Ionized form

Ion-exchange methods are based essentially on a reversible exchange of ions between an external liquid phase and an ionic solid phase. The solid phase consists of a polymeric matrix, insoluble, but permeable, which contains fixed charge groups and mobile counter ions of opposite charge. These counter ions can be exchanged for other ions in the external liquid phase. Enrichment of one or several of the components is obtained if selective exchange forces are operative. The method is limited to substances at least partially in ionized form. 

If a sample substance is introduced into a plasma, its constituent molecules experience a number of radiative, convective, and collisional processes that break the molecules into their constituent atoms, which appear mostly in ionized form. 

Thus nicotinoids that have the highest insecticidal action have the highest piC and, consequently, exist largely in the ionized form at physiological pH. This produces the anomaly that the compounds that are most highly ionized react most rapidly with the receptor protein, yet they are less able to penetrate through the ionic barrier surrounding the insect nerve synapse. 

In another attempt to relate degree of ionization with antibacterial activity, the effect of pH of the medium on the antibacterial activity was studied (27,28). Activity increased with increase in pH only up to the point at which the dmg was 50% ionized, and then decreased. The interpretation of this was that sulfonamides penetrate the bacterial cell in the unionized form, but once inside the cell, the equiUbrium between ionized and unionized forms is reestabhshed, and the activity is due to the ionized form. For optimum activity, a sulfonamide should have a p that provides half-dissociation at the physiologic pH in the area where it is absorbed. This observation also provided an explanation of the paraboHc relationship between piC and MIC (24). 

Multiple pathways are a major concern since depostion of PIC would have occurred. Specific soil conditions determine attenuation rates of penta PIC leachate. Once penta reaches the water table, other transport and fate processes become important. Penta exists in two forms ionized and non-ionized. The ionized form is soluble in water, while the non-ionized form is not. The ratio of the two forms in water is dependent on the pH of the aquifer. In alkaline environments penta PIC tend to be more soluble and more susceptible to advective transport and biological decay. Half-lives of penta leachate in groundwater have been estimated ranging from 27 days to 58 years. 

The proportion of ionized and unionized forms of a chemical compound can be readily calculated according to the above equation. It can be easily seen that pK is also a pH value at which 50% of the compound exists in ionized form. The ionization of weak acids increases as the pH increases, whereas the ionization of weak bases increases when the pH decreases. As the proportion of an ionized chemical increases, the diffusion of the chemical through the biological membranes is greatly impaired, and this attenuates toxicokinetic processes. For example, the common drug acetosalicylic acid (aspirin), a weak acid, is readily absorbed from the stomach because most of its dose is in an unionized form at the acidic pH of the stomach. 

The term is also called an ionization constant because it states the extent to which a substance forms ions in water. The relatively low value of for acetic acid reveals that the un-ionized form, CH3COOH, predominates over H and CH3COO in aqueous solutions of acetic acid. Viewed another way, CH3COO, the acetate ion, has a high affinity for H. ... 

The reactivity of the coordinated, deprotonated nucleophile is typically intermediate between that of the un-ionized and ionized forms of the nucleophile. Carboxypeptidase (Chapter 5) contains an active site Zn, which facilitates deprotonation of a water molecule in this manner. 

In the incompletely ionized form, edtaH, one of the oxygen atoms is no longer able to coordinate to the metal and the anion is quinquidentate. 

Of course, in aqueous solution the reactants and the products exist wholly or partly in their ionized forms the acid, nitrite, and salt exist as H+X , Na+N02, and Na+X , while the diazonium salts are practically completely ionized and the amine is in equilibrium with the corresponding ammonium ion, Ar—NH3. The question of which of these various species are involved in the substitution proper will be dealt with in Chapter 3. Although it is generally desirable to introduce ionized forms into equations, this is inappropriate for the overall equation for the diazotization process, as will become apparent in the discussion of the reaction mechanism (Ch. 3) and from the following remarks. 

To realize the reason for this result from a simple intuitive point of view it is important to recognize that the ionized form of Aspc is more stable in the protein-active site than in water, due to its stabilization by three hydrogen bonds (Fig. 7.7). This point is clear from the fact that the observed pKa of the acid is around 3 in chymotrypsin, while it is around 4 in solution. As the stability of the negative charge on Aspc increases, the propensity for a proton transfer from Hisc to Aspc decreases. 

Thus the reactant ions for chemical ionization formed in the methane plasma consists of approximately equal amounts of a strong gaseous Bronsted acid (CH5+) and ions which can act either as Lewis acids or Bronsted acids (C2H5+ + C3H5+). These reactant ions will effect the chemical ionization with an added substance by proton transfer or hydride ion transfer, both of which may be accompanied by fragmentation of the ion initially formed. 

The term phospho- is used for (H0)2P(0)- or ionized forms in a biochemical context (see recommendations for the nomenclature of phosphorus-containing compounds [24]). 

The reversible reactions are initiated by an equilibrium between neutral and ionized forms of the monosaccharides (see Fig. 6). The oxyanion at the anomeric carbon weakens the ring C-O bond and allows mutarotation and isomerization via an acyclic enediol intermediate. This reaction is responsible for the sometimes reported occurrence of D-mannose in alkaline mixtures of sucrose and invert sugar, the three reducing sugars are in equilibrium via the enediol intermediate. The mechanism of isomerization, known as the Lobry de Bruyn-... 

There is a large variety of atmospheric sulfur compounds, in the gas, solid, and liquid phases. Table 7-3 lists a number of gaseous compounds, range of concentration, source, and sink (where known). As this list illustrates, a significant number of these gases contribute to the existence of oxidized sulfur in the forms of SO2 and sulfate aerosol particles. Table 7-4 lists the oxy-acids of sulfur and their ionized forms that could exist in the atmosphere. Of these the sulfates certainly are dominant, with H2SO4 and its products of neutralization with NH3 as the most frequently reported forms. 

Table 7-4 Oxyacids, their salts, and ionized forms that could exist in atmospheric aerosol particles...

Excretion via the kidney can be a straightforward question of glomerular filtration, followed by passage down the kidney tubules into the bladder. However, there can also be excretion and reabsorption across the tubular wall. This may happen if an ionized form within the tubule is converted into its nonpolar nonionized form because of a change in pH. The nonionized form can then diffuse across the tubular wall into plasma. Additionally, there are active transport systems for the excretion of lipophilic acids and bases across the wall of the proximal tubule. The antibiotic penicillin can be excreted in this way. 

The ionized forms of polypeptides exhibit many characteristics in common therefore, we have studied them under various conditions. The most interesting observation is the transition of a polyelectrolyte brush found by changing the polyelectrolyte chain density. The brush layers have been prepared by means of the LB film deposition of an amphiphile, 2C18PLGA(48), at pH 10. Mixed monolayers of 2C18PLGA(48) and dioctadecylphos-phoric acid, DOP, were used in order to vary the 2C18PLGA(48) content in the monolayer. 

From Circular No. 200 of the Committee of Editors of Biochemical Journals Recommendations (1975) According to standard biochemical convention, the ending ate in, eg, palmitate, denotes any mixture of free acid and the ionized form(s) (according to pH) in which the cations are not specified. The same convention is adopted in this text for all carboxylic acids. 

This equation reduces to the original Hinton equation (15) when one takes into account the importance of the concentration of the ionized and nonionized forms of pectin. In the Hinton equation, the concentration of pectin is the difference between the total and the ionized form. 

It was assumed for a long time that molecules can only cross a membrane in their neutral form. This dogma, based on the pH-partition theory, has been challenged [42, 43]. Using cyclic voltammetry it was demonstrated that compounds in their ionized form pass into organic phases and might well cross membranes in this ionized form [44]. 

The difference between the log P of a given compound in its neutral form (log P ) and its fully ionized form (log P ) has been termed dialog P ) and contains series-specific information, and expresses the influence of ionization on the intermolecular forces and intramolecular interactions of a solute [44, 51, 52]. 

Of course, the shape of the lipophilicity profiles becomes much more complicated for molecules with multiple ionized forms. Nevertheless, the partition coefficients may be calculated from the distribution coefficient using the general equation [113] ... 

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