Diffusion bounded

Another important speciation refers to Mg bound to RNA, which is essential to the folding and function of this macromolecule. A computational approach to this analysis was presented for site-bound and diffusively bound Mg(II) ions in RNA. This method confirmed the locations of experimentally determined sites and pointed to potentially important sites not currently annotated as Mg binding sites but deserving experimental follow-up in that direction ". 

The micellar surface has a high charge density and the stability of the aggregate is heavily dependent on the binding of counterions to the surface. From the solution of the Poisson-Boltzmann equation one finds that a large fraction (0.4—0.7) of the counterions is in the nearest vicinity of the micellar surface300. These ions could be associated with the Stern layer, but it seems simpler not to make a distinction between the ions of the Stern layer and those more diffusely bound. They are all part of the counterions and their distribution is primarily determined by electrostatic effects. 

Rueda, D., Wick, K., McDowell, S. E., and Walter, N. G. (2003). Diffusely bound Mg2+ ions slightly reorient stems I and II of the hammerhead ribozyme to increase the probability of formation of the catalytic core. Biochemistry 42, 9924—9936. 

When a nucleic acid interacts with cations in aqueous solution, bound states form in the interfacial region of the macromolecule [10], These states are classified either as site bound or diffusely bound [9, 11-14], If the translational energy of the ions is unable to overcome the electrostatic forces between the ions and the nucleic acid, then the ions are viewed as trapped in a volume encasing the surface of the nucleic acid. These ions are site bound [9, 11, 13, 14]. 

If the diffusing bound particle is composed of many molecules, as, for example, in the case of a rigid macroscopic particle, rotational diffusion may influence the shape of the Mossbauer or neutron scattering spectra. Ofer et al. (1984) treat the case in which the particle participates in both bound translational and free rotational diffusion. The Mossbauer spectrum in this case can again be represented by a sum of Lorentzian lines. In the... 

Vibrational Feshbach resonances (VFRs) in a vibrational Feshbach resonance, the interaction of a slow electron takes the form of a virtual excitation of a vibrational level of the neutral molecule with capture of the electron (ffotop et al. 2003 Dessent et al. 2000). For the zero point vibration, the maximum probability of interaction of the electron with parent molecule occurs at zero energy, if the dipole moment of the neutral molecule exceeds the critical value of approximately 2 Debye, the impinging electron maybe trapped into the diffuse bound state, which provides a much longer timescale for the electron to stay near the molecule (fiotop et al. 2003 Dessent et al. 2000 lllenberger 1992), and VFRs may appear as shown in O Fig. 34-5. 

We assume in the following that the ligand is bound in a binding pocket of depth 6 —a = 7 A involving a potential barrier AU = 25 kcal/mol, similar to that of streptavidin (Chilcotti et al., 1995). We also assume that the diffusion coefficient of the ligand is similar to the diffusion coefficient of the heme group in myoglobin (Z) = 1 A /ns) as determined from Mofibauer spectra (Nadler and Schulten, 1984). 

Basis sets can be extended indefinitely. The highest MOs in anions and weakly bound lone pairs, for instance, are very diffuse maybe more so than the most diffuse basis functions in a spht valence basis set. In this case, extra diffuse functions must be added to give a diffuse augmented basis set. An early example of such a basis set is 6-31+G [26]. Basis sets may also be split more than once and have many sets of polarization functions. 

At very low concentrations of water, or in foods held below the free2ing point of water, physical conditions may be such that the available water may not be free to react. Under these conditions, the water may be physically immobi1i2ed as a glassy or plastic material or it may be bound to proteins (qv) and carbohydrates (qv). The water may diffuse with difficulty and thus may inhibit the diffusion of solutes. Changes in the stmcture of carbohydrates and proteins from amorphous to crystalline forms, or the reverse, that result from water migration or diffusion, may take place only very slowly. 

The absorption of sulfonylureas from the upper gastrointestinal tract is faidy rapid and complete. The agents are transported in the blood as protein-bound complexes. As they are released from protein-binding sites, the free (unbound) form becomes available for diffusion into tissues and to sites of action. Specific receptors are present on pancreatic islet P-ceU surfaces which bind sulfonylureas with high affinity. Binding of sulfonylureas to these receptors appears to be coupled to an ATP-sensitive channel to stimulate insulin secretion. These agents may also potentiate insulin-stimulated glucose transport in adipose tissue and skeletal muscle. 

Active Transport. Maintenance of the appropriate concentrations of K" and Na" in the intra- and extracellular fluids involves active transport, ie, a process requiring energy (53). Sodium ion in the extracellular fluid (0.136—0.145 AfNa" ) diffuses passively and continuously into the intracellular fluid (<0.01 M Na" ) and must be removed. This sodium ion is pumped from the intracellular to the extracellular fluid, while K" is pumped from the extracellular (ca 0.004 M K" ) to the intracellular fluid (ca 0.14 M K" ) (53—55). The energy for these processes is provided by hydrolysis of adenosine triphosphate (ATP) and requires the enzyme Na" -K" ATPase, a membrane-bound enzyme which is widely distributed in the body. In some cells, eg, brain and kidney, 60—70 wt % of the ATP is used to maintain the required Na" -K" distribution. 

Materials may be absorbed by a variety of mechanisms. Depending on the nature of the material and the site of absorption, there may be passive diffusion, filtration processes, faciHtated diffusion, active transport and the formation of microvesicles for the cell membrane (pinocytosis) (61). EoUowing absorption, materials are transported in the circulation either free or bound to constituents such as plasma proteins or blood cells. The degree of binding of the absorbed material may influence the availabiHty of the material to tissue, or limit its elimination from the body (excretion). After passing from plasma to tissues, materials may have a variety of effects and fates, including no effect on the tissue, production of injury, biochemical conversion (metaboli2ed or biotransformed), or excretion (eg, from liver and kidney). 

In practice, 1—10 mol % of catalyst are used most of the time. Regeneration of the catalyst is often possible if deemed necessary. Some authors have advocated systems in which the catalyst is bound to a polymer matrix (triphase-catalysis). Here separation and generation of the catalyst is easy, but swelling, mixing, and diffusion problems are not always easy to solve. Furthermore, triphase-catalyst decomposition is a serious problem unless the active groups are crowns or poly(ethylene glycol)s. Commercial anion exchange resins are not useful as PT catalysts in many cases. 

In the special case that A and B are similar in molecular weight, polarity, and so on, the self-diffusion coefficients of pure A and B will be approximately equal to the mutual diffusivity, D g. Second, when A and B are the less mobile and more mobile components, respectively, their self-diffusion coefficients can be used as rough lower and upper bounds of the mutual diffusion coefficient. That is, < D g < Dg g. Third, it is a common means for evaluating diffusion for gases at high pressure. Self-diffusion in liquids has been studied by many [Easteal AIChE]. 30, 641 (1984), Ertl and Dullien, AIChE J. 19, 1215 (1973), and Vadovic and Colver, AIChE J. 18, 1264 (1972)]. 

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