Solute exclusion

These observations can be extrapolated to the pyrrole series the 2-amino derivatives are very unstable whereas 3-aminopyrroles appear to be more stable. 3-Amino-l-tritylpyr-role (162) appears to exist in solution exclusively in the imino-A -pyrroline form (163) (83JCS(P1)93). 2-Aminoindole (164) is unusual in that it exists mainly as the 3//-tautomer (165). 4-Alkylaminoindoles (166) undergo an unexpected rearrangement to 4-amino-1-alkylindoles (167) when heated with p-toluenesulfonic acid hydrate (82CC1356). 

Equation (36) is certainly more accurate than equation (34) but still does not take into account any exclusion properties that the support may have. In addition, the particles are close-packed and touching so, in the interstitial volume, around the points of contact, some additional solute exclusion will almost certainly take place. The pore... 

Apparently, cyclization involves the formation of open-chain intermediates 342, 343, further closing up to imidazolidines 344 and oxazolidines 345 which eliminate the secondary amine, thus leading to imidazolines 346 and oxazolines 347. The latter exist in the solution exclusively in the enolic forms 348, 349 which are stabilized by conjugation and intramolecular hydrogen bonds. 

A derivative of 5-(indol-2 -yl)dihydropyridazine 44 (R = H) exists in DMSO-iig solution exclusively as tautomer 44a (within the limits of 400-MHz H NMR detection). However, the introduction of methyl groups both into the indole ring and into the pyridazine ring favors a shift of the tautomeric equilibrium... 

The exclusion effect of hard-spheres is illustrated in Figure lA., which shows a spherical solute of radius r inside an infinitely deep cylindrical cavity of radius a. Here the exclusion process can be described by straightforward geometrical considerations, namely, solute exclusion from the walls of the cavity. Furthermore, it can be shown thatiQJ... 

Fig. 5. Probabilities pn of observing n water-oxygen atoms in spherical cavity volumes v. Results from Monte Carlo simulations of SPC water are shown as symbols. The parabolas are predictions using the flat default model in Eq. (11). The center-to-center exclusion distance d (in nanometers) is noted next to the curves. The solute exclusion volume is defined by the distance d of closest approach of water-oxygen atoms to the center of the sphere. (Hummer et al., 1998a)... 

The current-potential curves that we have considered so far dealt exclusively with the Faradaic component of the current and concerned a reaction that takes place at one electrode, the potential of which is defined against a fixed reference. It was also assumed that the reactants were transported between the electrode and the bulk of the solution exclusively by diffusion. How the experiments should be carried out to approach this ideal situation is the object of this section. 

On the basis of results obtained from (in-situ-radiolysis [19]) electron spin resonance [16, 20] and pulse radiolysis with optical and conductance detection [19], a-alkoxyalkyl radicals react in aqueous solution exclusively via addition to give alkoxynitroxyl radicals (cf. Eq. 10). This is in contrast to the reactions of CHaCH OH (see Sect. 2.1.1) and 5,6-dihydropyrimidine-6-methyl-6-yl radicals (see Sect. 2.1.3), where addition and redox products are formed. 

The effect of added electrolyte and/or ionic strength on the rate constant(s) of a reaction in solution, exclusive of any role of that the electrolyte as a reactant or catalyst. 

Cyclopentadiene adducts (mono-, tetra- and hexa-adducts) of were stabilized against retro-reaction by selective hydrogenation and bromination of the pendant groups [21]. Utilization of Adam s catalyst and dilute bromine solutions exclusively leads to an addition to the cyclopentene double bonds, because itself is inert towards these reagents. The increased stability of the reduced cycloadducts can be demonstrated by mass spectrometry [21]. 

The capacity ratio of a solute, (k ), was introduced to develop a chromatographic measurement, simple to calculate, independent of flow-rate and one that could be used in solute identification. Although helpful, the capacity ratio is so dependent on the accurate measurement of extra column volume and on very limited solute exclusion by the support and stationary phase, that it is less than ideal for solute identification. An alternative measurement, the separation ratio (a) was suggested where, for two solutes (A) and (B),... 

In the simplest case where the oxidation reaction of a semiconductor material (42a) proceeds exclusively through the valence band and the reaction of reduction of the Ox component of the solution exclusively through the conduction band (see Fig. 13a), corrosion kinetics is limited by minority carriers for either type of conductivity. In fact, it can be seen from Fig. 12 that icorr(p) = i"m(p) and 1 ( ) = ipim(n), where ijj are the limiting currents of minority carriers (symbols in parentheses denote the type of conductivity of a sample under corrosion). Since the corrosion rate is limited by the supply of minority carriers to the interface, it appears to be rather low in darkness. The values of [Pg.283]

Lanthanide complexes of mono- and tetra-amide /1-cyclodextrin derivatives of DOTA have been characterized [140]. The proton NMR spectra of the Eu3+ complexes in methanol-d, show that, while the tetra-amide complex occurs in solution exclusively as a C4-symmetry SAP structure, the mono-amide complex, with less than C4-symmetry, occurs predominantly as two SAP isomers (A/XXXX and Al8885), with the presence of a small amount of the twisted SAP isomer. Luminescence and relaxivity measurements confirm that the Eu3+, Tb3+ and Gd3+ complexes of the eight-coordinate mono-amide ligand possess one bound water molecule, while the tetra-amide complexes have q = 0. The relaxivity of the /LCD mono-amide Gd3+ complex is enhanced when non-covalently bound to a second Gd3+ complex bearing two phenyl moieties (MS-325, AngioMARK , EPIX/Mallinckrodt). 

From gelfiltration studies, a molecular mass of 240 kDa was calculated for the tetrameric holoenzyme of PDCS.c., while the apoenzyme was found to be a dimer at pH 8.5 [78,79]. The pH-dependent dissociation/association of the tetramer was further investigated by X-ray solution scattering [80]. Native PDC from yeast is completely tetrameric in the range of 5.5 < pH < 6.5, between pH 7.5 and 9.5 there is an equilibrium between the dimers and the tetramers and above pH 9.5 the solutions exclusively contain dimers. The situation is different for PDCZ.m., where only tetramers are found, independent of the pH-value [72,81,82], CD-investigations revealed that the alkali-induced dissociation of tetrameric holo-PDCS.c. occurs in two steps, where the dissociation of cofactors is fast, and the dissociation of the tetramer into dimers is rate-limiting [83]. 

Most of these compounds exist predominantly in the amino form. However, there are exceptions. 3-Amino-l-tritylpyrrole 422 appears to exist in solution exclusively in the form 423. 2-Aminoindole 424 exists mainly as the 3H-tautomer 425, which is stabilized by amidine resonance. 

The accessibilities of cotton fibers have been measured by solute exclusion. A simplified mechanism is shown in Figure 5.42. 

The fiber saturation point (FSP) of cotton is the total amount of water present within the cell wall expressed as a ratio of water to solid content. It is equivalent to the water of imbibition of the fiber, also called its water retention value. The FSP has been measured using solute exclusion, centrifugation, porous plate, and hydrostatic tension techniques. It occurs at RVP greater than 0.997 and from the review of the papers, it has been concluded that the studies have yielded a value for FSP in the range of 0.43 to 0.52 g/g [303]. 

Timasheff s preferential interaction mechanism also explains the influence of solutes on the degree of assembly of multimeric proteins. Preferentially excluded solutes tend to induce polymerization and stabilize oligomers since the formation of contact sites between constituent monomers serves to reduce the surface area of the protein exposed to the solvent. Polymerization reduces the thermodynamically unfavorable effect of preferential solute exclusion. Conversely, preferential binding of solute induces depolymerization because there is greater solute binding to monomers than to polymers. 

The theory can be extended to include additional interactions. For example, if nonadsorbing polymer is present in solution, exclusion of this polymer from regions where the particles are closer together than the radius of gyration of the polymer molecules produces a potential W. pi that is roughly the osmotic pressure H times the volume of layers from which polymer is depleted. Thus (Patel and Russel 1987 Russel et al. 1989)... 

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