Transport agents

Doubly doped polymers can be used to enhance two functions. For example, Wu et al. [58] added Alq3 as an election transport agent and nile red as an emitter to hole transporting PVK. 

Stabilizers, transport agents, or problem-specific polymers to... 

BiTel has been grown by chemical transport from BizTea and Bils, using Br2 as the transport agent (218). 

The crystal compositions vary, depending on the transport agent used, from TiB,4,y (Ij) and TiBj <,4 (TeCl4). At process temperatures > 1000°C and with I2, the transport direction is reversed. Crystals grow in the cold zone of the tube, but only microscopic needle-like crystals are obtained. ... 

Single-crystal LaBg in well-formed parallelipipeds (0.15 X 0.15 X 2 m) grows when LaBrj is used as transport agent in an Ar atmosphere with T in the source and crystallization zones of 1150 and 900°C, respectively. The transport probably proceeds through the subhalide. The amount of transported LaB is increased by a factor of 10 when oxygen is replacing Ar, and LaClj is substituted for LaBrj. The more efficient transport in this case is due to the formation of LaOCl. 

Sediment may be added by bulk mixing via imbricate thnisting (Bebout and Barton 2002), dehydration (Class et al. 2000), or melting (Johnson and Plank 1999). The latter two may differ in their P-T conditions and, therefore, residual mineralogy as well as relevant partition coefficients. In general, fluids are less effective transport agents than melts (i.e., trace elements are more soluble in melt than in pure water or even brine), but fluid/solid partitioning can fractionate some elements, notably Ba-Th and U-Th, more than melt/solid. However, as pressure increases, the distinction between fluid and melt decreases as their mutual solubility increases and they approach a critical end-point. 

Iron transport agents may belong to the protein or non-protein class. In the former group are found the animal proteins transferrin (25), lactoferrin (26) and conalbumin (27). The low molecular weight iron carrying compounds from microorganisms, the siderochromes, may occur with or without a bound metal ion. Typically, severe repression of biosynthesis of these substances can be expected to set in at an iron concentration of ca. 2 x 10-5 g atoms/liter (28). Most, but not all, of these substances can be described as phenolates or hydroxamates (4). 

The data presented in Table 3, which includes the amino acid composition of baker s yeast and Candida krusei cytochrome c for comparison, show that Ustilago and Neurospora cytochrome c contain the same number of total residues. In seven instances, the number of residues of a particular amino acid/mole are identical. Thus, even in the absence of a sequence for the Ustilago cytochrome it can be concluded that this protein, unlike the siderochromes, has suffered little alteration in the progression from the Ascomycetes to the Basidiomycetes. This can be ascribed to the varying function of the two types of molecules. Cytochrome c must fit into a relatively specific slot bounded by a reductase and an oxidase and it has hence evolved much more slowly than the more freely acting transport agents where the specificity constraints are less demanding. 

The tellurate EIg2Te207 is dimorphic. Both forms have been prepared from HgO, Te02, and Te03 by chemical transport reactions with HgCl2 as transport agent.136 In both modifications [Te(VI)Oe] octahedra are connected to infinite... 

TABLE 2.1. Soil Parent Material Transporting Agents, The Name of the Material, and the Geomorphic Features They Form 1... 

The Van Arkel process can also be used to prepare actinide metals if the starting compound reacts easily with the transporting agent (I2). The thorium and protactinium carbides react with I2 to give volatile iodides above 350°C these are unstable above 1200°C and decompose into the actinide metals and iodine. Attempts to prepare other actinides, such as U and Pu, through the process were not successful, because from Th to Pu along the actinide series, the vapour pressure of the iodide decreases and the thermal stability increases. 

Schematically, in its simplest version, the reactant X (to be transported) is placed at the end of a vacuum sealed tube together with a small quantity of the transporting agent Z. Inside the tube a temperature gradient must be maintained.

The composition of low-volatility two-component compounds in the Ga-Se system as controlled using chemical transport reactions has been studied (Zavrazhnov et al. 2003). For long-term heat treatment of a low-volatility phase in the presence of a transport agent, the annealed phase composition was found to be determined only by the temperature and the nature of the sample. In the case of a two-temperature anneal of gallium selenides with added iodine, the sample composition ranges are presented as a function of the cold and hot zones temperatures. A scheme of their experimental arrangement is shown in Fig. 6.14. 

Oxic Diagenesis Metals remobilized from sediments lying in the oxic zone. Remobilization likely occurs in anoxic microzones adjacent to nodules. Bioturbation is an important metal transport agent. Some nodules now found in oxic sediments were likely formed during times when the redox boundary was closer to the seafloor. 10-50 Todorokite (high Cu and Ni content) 32% 5-10 15-20... 

Drug resistance in vitro and probably in vivo results both from inhibition of influx of the vinca alkaloids and, perhaps more frequently, from promotion of their efflux out of cells (34,35). Until relatively recently, the former mechanism was thought to predominate, and, indeed, certain acquired drug-resistant states are clearly associated with the loss of membrane proteins which can be shown to bind and transport agents into cells (34). However, other resistant states have been shown to be associated with the acquisition of membrane transport proteins which remove toxins (and, therefore, chemotherapeutic agents) both from normal and malignant cells. 

Corey EJ, Bhattacharyya S (1977) Total Synthesis of Enterobactin, a Macrocyclic Iron Transporting Agent of Bacteria. Tetrahedron Lett 3919... 

The van Arkel-De Boer process is widely used to refine metals. A transporting agent such as Ij reacts with the metal (M) to be refined to form a volatile iodide. This iodide is then decomposed at a higher temperature into the refined metal and I2, which becomes available again to react with the impure metal, thus sustaining the process ... 

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