摘要:

AbstractThe tellurides represent a class of compounds which exhibit a wide range of physical and chemical properties. In addition to the salt‐like compounds, some tellurides show metallic properties while others have strong covalent bonds. Like tellurium itself, many of these are semiconductors, e.g., the II‐VI compounds CdTe and MnTe, and in this respect are of special interest. Other tellurides, e.g., NbTe4and ZrTe5, have low‐dimensional electronic transport properties. Some ternary transition metal‐main group metal tellurides appear in an amorphous state with spin glass properties. The tellurium‐rich tellurides are characterized without exception by directed Te‐Te bonds. It turns out that general structural chemical relationships exist between them which allow these different compounds to be treated as a single comprehensive class of substances. They represent a link between the Zintl phases on the one hand, and molecular compounds o

ISSN:0570-0833    

DOI:10.1002/anie.198807593

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe translation of DNA via the genetic code into protein is a fundamental process carried out by all living systems. The extreme accuracy with which it is performed requires that the enzymes involved operate with extremely high specificity. It is for this reason that the structure and function of these proteins have been of special interest to molecular biologists and biochemists for the last twenty years. The discrimination between the different amino acids present in the cytosol and their esterification with the correct tRNA is the special task of the aminoacyl‐tRNA synthetases. One of the most difficult discriminations that has to be performed is the specificity for isoleucine. There are many amino acids in the cell that have a structure similar to isoleucine, in particular valine. To solve this problem, the aminoacyl tRNA synthetases have evolved unique proofreading mechanisms. The most detailed investigations of these mechanisms have been on the isoleucyl‐tRNA synthetases from yeast and fromEscherichia coli. Both enzymes are single‐chain polypeptides with molecular weights of about 110 000. Sequence homologies with other aminoacyl‐tRNA synthetases seem to occur only within narrow regions, for example in the ATP binding site. The identification of the correct amino acid is achieved in a four‐step process. In the first two steps the hydrophobic interaction of the amino acid side chain with the enzyme is utilized for discrimination. The two hydrolytic proofreading steps that follow make use of the steric hindrance associated with the amino acid side chain. The enzymes are characterized by the special property that slight variations in the reaction conditions cause conformational changes that affect the catalytic cycle. In this way the observed specificity of the enzymes can be changed to almost any value. In general, high specificity is associated with high energy cost, which is reflected in high ATP consumption. Under physiological conditions a medium level of specificity and energy consumption seems to be

ISSN:0570-0833    

DOI:10.1002/anie.198807731

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThree types of photochemical reaction of coordination compounds are well‐known and have often been discussed: redox reactions, substitutions, and isomerizations. Less well known is a fourth type of reaction, which occurs fairly frequently: reaction of the ligands themselves. It is the purpose of this article to fill the corresponding gap in the photochemical literature and to summarize our current knowledge of such reactions and of their mechanisms and possible application

ISSN:0570-0833    

DOI:10.1002/anie.198807891

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe chemical reactivity of cyclobutanones and cyclobutenones is considerably different from that of cyclic ketones with larger rings; this is due to their ring strain of ca. 25 kcal/mol. Detailed knowledge regarding the influence of this ring strain on regio‐, chemo‐ and stereoselective transformations of four‐membered ring ketones is of particular importance. While several reactions, such as the Baeyer–Villiger reaction, the Beckmann and Favorskii rearrangements andcine‐substitution often proceed in a manner specific to four‐membered rings, other reactions such as the facile ring‐opening by nucleophiles, the rearrangement to tropolones, the thermal [2+2]‐cycloreversion, the isomerization to vinylketenes and the photochemical formation of oxacarbenes are rather specific to cyclobutanones and cyclobutenones. The remarkable selectivity and the excellent yields of such transformations, which are favored or caused by ring strain as the inherent driving force, offer the synthetic chemist fascinating possibilities for the development of new strategies for the synthesis of natural products and biologically a

ISSN:0570-0833    

DOI:10.1002/anie.198807971

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198808281

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198808291

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198808331

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198808351

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198808371

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198808381

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY