摘要:

AbstractThe marine environment is a seemingly inexhaustible treasury of organisms whose secondary metabolites bear witness to the lavishness and inventiveness with which nature is able to manipulate molecular architecture. But to what purpose are these diverse and often grotesque compounds produced? This review is founded on the premise that some of them may be involved in the uptake and transport of metal ions present in the aquatic milieu. Many metabolites produced by terrestrial organisms are known to act as ionophores, but the case for similar behavior by their marine counterparts is far hazier. Notwithstanding the relative abundance of certain metal ions in the oceans, and of metabolite structures possessing features that should facilitate the chelation of metal ions, few attempts to establish a connection between these two phenomena have been reported. We have whittled down the voluminous literature of natural products derived from marine sources to expose a core of observations and speculations germane to our premise. These facts and fantasies are evaluated in this review. A mere handful of metal‐containing complexes has actually been isolated; furthermore, attempts to prepare such complexes in vitro are rare, and spectroscopic evidence for metal–metabolite interactions, whether in vivo or in vitro, is not common. Only with the vanadium‐sequestering tunichromes does a logical (but by no means complete) picture begin to emerge. In several other cases, the plausibility of metal chelation, though mooted by authors, remains unsupported by experimental evidence. However, continuing efforts to obtain structural, and particularly conformational, information on the metabolites by means of X‐ray crystallography, nuclear magnetic resonance spectroscopy, and molecular mechanics calculations would seem to provide the key to a rational approach to this neglected topic. On the basis of recent studies dealing with such structural aspects, we present a selection of candidate compounds, some of which are the targets of our own synthetic attentions, whose potential for binding to metal cations merits further r

ISSN:0570-0833    

DOI:10.1002/anie.199300013

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractTransition metal–mercury complexes were among the first compounds of study for the concept of direct metal–metal bonding which was established more than three decades ago. Since then, a large number of such systems have been synthesized and studied. The fact that mercury is readily attached to a large variety of main group or transition metals has stimulated its use as a general building block in the systematic synthesis of mixed‐metal clusters. The past decade has witnessed a rapid expansion of bimetallic cluster chemistry in which species containing mercury have played a prominent role, and which has led to the discovery of many unprecedented cluster structures and reactions. In particular, the ability of mercury to form multicenter metal–metal bonds with polynuclear cluster fragments has substantially extended its coordination chemistry which was thus far dominated by simple linear structural arrangements. Although certain structural motifs are found to be common to many of the transition metal–mercury clusters investigated to date and thus enable a relatively systematic synthetic approach, the multitude of surprising discoveries has kept the interest in the chemistry of the element itself alive. The recent discovery of the redox and photochemical reactivity of some of these systems has opened up an exciting and promising area of cluster research. Its significance for the synthetic methodology lies in the fact that the increasing redox activity of molecular carbonyl clusters on going to higher nuclearities appears to set a limit on the size of metal frameworks attainable by the standard preparative methods. On the other hand, their potential use as photochromes or redox mediaters in coupled electron‐transfer reactions provides an additional stimulus for future studies in

ISSN:0570-0833    

DOI:10.1002/anie.199300241

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractIn semiconductor particles of nanometer size, a gradual transition from solid‐state to molecular structure occurs as the particle size decreases. Consequently, a splitting of the energy bands into discrete, quantized levels occurs. Particles that exhibit these quantization effects are often called “Q‐particles” or, generally, quantized material. The optical, electronic and catalytic properties of Q‐particles drastically differ from those of the corresponding macrocrystalline substance. The band gap, a substance‐specific quantity in macrocrystalline materials, increases by several electron volts in Q‐particles with decreasing particle size. In Q‐particles there are approximately as many molecules on the surface as in the interior of the particle. Therefore, the nature of the surface as well as the particle size is also largely responsible for the physico‐chemical properties of the particle. Q‐particles of many materials can be prepared in the form of colloidal solutions or embedded in porous matrices and are stable over a long period of time. In sandwich colloids, in which Q‐particles of different materials are coupled, as well as in porous semiconductor electrodes containing Q‐particles in the pores, very efficient primary charge separation is observed. As a result, sandwich colloids have greatly enhanced photocatalytic activity relative to the individual particles, while electrodes modified with Q‐particles show high photocurrents. This article deals with the size quantization effect, the synthesis and characterization of Q‐particles, as well as with the spectroscopic, electrochemical, and electron‐microscopic inv

ISSN:0570-0833    

DOI:10.1002/anie.199300411

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractT lymphocytes (T cells) are the central cell type initiating all immune responses. They are able to recognize other cells in the body that have been invaded by foreign living or nonliving matter. In such cells, foreign peptides generated by intracellular breakdown are complexed with molecules of the major histocompatibility complex (MHC) specially designed for peptide binding. Peptide‐loaded MHC molecules appear on the surface of these cells and alert the immune system. The molecular complex which T cells use for recognition of peptide‐loaded MHC molecules is among the most sophisticated and versatile receptor systems in biology. It consists of specific and nonspecific transmembrane components which assemble to a functional signal transduction unit as the result of ligand binding. Correct assembly leads to activation and relocation of enzymes including membrane‐associated, tyrosin‐specific protein kinases and phosphatases. Transmembrane signaling in T cells depends on the correct assembly and cooperation among multiple molecular components. This may be related to a multitude of different cellular responses of T cells at different stages of differentiation, all elicited through the T cell receptor

ISSN:0570-0833    

DOI:10.1002/anie.199300541

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199300641

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199300671

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199300691

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199300721

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199300741

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199300761

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY