Isoionic point

An ampholyte is characterized by having more than one protolytic group, and by having protolytic groups of both acidic and basic nature in each molecule. An ampholyte molecule can be specified in terms of the pK values of these protolytic groups. 

There are two concepts which are intimately related to ampholytes— 

Experimentally, the isoionic point is defined as the pH of an ampholyte solution which will not change when the concentration is increased by dissolving more of the pure ampholyte. 

The mathematical treatment of the isoelectric conditions (the pH of zero mobility) for an ampholyte is a much more intricate problem than that of the isoionic state. Since the difference is without practical importance, the isoionic state is chosen for the discussions below. Therefore no distinction is made in the following pages between isoelectric and isoionic point. Isoelectric point will be used also for isoionic points and the symbol pi will be used for both. 

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Table 27 2 includes a column labeled pi which is the isoelectric point of the ammo acid The isoelectric point, also called the isoionic point, is the pH at which the ammo acid has no net charge It is the pH at which the concentration of the zwitterion is a maximum At a pH lower than pi the ammo acid is positively charged at a pH higher than pi the ammo acid is negatively charged For the ammo acids m Table 27 2 pi is the average of pA i and pK 2 and lies slightly to the acid side of neutrality... 

Coa.cerva.tlon, A phenomenon associated with coUoids wherein dispersed particles separate from solution to form a second Hquid phase is termed coacervation. Gelatin solutions form coacervates with the addition of salt such as sodium sulfate [7757-82-6] especially at pH below the isoionic point. In addition, gelatin solutions coacervate with solutions of oppositely charged polymers or macromolecules such as acacia. This property is useful for microencapsulation and photographic apphcations (56—61). 

Since the binding of extraneous ions considerably alters the value of lEP of an aminoacid or protein, this point is not a constant. The term isoionic point (IP) is used to designate the pH value of a pure protein in salt-free water. The direct determination of this constant is difficult and because many proteins are insoluble in the absence of salts. The isoionic point is usually determined indirectly, that is, by measuring the lEP at different concentrations of the neutral salts and extrapolating to zero concentration The value of the isoionic point may differ from lEP by more than a pH unit (Haurowitz 1963). 

Isoindolinones, 19 446 Isoionic point, 12 438-439 Isolait, molecular formula and structure, 5 lllt... 

The isoionic point (or isoionic pH) is the pH obtained when the pure, neutral polyprotic acid HA (the neutral zwitterion) is dissolved in water. The only ions are H2A+, A-, H+, and OH. Most alanine is in the form HA, and the concentrations of H2A+ and A are not equal to each other. 

For a protein, the isoionic pH is the pH of a solution of pure protein with no other ions except H+ and OH Proteins are usually isolated in a chaiged form together with counterions such as Na+, NH4, or C1. When the protein is subjected to intensive dialysis (Demonstration 27-1) against pure water, the pH in the protein compartment approaches the isoionic point if the counterions are free to pass through the semipermeable dialysis membrane that retains the protein. The isoelectric point is the pH at which the protein has no net charge. Box 10-2 tells how proteins can be separated on the basis of their different isoelectric points. 

Acid-base titration of the enzyme ribonuclease. The isoionic point is the pH of the pure protein with no ions present except H+ and OH. The isoelectric point is the pH at which the average charge on the protein is 0. [C. I Tanford and J. D. Hauenstein, Hydrogen Ion Equilibria of Ribonuclease." J. Am. Chem. Soc. 1956, 78.5287.]... 

The opening page of this chapter shows the titration curve for an enzyme. Is the average charge of the protein positive, negative, or neutral at its isoionic point How do you know ... 

The base Na+A. whose anion is dibasic, was titrated with HC1 to give curve b in Figure 11-4. Is point H, the first equivalence point, the isoelectric point or the isoionic point ... 

The isoionic points of a number of the milk proteins are given in Table 3.5. 

Isoenzymes (isozymes) 536, 538 Isoionic point 106 Isolation of compounds 98-108 Isoleucine (He, I) 52s, 539 biosynthesis 540 branched fatty acids from 381 configuration 43 Isologous interactions 337-353 in oligomers 349 - 353 square 352s... 

V8, as indicated by two-dimensional peptide maps of tryptic digests (44)- It appears, therefore, that various strains of S. aureus produce a major extracellular nuclease with covalent structures extremely similar to that of nuclease V8. Staphylococcus aureus has been reported, however, to produce several minor extracellular nucleases with different isoionic points and heat stabilities 45)- The covalent structures of these materials have not been studied. 

The isoionic point of RNase-A determined as the pH of a concentrated salt-free solution is 9.60 (268). Estimates of the isoelectric point in buffers not containing phosphate all give values above 9, Anderson and Alberty (269) reporting 9.45. In phosphate buffers specific anion binding dramatically reduces the measured isoelectric point to an extent dependent on the total phosphate concentration. Values below 6 have been measured (270). 

For example, a - casein contains one major subfraction (a -Cn) plus several minor components. The four genetic variantssof < si-casein have similar isoionic points, which would be expected from their similar primary structures. The 3-caseins, K-caseins, and "whole -caseins" have also been fractionated and their subfractions characterized (Table 1). The isoionic points for all of these casein subunits range from about 4.9 to 6.0 and their molecular weights range from about 11,500 to 24,000. 

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