Channel-forming

Urea has the remarkable property of forming crystalline complexes or adducts with straight-chain organic compounds. These crystalline complexes consist of a hoUow channel, formed by the crystallized urea molecules, in which the hydrocarbon is completely occluded. Such compounds are known as clathrates. The type of hydrocarbon occluded, on the basis of its chain length, is determined by the temperature at which the clathrate is formed. This property of urea clathrates is widely used in the petroleum-refining industry for the production of jet aviation fuels (see Aviation and other gas-TURBINE fuels) and for dewaxing of lubricant oils (see also Petroleum, refinery processes). The clathrates are broken down by simply dissolving urea in water or in alcohol. 

Fig. 18. Crystal structures of recent clathrate design (a) coordinatoclathrate between host (39) (Fig. 17) and / -butanol (host—guest hydrogen bonding in the shaded area) (b) perspective view of the hehcal inclusion channel formed by diol host (43) (Fig. 17 all except one host molecule are represented...

Fig. 3. Model of the crystal structure of the mineral mordenite showing the main channel formed by 12-membered ring and small channels which contain some of the sodium cations. Synthetic types of mordenite exhibit the adsorption behavior of a 12-membered ring, whereas the mineral does not, probably...

The diffusion, location and interactions of guests in zeolite frameworks has been studied by in-situ Raman spectroscopy and Raman microscopy. For example, the location and orientation of crown ethers used as templates in the synthesis of faujasite polymorphs has been studied in the framework they helped to form [4.297]. Polarized Raman spectra of p-nitroaniline molecules adsorbed in the channels of AIPO4-5 molecular sieves revealed their physical state and orientation - molecules within the channels formed either a phase of head-to-tail chains similar to that in the solid crystalline substance, with a characteristic 0J3 band at 1282 cm , or a second phase, which is characterized by a similarly strong band around 1295 cm . This second phase consisted of weakly interacting molecules in a pseudo-quinonoid state similar to that of molten p-nitroaniline [4.298]. 

Colicins are pore-forming proteins, produced by certain strains of E. coli, that kill or inhibit the growth of other, competing bacteria and even other strains of E. coli (a process known as allelopathy). Channel-forming colicins are released as soluble monomers. Upon encountering a host cell, the colicin molecule traverses the bacterial outer membrane and periplasm, then inserts itself... 

FIGURE 10.33 The structure of the heptameric channel formed by o -hemolysin. Each of the seven subunits contributes a /3-sheet hairpin to the transmembrane channel. 

Chri.sten.sen, B., et al., 1988. Channel-forming propertie.s of cecropin.s and related model compound.s incorporated into planar lipid mem-brane.s. Proceedings of the National Academy of Sciences U.S.A. 85 5072—5076. 

The structures of boron-rich borides (e.g. MB4, MBfi, MBio, MB12, MBe6) are even more effectively dominated by inter-B bonding, and the structures comprise three-dimensional networks of B atoms and clusters in which the metal atoms occupy specific voids or otherwise vacant sites. The structures are often exceedingly complicated (for the reasons given in Section 6.2.2) for example, the cubic unit cell of YB e has ao 2344 pm and contains 1584 B and 24 Y atoms the basic structural unit is the 13-icosahedron unit of 156 B atoms found in -rhombohedral B (p. 142) there are 8 such units (1248 B) in the unit cell and the remaining 336 B atoms are statistically distributed in channels formed by the packing of the 13-icosahedron units. 

Spiral-wound modules consist of several flat membranes separated by turbulence-promoting mesh separators and formed into a Swiss roll (Figure 16.18). The edges of the membranes are sealed to each other and to a central perforated tube. This produces a cylindrical module which can be installed within a pressure tube. The process feed enters at one end of the pressure tube and encounters a number of narrow, parallel feed channels formed between adjacent sheets of membrane. Permeate spirals roward the perforated central tube for collection. A standard size spiral-wound module has a diameter of about 0.1m, a length of about 0.9 m and contains about 5 m2 of membrane area. Up to six such modules may be installed in series in a single pressure tube. These modules make better use of space than tubular or flat sheet types, but they are rather prone to fouling and difficult to clean. 

D, shown in bold in Fig. 1 bottom) in the pore loop of TRPV5, which is conserved in TRPV6, but not in the other members of the TRPV subfamily, has been shown to be responsible for the high Ca2+selectivity of channels formed by TRPV5 [3]. 

A spiral plate exchanger is illustrated in Figure 9.90 in which two fluids flow through the channels formed between the spiral plates. With this form of construction the velocity may be as high as 2.1 m/s and overall transfer coefficients of 2.8 kW/m2 K are frequently obtained. The size can therefore be kept relatively small and the cost becomes comparable or even less than that of shell and tube units, particularly when they are fabricated from alloy steels. 

Bechinger, B. Structure and functions of channel-forming peptides Magainins, cecropins, melittin, and alamethicin. /. Memhr. Biol. 1997, 756, 197-211. 

FIG. 23 SPFM image of a network of interconnected water channels formed after 5 seconds of tip contact at 40% RH, with a mica surface contaminated as a result of exposure to the ambient air for about 2 hours. Notice that many angles between segments are close to 120°. The area covered by the water structures increases with contact time. (From Ref. 51.)... 

In analogy to a series of polypeptide channel forming quasi ionophores, a model of pardaxin tetramer in water and in the membrane is presented ... 

---