摘要:

AbstractIn addition to the static parameters of the chemical shifts and coupling constants, which serve as a source of knowledge for molecular structure and stereochemistry, an NMR spectrum can frequently furnish dynamic quantities characterizing relaxation and exchange phenomena. The information about nuclear switching processes has proved to be particularly useful in practice for the detection of internal molecular motions and for the estimation or determination of the corresponding energy barriers. A plethora of studies of this nature has in the past been performed on simple proton spectra. Methodological developments of recent years have led to a significant reduction of the effort required for the quantitative dynamic evaluation of NMR spectra arising from complex spin systems or involving other nuclei. In many cases it has, moreover, become possible to extract detailed mechanistic information inaccessible by other means. The practical execution of such analyses will be explained and illustrated by a selected number of applications.

ISSN:0570-0833    

DOI:10.1002/anie.198004113

出版商:Hüthig&Wepf Verlag    

年代:1980

数据来源: WILEY

摘要:

AbstractIn this essay, the classical question of “the influence of the number and kind of substituents on the strength of the CC bond”, is pursued with the modern tools of contemporary physical organic chemistry. Based on the work ofKarl Ziegler, the products and kinetics of thermolysis of a large number of highly branched aliphatic hydrocarbons and phenyl‐ or cyano‐substituted derivatives were investigated. For each class of compounds, a linear relationship was found between the free enthalpy of activation of the homolytic cleavage of the weakest CC bond and the strain energy in the ground state. These relationships permitted aquantitativeseparation of steric and electronic effects on the cleavage of CC bonds. The influence of the size of the substituent groups on bond angles, bond lengths, and the conformational behavior of model compounds was studied by means of experimental structure determinations and force field calculations. CC bond lengths up to 164 pm, bond angles at tetracoordinate carbon as large as 126°, and unusual eclipsed andgauchepreferred conforma

ISSN:0570-0833    

DOI:10.1002/anie.198004291

出版商:Hüthig&Wepf Verlag    

年代:1980

数据来源: WILEY

摘要:

AbstractGlycogen phosphorylases catalyze the degradation of glycogen by phosphate (or arsenate) to glucose 1‐phosphate (or glucose + arsenate). All glycogen phosphorylases that have been studied so far contain pyridoxal 5′‐phosphate, a vitamin B6‐derivative, as cofactor. Removal of the cofactor results in an inactive apoenzyme. However, reduction of the azomethine bond linking pyridoxal phosphate to an ϵ‐aminolysyl side chain of the enzyme with NaBH4does not inactivate glycogen phosphorylase. If therefore the cofactor should be involved in catalysis in glycogen phosphorylase it must function differently from all other classical pyridoxal phosphate dependent enzymes, for these are inactivated by reduction.31P‐NMR spectroscopy has revealed that the 5′‐phosphate group of pyridoxal phosphate is present in catalytically active forms of glycogen phosphorylases as dianion in a hydrophobic environment shielded from aqueous solvent. Covalent and/or allostericactivationof muscle glycogen phosphorylases is accompanied by a transition of the monoprotonated form to the dianionic form of the phosphate group of the cofactor. We now report on such ionization changes in unregulatedactivepotato‐ andE. colimaltodextrin phosphorylases on binding of glucose and oligosaccharides and following catalytic turnover,i.e.arsenolysis of α‐1,4‐glycosidic bonds. (Like glycogen phosphorylases, maltodextrin phosphorylases belong to the class of α‐glucan phosphorylases.) The results of experiments carried out by our group together with recent findings on the three dimensional structure of crystalline muscle glycogen phosphorylases indicate a participation of the dianionic phosphate group as proton acceptor for the glucosyl transfer to and from the glucosyl acceptor. Although other interpretations are not excluded, at present little doubt remains that in the case of glycogen phosphorylases the dianionic phosphate group of the cofac

ISSN:0570-0833    

DOI:10.1002/anie.198004411

出版商:Hüthig&Wepf Verlag    

年代:1980

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198004561

出版商:Hüthig&Wepf Verlag    

年代:1980

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198004571

出版商:Hüthig&Wepf Verlag    

年代:1980

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198004581

出版商:Hüthig&Wepf Verlag    

年代:1980

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198004591

出版商:Hüthig&Wepf Verlag    

年代:1980

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198004601

出版商:Hüthig&Wepf Verlag    

年代:1980

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198004611

出版商:Hüthig&Wepf Verlag    

年代:1980

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198004631

出版商:Hüthig&Wepf Verlag    

年代:1980

数据来源: WILEY