2 ± 1.9	99.8 ± 0.4	(at 314.6 K)
m-t-butylbenzoic acid	504.3 ± 1.6	103.0 ± 0.5	(at 326.5 K)
p-t-butylbenzoic acid	502.9 ± 1.7	103.8 ± 0.4	(at 334.1 K)

Massenspektren neuer Phloroglucide,insbesondere solcher mit Valerylseitenketten

Mass spectra of phloroglucinol derivatives recently isolated from African Dryopteris species are presented. Phloroglucides with n-valeryl side chain were found in these ferns for the first time. In the mass spectra they show strong peaks corresponding to loss of propene (C₃H₆) produced by McLafferty rearrangement, as shown in model compounds. Aside of the rottleron change (discussed formerly) this must be considered in order to avoid wrong interpretation, particularly when mass spectra are used to analyse mixtures of homologues, as usually present in the plants. If valeryl side chains are present other methods must be used therefore to measure the amount or show the absence of lower homologues with acetyl side chains.     

A kinetic approach to homogeneous Ziegler type polymerization. Steady state

In this paper we present a kinetic approach to the analysis of steady-state homogeneous Ziegler-Natta polymerization activity data. The influence of the number of monomeric species that are coordinated to the active site on the apparent rate law is discussed and the equations are fitted to the experimental results.     

Identification of soy protein in meat by pyrolysis-high-resolution gas chromatography

Pyrolysis-gas chromatography was applied to the characterization of ground beef and soy protein isolate and its mixtures at the 50, 30 and 10% levels. A cryogenic trap pyrolysis mode was employed and the volatile pyrolysates were analysed by a high-resolution capillary gas chromatograph. Two phenolic derivatives, o-methoxy- and dimethoxyphenol, as well as 2,3-dithiabutane are uniquely observed at a large concentration in the soy pyrolysate. Quantification of these characteristic compounds allow the estimation of the level of soy inclusion in the mixture. This approach effectively denotes soy addition up to the 10% level in comminuted beef.     

Molecular structures of two unexpected metal complexes obtained from additions of coordinated phosphines to acetylenes

The syntheses and structures of two new compounds are reported. The first compound, [Cr(CO)₄]₂[C₄F₂(PPh₂)₄], obtained from Cr(CO)₄(PPh₂H)₂ and CF₃CCF₃ in the presence of one equivalent of BuLi has a structure with the ligand 1,2,3,4-tetrakis(diphenylphosphino)-1,4-difluorobutadiene chelating to two Cr(CO)₄ groups via the 1,4 and 2,3 phosphine groups. A mechanism for the formation of this compound is suggested which involves sequential deprotonation of a phosphine, nucleophilic attack on the fluorocarbon, and fluoride ion elimination. The second compound, CrC₃₆H₂₈P₂O₇, arises from a similar base promoted reaction of Cr(CO)₄-(PPhH₂)₂ and PhCCCOOEt. Here the expected initial product from cyclization of these reactants acts as a nucleophile to attack a second equivalent of the acetylene. The intermediate carbanion from this reaction can undergo a ring closure by displacement of OEt^-, giving the observed product.