摘要:

AbstractRestriction endonucleases are enzymes commonly found in bacteria which can fragment DNA. Cleavage of the DNA molecule takes place after recognition of a nucleotide sequence which is specific for each restriction endonuclease. Some of the enzymes split the DNA within the recognition sequence. This property allows these enzymes to be used for the systematic fragmentation of DNA molecules in structural and functional analyses of genetic material. The biological function of restriction endonucleases could be to provide a sort of primitive immune system which enables the cells to destroy foreign genetic material. Methylation of nucleotides lying within the recognition sequence serves to protect the cell's own DNA from being degraded by its restriction endonucleases. Restriction endonucleases were discovered during the investigation of the restriction of viral growth observed on changing hosts.

ISSN:0570-0833    

DOI:10.1002/anie.197800733

出版商:Hüthig&Wepf Verlag    

年代:1978

数据来源: WILEY

摘要:

AbstractThe uses in applied quantum chemistry of computer‐produced three‐dimensional diagrams of electron‐density distributions are discussed. The two major catagories of such diagrams are contrasted; and advantages are described for plots showing as the third dimension the point‐by‐point electron density in a cross‐sectional cut (corresponding to the other two dimensions) through a molecule. Examples are presented to show how these plots may be used (1) to assess the adequacies of a given mathematical representation employed in the calculation of a wavefunction, (2) to clarify the interrelationship between an individual molecular orbital and the atomic orbitals of the constituent atoms, as well as (3) to explain the canonical set of molecular orbitals of any selected molecule. Furthermore these plots serve(4) to demonstrate clearly a replication of key characteristics between certain molecular orbitals in different molecules. These examples are accompanied by others which indicate how the electron‐density plots may be used(5) to understand and clarify accepted chemical dogma. Finally, the possibility is discussed of employing quantum calculations on a wider scale so as to be of value in the more practical aspects

ISSN:0570-0833    

DOI:10.1002/anie.197800801

出版商:Hüthig&Wepf Verlag    

年代:1978

数据来源: WILEY

摘要:

AbstractHigh‐energy radiation can give rise to pairs of complementary defects in nonmetallic solids by the transfer of electrons between various types of atoms. These “color centers” which are generally paramagnetic, can usually be described as unusual valence states of an element. They are destroyed by heating and in most cases regenerated by renewed irradiation. In a heteropolar solid the formation of color centers usually leads to cancellation of point charges due to foreign ions of other valence or to vacancies. This is shown by the examples of kunzite, brazilianite, smoky quartz, and citrine; the most important methods for the structural elucidation of color centers are also described. Application of the principle of charge balance opens up possibilities for the production of unusual valence states,e.g.Al2+, F2−, Fe4+, and O−. Moreover, the type of the color center often permits far‐reaching conclusions to be drawn about the defect structure of real crystals, which could hardly be clarified in

ISSN:0570-0833    

DOI:10.1002/anie.197800891

出版商:Hüthig&Wepf Verlag    

年代:1978

数据来源: WILEY

摘要:

AbstractBesides mechanical blending and copolymerization there is a third possible way in which two polymers can be combined. Each polymer forms its own network, while both networks interpenetrate each other. There are no covalent bonds between the polymers. Such interpenetrating networks have been synthesized sequentially (from polymer A and monomer B) and simultaneously (from monomer A and monomer B). It is preferable that the polymers be of different chemical type; usually, an elastomer and a glass are combined,e.g.a polyurethane and a polyacrylate. Depending upon the ratio of component polymers either a strengthened elastomer or a glass having impact strength is formed. So far, there are no direct methods for establishing the degree of interpenetration.

ISSN:0570-0833    

DOI:10.1002/anie.197800971

出版商:Hüthig&Wepf Verlag    

年代:1978

数据来源: WILEY

摘要:

AbstractHomoaromaticity has been the subject of many investigations and continues to remain an extraordinarily rich area of chemistry. Quantitative experimental evaluation of the level of long range delocalization and assessment of interactions in molecules having two or more insulating atoms, together with various theoretical estimates of energies and geometries, are areas currently being examined. The primary intention of this review is to provide a critical up‐to‐date survey of relevant spectral and structural data, as well as an analysis of the many reactions believed to involve generation of homoaromatic ions. Extensively discussed are mono‐,bis‐, and trishomoaromatic systems arranged on the basis of their cationic, anionic, or uncharged nature. Presentation of these individualized segments is preceded by a brief analysis of homoaromatic

ISSN:0570-0833    

DOI:10.1002/anie.197801061

出版商:Hüthig&Wepf Verlag    

年代:1978

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.197801171

出版商:Hüthig&Wepf Verlag    

年代:1978

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.197801191

出版商:Hüthig&Wepf Verlag    

年代:1978

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.197801201

出版商:Hüthig&Wepf Verlag    

年代:1978

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.197801211

出版商:Hüthig&Wepf Verlag    

年代:1978

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.197801231

出版商:Hüthig&Wepf Verlag    

年代:1978

数据来源: WILEY