摘要:

AbstractNatural products have been synthesized for billions of years in animals, plants, and microorganisms. As a rule they occur enantiomerically pure. Their chiral character corroborates their use in metabolism or as biologically active agents. Natural products may be insufficient in quality or quantity. They have recently begun to become accessible, either unchanged or modified, by biological synthesis; here, too, they are obtained enantiomerically pure. In the last twenty years chemical synthesis has become a major concern of organic chemists. Their target compounds are primarily enantiomerically pure natural products or biologically active variants thereof.

ISSN:0570-0833    

DOI:10.1002/anie.198306373

出版商:Hüthig&Wepf Verlag    

年代:1983

数据来源: WILEY

摘要:

AbstractBilirubin and biliverdin dimethyl esters (BRE and BVE, respectively) and related linear tetrapyrroles have been studied using a combination of photochemical and spectroscopic techniques, the latter including absorption, fluorescence fluorescence excitation, medium‐induced circular dichroism, and proton magnetic resonance. Both types of tetrapyrroles form mixtures of different topological isomers in very dilute solutions. In the case of the bilirubins the heterogeneity of the solutions is caused by two coexisting conformers with different orientations of the A/B and C/D pyrromethenone moieties with repect to each other. The spectral properties of one conformer resemble the isolated parent pyrromethenone, whereas those of the other result from electronic coupling of the two subchromophores presumably held in a “ridge tile” ‐like orientation. CC rotations at the C‐5 and C‐15 bridges substantially compete in both components with the photochemical channels (E→Zisomerization and lumirubin formation) for the radiationless deactivation of the excited singlet state. The more rigid “ridge tile” component additionally undergoes hydrogen bond‐mediated deactivation, and it photoisomerizes more efficiently. The situation is markedly more complex with the biliverdins. In order to obtain a more detailed insight into the mechanisms of the radiationless excited‐state processes, time‐resolved optoacoustic spectroscopy and ultrafast absorption (pump‐probe) and fluorescence detection (single‐photon‐timing) techniques were used to supplement the stationary methods. The solution mixtures are composed of a (family of) helically coiled all‐Z, all‐synspecies, and of species differing from the former by stretched arrangements of the rings B and C around the central C‐10 bridge (E‐anti,E‐syn, andZ‐anti). Two excited singlet states with picosecond lifetimes are attributed to either one or two coiled ground‐state forms, and two remarkably long‐lived nanosecond excited states arise each from a stretched ground state. The radiationless deactivation of the shorter‐lived of the picosecond states is brought about by ultrafast intramolecular proton transfer between the B/C nitrogen atoms, in addition to the CC rotational modes operative in both.Z→Ephotoisomerization is also an appreciable deactivation channel of excited biliverdin dimethyl ester. It is confined to the central C‐10 double bond and selectively affords a stretched isomer (10E‐anti), which thermally reforms the coiled starting meterial at room temperatureviaa sequence of tautomerization and CC rotation. Heating or ultrasonic treatment can reverse this sequence and drive it farther to populate another stretched isomer (10E‐syn) which is thermally stable at room temperature. This stretched form aggregates (presumably to dimers) already at concentrations at

ISSN:0570-0833    

DOI:10.1002/anie.198306561

出版商:Hüthig&Wepf Verlag    

年代:1983

数据来源: WILEY

摘要:

AbstractRecent advances in the synthesis and spectroscopic characterization of ligand‐free metal clusters immobilized in cryogenic rare gas matrices have contributed greatly to the understanding of electronic, geometric, dynamic, and chemical bonding properties of a wide range of uni‐ and bimetallic clusters as a function of nuclearity and metal type. The knowledge accumulated on molecular metal aggregates devoid of ligands and isolated on various supports will form an important data base for gauging the reliability of quantum chemical calculations on metal clusters, as well as for comprehending certain aspects of chemisorption on, and catalysis by, supported metal clusters. It can be envisaged that information on ligand‐free metal clusters entrapped in a wide range of matrix environments in conjunction with the data for these same metal clusters in the gas phase and in molecular beams will probably contribute towards understanding metal‐support interactions and to the designed synthesis of a new breed of high‐technology heterogeneous catalysts in the not too dista

ISSN:0570-0833    

DOI:10.1002/anie.198306741

出版商:Hüthig&Wepf Verlag    

年代:1983

数据来源: WILEY

摘要:

AbstractProbably the most fascinating biological phenomenon is the movement which is so impressively observed in higher organized plants when,e.g., in a sort of “quick retreat”, the sensitive plant apparently vanishes on touching, or when all the pinnules of an acacia fold together pairwise at dusk, as though the whole tree were going to “sleep”. Such plants and trees do not possess muscles for the movement of their organs; instead of the contraction of a “still primitive” actomyosin system in this movement the hydrostatic internal pressure of the vacuoles—the turgor—in the parenchymal cells of the motile organs is often drastically and unilaterally reduced. After preception of an external stimulus there follows a gradual or sudden change in the semipermeability of the boundry plasma layers, and, mediated by a stimulus conduction, also of all the symplasts of a multicellular plant organism. The phytohormones which form the molecular basis of the physiology of movement discussed here have now been isolated and structurally elucidated. The chemical agonists of phytodynamics, referred to as turgorins, elicit leaf movement; moreover, they very probably also regulate—perhaps together with other phytohormones—the mechanism of change of aperture in stomatal transpiration. Hence, the turgorins facilitate not only a regulation of temperature, but even their own transport in the sap of t

ISSN:0570-0833    

DOI:10.1002/anie.198306951

出版商:Hüthig&Wepf Verlag    

年代:1983

数据来源: WILEY

摘要:

AbstractThe complete manuscript of this communication appears in:Angew. Chem. Suppl. 1983, 1025. DOI:10.1002/anie.198310250

ISSN:0570-0833    

DOI:10.1002/anie.198307112

出版商:Hüthig&Wepf Verlag    

年代:1983

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198307121

出版商:Hüthig&Wepf Verlag    

年代:1983

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198307131

出版商:Hüthig&Wepf Verlag    

年代:1983

数据来源: WILEY

摘要:

AbstractThe complete manuscript of this communication appears in:Angew. Chem. Suppl. 1983, 957. DOI:10.1002/anie.198309570

ISSN:0570-0833    

DOI:10.1002/anie.198307151

出版商:Hüthig&Wepf Verlag    

年代:1983

数据来源: WILEY

摘要:

AbstractThe complete manuscript of this communication appears in:Angew. Chem. Suppl. 1983, 1005. DOI:10.1002/anie.198310050

ISSN:0570-0833    

DOI:10.1002/anie.198307161

出版商:Hüthig&Wepf Verlag    

年代:1983

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198307171

出版商:Hüthig&Wepf Verlag    

年代:1983

数据来源: WILEY