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11.
Hopanoids are pentacyclic triterpenoid lipids occurring in bacteria. They are synthesized from isopentenyl units which are
formed in a new biosynthetic route leading to isopentenyl diphosphate. Six C5 units are joined to form squalene, the immediate precursor in hopanoid synthesis. In a highly complex cyclization reaction
that shares considerable similarities with that of oxidosqualene to sterols, the hopane skeleton is formed from squalene by
the squalene-hopene cyclase. Recent elucidation of the X-ray structure of this membrane-bound cyclase has shed some light
on the properties of this unusual enzyme. The active site is located in a cavity within the enzyme. The squalene substrate
diffuses through a channel structure from the membrane into this cavity and is there transformed into hopene. Polar side chains
are attached to hopene resulting in the amphiphilic molecular structure of many hopanoids. These hopanoids are membrane components
involved in regulating membrane fluidity and stability. However, the many structural variants of hopanoids indicate that they
may have other interesting but as yet unknown functions. 相似文献
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Erich Huster 《Die Naturwissenschaften》1977,64(9):448-460
This paper deals with the history of exoelectron emission (EE), as well as with the experimental results and the attempts to explain them. The author concludes that thermally and optically stimulated EE (TSEE, OSEE) are different phenomena and that OSEE is mere photoeffect. The source of the energy causing the emission has been heatedly discussed. The author believes that most of the models can be ruled out by the fact that the activation energies do not depend on the nature of the metal nor on its oxide. He supposes the simplest model—direct thermal emission from surface traps—to be right. The author concludes that none of the excitation processes produces electron traps, as is often assumed. They only fill existing traps with electrons, e.g., radiation, mechanical treatment or a simple dip in water. 相似文献
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