摘要:

Serum albumins from several species, which specifically bind and transport several divalent metals in blood, contain an Asp-X-His or Glu-X-His amino acid triad as their NH2-terminal sequence. We have synthesized a tripeptide, Asp-Ala-His-N-methylamide, corresponding to the native sequence of the human serum albuminN-terminus, and examined its interaction with Zn(II) ions in aqueous solution using carbon-13 nmr techniques. Variations in carbon chemical shifts and spin–lattice relaxation times (T1's) as a function of pH and increments of added Zn(II) were used to delineate sites of Zn(II):peptide interactions. Trends inT1values indicated that local motions of several of the tripeptide carbons were restricted by an induced order accompanying metal binding. Analysis of spectral data suggested that Zn(II) is coordinated to the His imidazole nitrogen atom and the Asp β-carboxylate oxygen atom(s) in each peptide molecule. Similar experiments using a tripeptide analogue, Gly-Gly-His-N-methylamide, demonstrated that in the absence of a side chain carboxylate group, Zn(II) ions will complex the peptide through the His imidazole group and theN-terminal Gly α-amino group. These results support the possibility of a biologically functional role for the Asp-X-His triad as a liganding template for Zn(II)/protein binding.

ISSN:0008-4042    

DOI:10.1139/v80-118

出版商:NRC Research Press    

年代:1980

数据来源: NRC

摘要:

Magnetic susceptibilities of small hydrocarbons and water are calculated by employing gauge-invariant atomic orbitals within MINDO/3 approximation. A comparison with available INDO calculations indicates that MINDO/3 gives reasonable values for diamagnetic contribution, but in general overestimates the paramagnetic contribution to magnetic susceptibility.

ISSN:0008-4042    

DOI:10.1139/v80-119

出版商:NRC Research Press    

年代:1980

数据来源: NRC

摘要:

The consequences are examined of fitting rate constants and temperatures for a solvolytic reaction to the equation logk = A/T + BlogT + C. The coefficientsA,B, andCfor 150 reactions involving organic derivatives are linearly dependent. Possible mechanistic reasons for this underlying pattern of behaviour are considered. The linearity stems from the least squares analysis used to obtain the coefficients and as a result these quantifies are not truly independent. Consequently, the validity of employing this type of analysis based on the three parameter equation is questioned.

ISSN:0008-4042    

DOI:10.1139/v80-120

出版商:NRC Research Press    

年代:1980

数据来源: NRC

摘要:

The crystals belong to the orthorhombic space groupC222, witha = 9.226,b = 12.092,c = 16.513 Å,Z = 4. A single crystal X-ray diffraction analysis has shown that the title compound, in which all carbon atoms aresp2hybridized, exists with the ten membered ring in a slightly twisted tub conformation. The13C nmr spectrum is also reported.

ISSN:0008-4042    

DOI:10.1139/v80-121

出版商:NRC Research Press    

年代:1980

数据来源: NRC

摘要:

A reexamination of Linnett's nonpairing spatial orbital theory, as it is qualitatively employed, has led to a number of conclusions. Firstly, in our view the theory has been inconsistently applied to the prediction of molecular geometries and may be ambiguous in dealing with saturated systems. Secondly, it does not provide a guide to chemical reactivity, in contrast to claims made for it. Finally, NPSO theory does provide a useful extension of resonance theory as it is qualitatively employed in organic chemistry, contrary to an existing claim that NPSO theory makes resonance theory obsolete.

ISSN:0008-4042    

DOI:10.1139/v80-122

出版商:NRC Research Press    

年代:1980

数据来源: NRC

摘要:

The study of the specific reactivity of the ion pairs of alkaline enolates of β-dicarbonyl compounds shows that the lithium enolates, in spite of their characteristically strong enolate–cation interactions display, especially when compared to sodium enolates, a marked tendency towards O-alkylation. This strong O-/C-nucleophilicity of the lithium ion pairs is not very sensitive to changes in medium polarity (from THF or DME to DMF) but depends on the nature of the alkylating agent, being enhanced with moderately electrophilic alkyl sulfonates or sulfate but lessened with alkyl halides or with very electrophilic sulfonates (triflate). It is suggested that the lithium enolate tendency towards O-alkylation is partly the result of a Li+– leaving group interaction (especially in the case of the sulfonates) and partly an intrinsic property of the enolate–lithium ion pair; this latter property is discussed in terms of the respective solvation abilities towards cations of the C- and O-alkylation transition states.Some of our results about the nucleophilic reactivity of ion pairs in DMF are at variance with previous reports in the literature. The origin of the discrepancy is discussed.

ISSN:0008-4042    

DOI:10.1139/v80-123

出版商:NRC Research Press    

年代:1980

数据来源: NRC

摘要:

The isothiouronium salt derived from dimethyl 4-chloromethyl-1,4-dihydro-2,6-dimethylpyridine-3,5-dicarboxylate reacted with ammonium hydroxide to give dimethyl 7-thia-2-aza-1,3-dimethylbicyclo[3.2.1]oct-3-ene-4,8-dicarboxylate, which could be converted back to a dihydropyridine using iodomethane. Replacement of ammonium hydroxide by sodium hydroxide afforded a tetrahydro- and a dihydrothieno[3,4-c]pyridine as by-products. 3,5-Diacetyl-4-chloromethyl-1,4-dihydro-2,6-dimethylpyridine reacted with thiourea, followed by ammonium hydroxide, to give 3-acetyl-2-methylpyrrole, 4-acetyl- and 4,8-diacetyl-7-thia-2-aza-1,3-dimethylbicyclo[3.2.1]oct-3-ene. The structure and stereochemistry of the novel 7-thia-2-azabicyclo[3.2.1]octenes are established using spectroscopic methods. The mechanism of their formation is discussed in terms of Baldwin's rules.

ISSN:0008-4042    

DOI:10.1139/v80-124

出版商:NRC Research Press    

年代:1980

数据来源: NRC

摘要:

Two spin traps, 4-(N-methylpyridinium)tert-butyl nitrone (4-MePyBN) and 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) were examined for their effectiveness at trapping hydroxyl in water. At pH 6 and 22 °C, rate constants for trapping hydroxyl were found to be 0.5 × 109 M−1 s−1(4-MePyBN) and 2.0 × 109 M−1  s−1(DMPO). The hydroxyl adduct of DMPO decayed by a first order process with a half-life of 870 s. This was at least three times greater than that of the 4-MePyBN adduct. These and other results suggest that DMPO is the superior trap for this particular application and is well suited for use in biological systems.

ISSN:0008-4042    

DOI:10.1139/v80-125

出版商:NRC Research Press    

年代:1980

数据来源: NRC

摘要:

The bromination of 2,3-dimethylindole in acetic acid followed by hydrolysis has been shown to yield the known dimer16of 3-hydroxy-2,3-dimethylindolenine15and not 3-hydroxymethyl-2-methylindole12as reported previously by others. Bromination of 2,3-dimethylindole in the presence of triethylamine has yielded 3-bromo-2,3-dimethylindolenine17as a crystalline solid which is stable at room temperature in a nitrogen atmosphere in the presence of triethylamine but decomposes vigorously in the absence of base. Under mildly acidic conditions17reacts rapidly with methanol to yield 3-methoxy-2,3-dimethylindolenine3in good yield. Withtert-butyl hypochlorite in the presence of triethylamine, 2,3-dimethylindole gives 3-chloro-2,3-dimethylindolenine2which has chemical properties similar to17reacting readily with methanol to give3. Similarly tetrahydrocarbazole was converted to the analogous chloroindolenine5and bromoindolenine18both of which readily underwent methanolysis to give6in good yield. A mechanism for the acid catalyzed methanolysis of 3-chloro- and 3-bromoindolenines is proposed.

ISSN:0008-4042    

DOI:10.1139/v80-126

出版商:NRC Research Press    

年代:1980

数据来源: NRC

摘要:

3,4,5-Trichloroguaiacol, which is formed during bleaching of chemical pulp and shown to bioaccumulate in fish, has been synthesized. The structure of the compound has been determined by means of X-ray analysis. The values of the13C nmr chemical shifts and melting point differ from those previously reported. A reaction mechanism is suggested for the formation of 3,4,5- and 4,5,6-trichloroguaiacol.

ISSN:0008-4042    

DOI:10.1139/v80-127

出版商:NRC Research Press    

年代:1980

数据来源: NRC