摘要:

AbstractCertain aromatic diamines (the “proton sponges”) are found to have exceptionally high basicity constants: this is due to spatial interaction of the basic centers, which are in close proximity. The two factors which are most important in causing this effect are, on the one hand, the extreme steric strain in these systems and the destabilizing effect of the overlap of the nitrogen lone pairs of the neutral diamines and, on the other, the strong NċHċN hydrogen bonds which are formed onmonoprotonation and which lead to a considerable relaxation of the steric strain. By the systematic variation of the structures of such aromatic diamines we have been able to study these effects as a function of steric factors, in particular of the geometry and the bond length of the NċHċN hydrogen bonds, by means of X‐ray structural analysis. The hydrophobic shielding of the basic centers and the NċHċN hydrogen bonds, which was characteristic of the “proton sponge” compounds studied previously, is indeed responsible for the extremely low rate of protonation and deprotonation of these compounds; however, it apparently has no influence on their high thermodynamic basicity. The recent synthesis and basicity determination of a new type of “proton sponge” with no hydrophobic shielding whatever show that not only very strong but also kinetically active bases are accessible using the “proton sponge” concept. Their unusual properties, which are discussed here as the result of steric interactions between two basic centers, provide examples of the fact that cooperative steric interactions of reactive structural elements can lead to properties which cannot be derived from an isolated consideration of the various functional groups. Such “proximity effects” are certainly of general importance in chemistry and biochemistry; the study of their structure‐function relationships is w

ISSN:0570-0833    

DOI:10.1002/anie.198808653

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe information provided by modern crystal structure analyses is not limited to the atomic arrangement. It also includes, for each atom, a set of quantities known as anisotropic Gaussian displacement parameters (ADP's), which provide information about averaged displacements of atoms from their mean positions. From analysis of these quantities, conclusions can be drawn about the rigid‐body motion of molecules, about large‐amplitude internal molecular motions, and about the identification of any disorder present in the crystals as being mainly dynamic or mainly static in nature. For some crystals, such analyses yield energy barriers to rotation of rigid molecules or molecular fragments that are in good agreement with values obtained by other physical meth

ISSN:0570-0833    

DOI:10.1002/anie.198808801

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe pentazadienide ion is a surprisingly versatile ligand. The planar, zigzag N5chain can utilize its atoms N1, N3, and N5 to complex one, two, or three metal atoms in a monodentate or bidentate fashion. Of particular interest is the ligand's ability to promote short metal‐metal contacts in complexes with d10metal ion

ISSN:0570-0833    

DOI:10.1002/anie.198808961

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractIn this article we shall describe processes and phenomena which may appear startling and unusual for someone who is familiar with the behavior of solutions of low molecular weight solutes. Some of these phenomena might even be surprising for someone who works with surfactants. The authors still remember their astonishment when they first became aware of a viscoelastic surfactant solution. This was many years after our group had started investigations on surfactants. But all of the solutions we had encountered until that time had behaved normally. Some of the phenomena which will be discussed are also encountered in solutions of polymers. Due to their high molecular weight one expects to find properties which are different from the properties of low molecular weight solutions and does not find them so surprising. Surfactant solutions, however, are aqueous solutions of small molecules with typical molecular weights of 200 to 400. For some of our experiments even dilute solutions were used which contained only up to one percent of surfactant.

ISSN:0570-0833    

DOI:10.1002/anie.198809021

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractProtein engineering has as its goals the design and construction of new peptides and proteins with novel binding and catalytic properties. In one approach to protein engineering, new active sites have been introduced into naturally occurring proteins either by site‐directed mutagenesis or by chemical modification. Providing that important changes in the tertiary structures do not result from such alterations, at least a portion of the binding site of the original protein should be available for the formation of complexes between the altered enzyme and its substrates. Many examples of active‐site mutations have been described, including the generation by us of a cysteine mutant of alkaline phosphatase. A fundamental limitation of the site‐directed mutagenesis methodology is that replacements of residues are restricted to the twenty naturally occurring amino acids. The alternative, chemical modification, is difficult to carry out for the specific replacement of one amino acid by another. However, we have shown that through such modification coenzyme analogues can be introduced covalently into appropriate positions in proteins, allowing us to produce semisynthetic enzymes with catalytic activities radically altered from those of their precursor proteins. In another approach to protein engineering efforts have focused on the construction of systems where, as a first approximation, folding can be neglected and the preparation of secondary structural units is the target. Examples of the successful design of biologically active peptides and proteins along such lines, taken from our own work, include molecules mimicking apolipoproteins, toxins, and many hormones. In recent studies we have progressed to the stage where we are starting to combine the two general approaches to protein engineering we have described and are able to construct small enzymes like ribonuclease T1and its structural anal

ISSN:0570-0833    

DOI:10.1002/anie.198809131

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198809231

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198809251

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198809261

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198809271

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198809291

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY