Isoelectric species

The isoelectric species is the form of a molecule that has an equal number of positive and negative charges and thus is electrically neutral. The isoelectric pH, also called the pi, is the pH midway between pA values on either side of the isoelectric species. For an amino acid such as alanine that has only two dissociating groups, there is no ambiguity. The first pK (R—COOH) is 2.35 and the second pK (R—NH3+) is 9.69. The isoelectric pH (pi) of alanine thus is... 

The isoelectric point, determined by Sumner and Hand (50) to have a value of 5.0-5.1, has been redetermined by the electrofocusing technique (43). The value obtained was 4.8. The solubility is extremely small at this pH, but the urease can be located by its enzymic action. It is a point of interest that the solubility of the isoelectric species increases spectacularly if the sulfhydryl groups are substituted with A-ethylmaleimide (NEM) (43). 

Thus, as a general rule, the isoelectric point of an amino acid is the average of the pKa values of the protonation transitions on either side of the isoelectric species this implies a knowledge of the charges of the various forms. 

Since the isoelectric species is flanked by transitions whose pKa values are 2.1 and 3.9, respectively,... 

The isoelectric point, pJ, is the pH at which the amino acid or peptide carries no net charge that is, the predominant ionic form is the isoelectric species, AA , and (because the isoelectric form ionizes both as an acid and as a base) there are equal. a mounts of the ionic forms AA" and AA. (The ionization of AA to form AA and AA is a disproportionation reaction, as described earlier.) The P/ may also be thought of as the pH of a solution of the isoelectric form of the amino acid. 

Isoelectric species in the organophosphorus, organosilicon, and organoaluminium series... 

The pi is the pH at which the average charge is 0, and this exists at the average of the p7f values for the reactions that occur on either side of the isoelectric species. From Table 3-9, the pK values for this tripeptide are 9.7 (alanine N terminus), 4.3 (glutamate side chain), and 2.3 (glycine C terminus). The isoelectric species is ... 

Isobutyl group (Section 2 13) The group (CH3)2CHCH2— Isoelectric point (Section 27 3) pH at which the concentration of the zwittenonic form of an amino acid is a maximum At a pH below the isoelectric point the dominant species is a cation At higher pH an anion predominates At the isoelec tnc point the ammo acid has no net charge Isolated diene (Section 10 5) Diene of the type... 

Bacteria produce chromosomady and R-plasmid (resistance factor) mediated P-lactamases. The plasmid-mediated enzymes can cross interspecific and intergeneric boundaries. This transfer of resistance via plasmid transfer between strains and even species has enhanced the problems of P-lactam antibiotic resistance. Many species previously controded by P-lactam antibiotics are now resistant. The chromosomal P-lactamases are species specific, but can be broadly classified by substrate profile, sensitivity to inhibitors, analytical isoelectric focusing, immunological studies, and molecular weight deterrnination. Individual enzymes may inactivate primarily penicillins, cephalosporins, or both, and the substrate specificity predeterrnines the antibiotic resistance of the producing strain. Some P-lactamases are produced only in the presence of the P-lactam antibiotic (inducible) and others are produced continuously (constitutive). 

From Example 23.8 or Figure 23.7, it is dear that the zwitterion of glycine is the principal spedes over a wide pH range, certainly from pH 3 to pH 9. The maximum concentration of zwitterion occurs at about pH 6 this is referred to as the isoelectric point of glycine. At this pH, glydne does not migrate in an electric field, since the zwitterion is a neutral species. At... 

Relative concentrntions of different glycine species as a function of pH. Between about pH 3 and 9, the zwitterion is the principal species it has its maximum concentration at pH 6. the isoelectric point Below pH 2. the cation dominates above pH 10. the anion is the principal species. 

Epiactis prolifera. Extracts of this relatively common species contain two toxins, epiactolysins A and B (13). They have the same molecular weight, namely about 19,500, but differ in pi values as determined by isoelectric focusing. Both lack... 

Evidence of pectic enzymes secreted by FORL species has been obtained (3). The total proteins of FORL subjected to isoelectric focusing were resolved into several bands widely distributed between pi 5.9 and 7.45. FORL (rj) presented one mayor band of activity with a pi of 7.0 however, FORL (r ) presented one characteristic and principal band at 7.45, slightly more basic than these reported by others authors (4). 

One solution-based approach that works for gold catalysts, in that it produces highly active catalysts, is the deposition-precipitation (DP) method [8]. The DP method entails adjusting the pH, temperature, and gold concentration of an HAUCI4 solution to form a gold hydroxide species which is then deposited onto the support material [8]. This catalyst precursor is washed, dried, and annealed to form small (<5nm) catalyst particles [9]. The DP method has a number of limitations for example, DP cannot produce Au particles with diameters less than 5 nm on support materials with low-isoelectric points (lEPs) like SiOz and WO3 [5,10,11]. 

True zwitterionic compounds are rare among drugs. The oral absorption of truly zwitterionic compounds is poor unless the compound is a substrate for an absorptive biological transporter as in an a-amino acid which is a substrate for the PepTl nutrient transporter. The aqueous solubility of a true zwitterionic compound will be at a minimum at the isoelectric point which unfortunately for many compounds happens close to the neutral pH at which oral absorphon occurs. Species extrapolation predicting oral absorphon and pk/pD from preclinical animal tests to man are difficult for zwitterions. 

When the acid and basic pftjS of a zwitterion differ by only 3 or 4 units the compound may exist at isoelectric pH as an equilibrium between the true zwitterion and the formally neutral species. Oral absorption is much improved if this type of equilibrium exits. Charge proximate zwitterions are better absorbed than charge separated zwitterions. When the posihve and negative charge centers are close in space there is an overlap in the polarized aqueous salvahon shells so that the compound is less polarized than if the charges were far apart. 

It should finally be noted that the amount of the neutral and zweitterionic forms of a compound in solution is determined by its tatuomeric equilibrium constant, defined as Kz = cz/cn. Therefore, the neutral species and the zwitterion coexist around the isoelectric pH, and membrane permeation is conditioned by Kx and by the partition coefficient values of both tautomers. 

Three bacterial species (E. coli, P. putida, and S. rubidae) were separated on isoelectric focusing in methylcellulose coated capillaries, and three bacterial species (P. fluorescens, E. aerogenes, and M. luteus) and the yeast S. cerevisae, were separated by capillary electrophoresis in the presence of polyethylene oxide.101 The polymers served to minimize adsorption to the walls without causing cellular lysis. 

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