摘要:

AbstractIn investigations concerned with the phenomenon of molecular chirality, the use of gas chromatography for the enantiomeric analysis of stable, volatile compounds is a technique of steadily growing importance.[1]In the last three years an important breakthrough in gas‐chro‐matographic separation of enantiomers has been achieved by using alkylated cyclodextrins (α, β, and γ) as chiral stationary phases in high‐resolution capillary columns. In academic and commercial practice two different and complementary strategies have been adopted up to now. In the first, alkylated cyclodextrins are diluted with polysiloxanes and coated on glass or fused silica capillary columns. In the second, lipophilic per‐n‐pentylcyclodextrins and hydrophilic di‐n‐pentyl‐ and hydroxyalkylpermethylcyclodextrins are coated directly in the form of liquid phases onto suitably pretreated glass or fused silica surfaces. These techniques permit enantiomer separations not only for polar diols and alcohols, derivatized hydroxycarboxylic acids, amino acids, sugars, and alkyl halides, but also for nonpolar alkenes, cyclic saturated hydrocarbons, and metal π complexes. An important aspect for practical applications is that in many cases the enantiomers can be separated without previous derivatization. Whereas the resolution of racemates of unfunctionalized hydrocarbons is attributed to an enantioselective host–guest inclusion complex, some observations indicate that for polar guest molecules additional enantioselective interactions are also involved. The new chiral stationary phases can be used over a wide range of temperatures (25 to 250°C). The technique described is likely to become widely adopted as a simple, accurate and highly sensitive method for the enantiomeric analysis of chiral compounds that can be vaporized without decomposition. It will also stimulate future research aimed at finding universal cyclodextrin phases and elucidating the mechanisms

ISSN:0570-0833    

DOI:10.1002/anie.199009393

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractThe neural sheaths that surround the nerve fibers (axons) are composed of myelin‐specific complex lipids and are assembled during the myelination phase either by the oligodendrocytes in the central nervous system (CNS) or by the Schwann cells in the peripheral nervous system. These multilayered myelin membranes insulate the axons and permit a rapid, saltatory conduction of excitation and a reduced axon diameter in comparison with noninsulated axons. Myelination was hence the decisive evolutionary event in miniaturization of the central nervous system (brain and spinal cord). The morphology of the myelin membrane has been studied in detail mainly by electron microscopy. Most of its biochemistry has been elucidated in recent years by molecular‐level analysis of both the lipid components (cholesterol, phospholipids and sphingolipids) and the constituent proteins. The multilamellar system is distinguished by a characteristic periodicity due to the 5‐nm‐thick bilayer formed by the myelin‐specific lipids. The bilayer interacts with the myelin basic protein (MBP) on the cytosolic side of the plasma membrane process, while the integral membrane protein proteolipid protein (PLP) has hydrophilic domains exposed on both the cytosolic and extracytosolic faces of the bilayer. Numerous protein‐chemical and ‐immunotopochemical findings have been summarized in a model of the myelin membrane. Through molecular biological studies, the genetic structure and chromosomal location of the myelin proteins have been determined. By employing techniques of molecular and cell biology together, it is now possible to analyze the process of myelinogenesis, the time‐ and location‐specific expression of myelin‐specific genes in the brain. Gene‐technological methods have been used to define the mutations in the models jimpy mouse and myelin‐deficient rat. These are animal models that correspond to genetically determined myelin defects (dysmyelinoses) in humans. Using them, it will be possible to study the cell death of oligodendrocytes on a molecular level; this process is the result of expression of mutant myelin proteins and is incompatible with life. Oligodendrocytes and the myelin structures they synthesize are the target structures of cytotoxic lymphocytes (Tc). In the course of the demyelination process in multiple sclerosis, these cause the breakdown of the myelin sheaths, in gradually appearing inflammations. Tclymphocytes recognize myelin structures as epitopes and destroy them. The picture of the myelin membrane's molecular composition, which we are now perfecting, will also lead to a better understanding of demyelination on a molecular level, and hence to new th

ISSN:0570-0833    

DOI:10.1002/anie.199009581

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractThe preparation and utilization of specific biaryl systems, particularly those which suffer hindered rotation, is a demanding goal not only in the synthesis of natural products and Pharmaceuticals, but also for example in the discovery of new reagents for asymmetric synthesis. This article will attempt to provide a topical review of modern, efficient processes for the specific preparation of biaryls. This appears to be of particular urgency, since, under the pressure of continually increasing demand, a wealth of new or modified synthetic approaches to the ever more important biaryl systems has been realized in recent years. The high standard which has been reached in this field is impressively demonstrated by regio‐ and stereoselective syntheses of important biaryl natural products such as steganone, ancistrocladine, ancistrocladisine, and dioncophylline A. Besides the utilization of asymmetric induction in the actual coupling step, the thermodynamically or kinetically controlled torsion of biaryl axes belongs to the most important concepts discusse

ISSN:0570-0833    

DOI:10.1002/anie.199009771

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractDuring recent decades it has become feasible to simulate the dynamics of molecular systems on a computer. The method of molecular dynamics (MD) solvesNewton'sequations of motion for a molecular system, which results in trajectories for all atoms in the system. From these atomic trajectories a variety of properties can be calculated. The aim of computer simulations of molecular systems is to computemacroscopicbehavior frommicroscopicinteractions. The main contributions a microscopic consideration can offer are (1) the understanding and (2) interpretation of experimental results, (3) semiquantitative estimates of experimental results, and (4) the capability to interpolate or extrapolate experimental data into regions that are only difficultly accessible in the laboratory. One of the two basic problems in the field of molecular modeling and simulation is how to efficiently search the vast configuration space which is spanned by all possible molecular conformations for the global low (free) energy regions which will be populated by a molecular system in thermal equilibrium. The other basic problem is the derivation of a sufficiently accurate interaction energy function or force field for the molecular system of interest. An important part of the art of computer simulation is to choose the unavoidable assumptions, approximations and simplifications of the molecular model and computational procedure such that their contributions to the overall inaccuracy are of comparable size, without affecting significantly the property of interest. Methodology and some practical applications of computer simulation in the field of (bio)chemistry will be reviewed.

ISSN:0570-0833    

DOI:10.1002/anie.199009921

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199010241

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199010251

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199010261

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199010281

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199010301

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199010321

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY