Transport Processes and Carrier Design

The chemistry of transport systems has three main goals to design transport effectors, to devise transport processes, and to investigate their applications in chemistry and in biology. Selective membrane permeability may be induced either by carrier molecules or by transmembrane channels (Fig. 10). 

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Transport is a three-phase process, whereas homogeneous chemical and phase-transfer [2.87, 2.88] catalyses are single phase and two-phase respectively. Carrier design is the major feature of the organic chemistry of membrane transport since the carrier determines the nature of the substrate, the physico-chemical features (rate, selectivity) and the type of process (facilitated diffusion, coupling to gradients and flows of other species, active transport). Since they may in principle be modified at will, synthetic carriers offer the possibility to monitor the transport process via the structure of the ligand and to analyse the effect of various structural units on the thermodynamic and kinetic parameters that determine transport rates and selectivity. 

Photosynthetically active quinones include plastoquinone of green-plant photosystem II, ubiquinone and menaquinone in photosynthetic bacteria, and phylloquinone in photosystem I. Plastoquinone is present in green-plant photosystem II both as a tightly-bound and a loosely-bound electron carrier, designated Qa and Qb, respectively. Qa is photoreduced only to the semiquinone (PQ ) but Qb can accept two electrons, forming the plastohydroquinone (PQ-Hj) [see Chapters 5, 6 and 16 for further discussion]. Plastohydroquinone PQb H2 is the final reduction product of photosystem II and goes on to reduce the cytochrome bj complex as part of the electron transport and proton translocation processes [see Chapter 35 for detailed discussions]. 

Various alternative precursor delivery processes have been designed specifically to circumvent the low volatility and low thermal stability problems associated with (Ba(dpm)2 (see Sect. 2.4.1.2). The first method involves the dissolution of Y(dpm)j, Ba(dpm)2 and Cu(dpm)2 precursors in solvents such as butylacetate [188], THF [153, 156], toluene [189], decane [190] and supercritical carbon dioxide [191]. According to this process, termed aerosol-assisted CVD (AACVD), the multicomponent precursor solution is atomized or vaporized into a carrier gas stream or directly into the reaction chamber, with deposition occurring on a heated substrate. Some attractive features of AACVD include deposition at atmospheric pressure, the ability to use thermally sensitive precursors, and a high precursor transport rate [189]. Figure 2-30 shows a sum-... 

Development of additional experimental procedures for measuring transport rates. Most of the Instrumental designs available are elaborate and/or expensive. Transport of permeates contained within liquid phases as well as in the gas phase roust be considered. Development of membrane support materials that provide greater stability than ILM s but still allow the systematic incorporation of carriers. Far greater understanding of dlffuslonal processes and chemical reactions within lEM s Is essential. The necessity for In situ experimental techniques was discussed In the previous section. 

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