Cyclic transfer

The third alternative is a more robust, sensitive and specialized fonn of the first, in that only hydrogen nuclei indirectly spin-spin coupled to in a specific molecular configuration are imaged. In achieving selectivity, the technique exploits the much wider chemical shift dispersion of compared to H. The metliod involves cyclic transfer from selected H nuclei to indirectly spin-spin coupled C nuclei and back according to the sequence... 

Hydrological cycle The cyclic transfer of water vapor from the Earth s surface via evapotranspiration into the atmosphere, from the atmosphere via precipitation back to earth, and through runoff into streams, rivers, and lakes, and ultimately into the oceans (U.S. Geological Survey, 2003). 

RNR catalyzes the conversion of ribonucleotides to deoxyribonucleo-tides. Because of this central position in generating precursors for DNA synthesis, the enzyme has been the target for pharmacological modification. The reaction has been proposed to proceed through a radical mechanism involving the cyclic transfer of a preexisting radical in the enzyme to the substrate and back (Stubbe, 1989). Several classes of RNR have been identified on the basis of the type of radical and associated metallocenter. For the purposes of this chapter, only the E. coli enzyme, which has a tyrosyl radical and a binuclear iron cluster, is considered. 

The identification of alkaloids of this type has been greatly facilitated by the introduction of mass spectrometry (165,162,163). By examination of a large number of derivatives, the breakdown pattern has been established, the principal peaks being represented by the fragments v and w which are formed by a cyclic transfer of electrons in ring D... 

As mentioned in Chapter 35, the Cyt b(Jcomplex is involved not only in noncyclic, or linear, electron transport but also in cyclic transfer around PS I. In the latter case, the electrons received from photosystem I by Fd, instead of going to reduce NADP, are transferred to the plastoquinone pool via b f. During this cyclic process, protons are translocated across the thylakoid membrane, contributing to the transmembrane proton gradient. This cyclic electron-transfer pathway, which is independent of PS II, functionally resembles that of the bacterial photosynthetic system. The existence of a cyclic electron-transfer pathway also helps to account for the observation that chloroplasts often require more than 8 photons for the evolution of one O2 molecule. The physiological function of the cyclic pathway, just as it is for the Q-cycle, is to increase the amount of ATP produced relative to the amount of NADPH formed, and thus provide a mechanism for the cell to adjust the relative amounts of the two substances according to its needs. 

Gawrzynski, Z. 1987. Sugar cooling in a pulsed fluidized bed with cyclically transferred airflow. Zuckerind, 112(10) 875-882 (in German). 

In Schemes 8—12 above, the syntheses of some dioxabicyclo (3.2.1) octyl pheromones by acyclic transfer were discussed. We now consider another member of this series which has been prepared by cyclic transfer . 

The ylide mechanism (the transition state of which has been eu ed to possess especial stability in view of the delocalisation of electrons possible in the cyclic transfer) , has been generally accepted. An ion-pair mechanism similar to that proposed for the Stevens reaction, seems unlikely as the rotation within the pair necessary to lead to ortho migration would be expected to produce some para migration, which has never been observed. When no o-hydrogen is available as in 55, the expected intermediate 56 can be isolated -and this breaks down on heating or on acid treatment as shown in reaction (59), but attempts to capture the methylene ylide (57) as a... 

This is the condition for a cyclical transfer O [transforming input energy (coding x) to output energy (coding y)] to function, physically as expressed by (26) and informationally as expressed by the inequality in (40),... 

Assuming that H(Y X) < 0 [information variant of Kelvin s formulation of the 11. Principle of Thermodynamics for irreversible cyclical transfer O when the respective relation (34) is valid], we also have the information variant of the second part of Carnot s theorem... 

Lithium ion batteries, based on a carbonaceous anode and a lithium metal oxide cathode, are high-energy power sources that are well established in the consumer electronics market. The lithium ion concept, however, can be extended to any electrode combination that assures a cyclic transfer of lithium ions across the cell. In general, a lithium ion cell can be considered as based on a lithium-rich L M Y electrode and a lithium-accepting electrode. The electrochemical process is ... 

It should be noted that the electrochemical process of polypyrrole can be modified by pre-doping it with a large anion, such as the dodecylsul-phate anion. In such a case, the large anions are somewhat immobilized in the polymer matrix, so that it is mainly the lithium ion that participates in the electrodic process [13]. Therefore, under these conditions the cell may operate on the cyclic transfer of lithium from the graphite electrode to the polypyrrole electrode, according to ... 

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