Cations cation diffusion facilitator

For Zn(II) export, there are at least 3 membrane transporters that have been identified in E. coli. ZntA, the high specificity Zn(II) efflux pump, is transcriptionally-regulated by ZntR, a MerR homolog that binds Zn(II) (79-27). ZitB is a member of the cation diffusion facilitator (CDF) family that is specifically induced by Zn(U) (22) however, it is not clear if this transporter is specific only for Zn(II). A second CDF exporter, YiiP, is also present in E. coli, and a recent crystal structure on this exporter revealed details about how Zn(II) binds 23). Although work still remains to be performed to fully understand how... 

Growth of surface oxide films takes place only if cations, anions, and electrons can diffuse through the oxide layer. The growth kinetics of very thin films ( 10-50 A) often follow the Mott or Cabrera-Mott mechanisms in which electrons tunnel through the film and associate with oxygen atoms to produce oxide ions at the surface. A large local electric field (10 -10 V/cm) results at the surface which facilitates cation diffusion from the metal-oxide interface to an interstitial site of the oxide. The film thickness Z at time t is given by... 

Calcium is absorbed from the intestine by facilitated diffusion and active transport. In the former, Ca " moves from the mucosal to the serosal compartments along a concentration gradient. The active transport system requires a cation pump. In both processes, a calcium-binding protein (CaBP) is thought to be required for the transport. Synthesis of CaBP is activated by 1,25-DHCC. In the active transport, release of Ca " from the mucosal cell into... 

It is suggested that the movement of the front by migration (electrical potential), diffusion (chemical potentials), and advection (hydraulic potentials) will cause desorption of cations and other species from clay surfaces and facilitate their release into the fluid.34... 

There appear to be two major ways by which ionophores aid ions to cross membrane barriers. Ionophores such as valinomycin and nonactin enclose the cation such that the outside of the complex is quite hydro-phobic (and thus lipid-soluble). The transport behaviour thus involves binding of the cation at the membrane surface by the antibiotic, followed by diffusion of the complexed cation across the membrane to the opposite surface where it is released. Such carrier type ionophores can be very efficient, with one molecule facilitating the passage of thousands of ions per second. A prerequisite for efficient transport by this type of ionophore is that both the kinetics of complex formation and dissociation be fast. 

The results (Table 10) show that the cryptands could act to produce carrier-mediated facilitated diffusion and there was no transport in the absence of the carrier. The rate of transport depended upon the cation and carrier, and the transport selectivity differed widely. The rates were not proportional to complex stability. There was an optimal stability of the cryptate complex for efficient transport, logKs 5, and this value is similar to that for valinomycin (4.9 in methanol). [3.2.2] and [3.3.3] showed the same complexation selectivity for Na+ and K+ but opposing transport selectivities. The structural modification from [2.2.2] to [2.2.C8] led to an enhanced carriage of both Na+ and K+ but K+ was selected over Na+. The modification changes an ion receptor into an ion carrier, and indicates that median range stability constants are required for transport. Similar, but less decisive, results have been found in experiments using open-chain ligands and crown ethers.498... 

Biological membranes present a barrier to the free transport of cations, as the hydrophilic, hydrated cations cannot cross the central hydrophobic region of the membrane which is formed by the hydrocarbon tails of the lipids in the bilayer. Specific mechanisms thus have to be provided for the transport of cations, which therefore allow for the introduction of controls. Such translocation processes may involve the active transport of cations against the concentration gradient with expenditure of energy via the hydrolysis of ATP. These ion pumps involve enzyme activity. Alternatively, facilitated diffusion may occur in which the cation is assisted to cross the hydrophobic barrier. Such diffusion will follow the concentration gradient until concentrations either side of... 

Intercalation of cations into a framework of titanium dioxide is a process of wide interest. This is due to the electrochromic properties associated with the process (a clear blue coloration results from the intercalation) and to the system s charge storage capabilities (facilitated by the reversibility of the process) and thus the potential application in rocking-chair batteries. We have studied alkali-metal intercalation and ion diffusion in the Ti02 anatase and spinel crystals by theoretical methods ranging from condensed-phase ab initio to semiempirical computations [65, 66]. Structure relaxation, electron-density distribution, electron transfer, diffusion paths and activation energies of the ion intercalation process were modeled. 

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