Amino acid isoionic points

ISOIONIC POINT AND TITRATION OF a-AMINO ACIDS Isoionic Points of Amino Acids... 

Table 27.2 includes a column labeled pi, which is the isoelectric point of the anino acid. The isoelectric point, also called the isoionic point, is the pH at which the anino 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 amino acid is positively charged at a pH higher than pi, the amino acid is negatively charged. For the anino acids in Table 27.2, pi is the average of and pK 2 and lies slightly to the acid side of neutrality. 

Ribonuclease is an enzyme with 124 amino acids. Its function is to cleave ribonucleic acid (RNA) into small fragments. A solution containing pure protein, with no other ions present except H+ and OH- derived from the protein and water, is said to be isoionic. From this point near pH 9.6 in the graph, the protein can be titrated with acid or base. Of the 124 amino acids, 16 can be protonated by acid and 20 can lose protons to added base. From the shape of the titration curve, it is possible to deduce the approximate pATa for each titratable group.1-2 This information provides insight into the environment of that amino acid in the protein. In ribonuclease, three tyrosine residues have "normal values of pATa(=10) (Table 10-1) and three others have pA a >12. The interpretation is that three tyrosine groups are accessible to OH, and three are buried inside the protein where they cannot be easily titrated. The solid line in the illustration is calculated from pA"a values for all titratable groups. 

If the acid dissociation constants of the amino acid serine CHoOH CHNH2 COOH) are 6.2 x 10 and 7.1 x 10" state which ionic form will predominate at the following pH values 2.0, 5.0, 8.0, and 11.0. What is the isoionic point ... 

The amino acid composition of some )S-lg variants is shown in Table 4.4. It is rich in sulphur amino acids which give it a high biological value of 110. It contains 2 moles of cystine and 1 mole of cysteine per monomer of 18 kDa. The cysteine is especially important since it reacts, following heat denatura-tion, with the disulphide of K-casein and significantly affects rennet coagulation and the heat stability properties of milk it is also responsible for the cooked flavour of heated milk. Some /S-lgs, e.g. porcine, do not contain a free sulphydryl group. The isoionic point of bovine j3-lgs is c. pH 5.2. 

The amino acid composition is shown in Table 4.4. a-La is relatively rich in tryptophan (four residues per mole). It is also rich in sulphur (1.9%) which is present in cystine (four intramolecular disulphides per mole) and methionine it contains no cysteine (sulphydryl groups). The principal a-la s contain no phosphorus or carbohydrate, although some minor forms may contain either or both. The isoionic point is c. pH 4.8 and minimum solubility in 0.5 M NaCl is also at pH 4.8. 

Because bleaching increases the ratio of acidic to basic amino acids [122], the isoionic point should decrease with increasing oxidation. One might also anticipate a similar decrease in the isoelectric point of hair with bleaching because the A-layer of the cuticle cells is rich in cystine. 

The minimum in swelling is consistent with the observation of Steinhardt and Harris [106]. In the absence of added electrolyte, there is no combination of wool hber with mineral acid or alkah from pH 5 to 10. This is in the vicinity of the isoionic point of hair. The large increase in swelhng above pH 10 is largely due to ionization of diacidic amino acid residues in the hair and partly due to keratin hydrolysis. The increase in swehing from pH 3 to 1 is due to the combination of acid with the dibasic amino acids. Brener and Prichard [57] attribute the decrease in swelhng below pH 1 to an irreversible structural change. 

The isoionic point of an amino acid in aqueous solution is defined as the pH value at which the net charge is zero, due to the presence of equal concentrations of the ionic species with net charge -I-1 and - 1. For neutral amino acids including cysteine and tyrosine the isoionic point is equal to i(pX, + pXj). 

The same holds true (with a negligible simplification) for the acidic amino acids, whereas the isoionic point for the basic amino acids is equal to KpA 2 + P s)- The isoelectric point of an amino acid is defined as the pH value at which the amino acid will move neither towards the positive nor negative pole when subjected to an electric current. The isoelectric points may depend on the presence of other ions but for all practical purposes the isoelectric point and isoionic point may be considered identical for amino acids (except when complexes between amino acids and metal ions are involved (see Section IV,G)). For proteins there may be substantial differences. The isoelectric point is not equal to the pH value of a solution of the amino acid in water, the latter being somewhere between the isoelectric point and pH 7. The difference will normally not be great, but neutral amino acids have a very small buffering capacity at the isoelectric point and the pH values found in their aqueous solutions are therefore variable. 

Thus, when we dissolve ciystalllne alanine In pure water, the pH of the solution Is 6.02, Or if an alanine solution is brou t to the pH 6.02, alanine will exist as a zwltterlon. This pH Is known as the isoionic point of alanine as alanine at this pH does not possess any net charge. This pH is also known as the isoelectric pH of alanine as at this pH alanine will be electrophoretically inunobile. In general the pH at which an amino acid exists as a zwltterlon is known as the Isolonlc or Isoelectric point of that amino acid. 

All the properties of amino acids which depend upon their ionization would naturally be affected by the prevailing pH. We can cite the exaimple of solubility of amino acids. Amino acids are least soluble at their isoionic pHs. They are much more soluble at pHs lower and higher than their isoionic pHs. Thus, at a given pH different amino acids will have different solubilities depending upon how far removed is the pH of the solution from their own isoionic points. 

Isoelectric point pH at which the concentration of the zwitterionic 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 isoelectric point the amino acid has no net charge. Isoionic point Synonymous with isoelectric point. 

This suggests that in solution there will be an acidity (pH) at which the amino acids will exist in the zwitterionic form, or have no net charge. The pH at which this occurs as a result of the proton condition is referred to as the isoionic point pL. Similarly, when it is observed that there is no net charge on the molecule within the system as judged by experimental conditions (i.e., no mobility during an electrophoresis experiment) the pH at which this occurs is referred to as the isoelectric point (p/J. For an aqueous solution of amino acids ... 

The isoelectric point is close to the isoionic point. It is obvious that the approximation used above in order to calculate the pH of an amino acid solution is equivalent to confuse these two points. Also, notice that the more diluted the amino acid solution is, the more the isoionic point tends toward the isoelectric point, as shown by Eq. (5.18). 

The isoionic point, pHp is the pH of the solution at which the concentration of the dipolar ion is a maximum. The relation of pHj to the concentrations of the various ionic forms of an amino acid are as follows. 

The isoionic point of an amino acid equals one-half the sum of the pA values of the carboxylate group and the amino group if it does not have an ionizing side chain. For example, the pAT of the carboxyl group of alanine is 2.4, and the pA value of its amino group is 9.9. The isoionic point of alanine is 6.1. Table 27.2 hsts the isoionic points of some amino acids. 

The isoionic point of a protein depends upon its amino acid composition. At its isoionic point, a protein has no net charge, and its solubility is at a minimum. As a consequence, a protein tends to precipitate form solution at its isoionic point. For example, casein, a protein in milk, has a negative charge at pH 6.3. Casein has many glutamic acid and aspartic acid residues. If acid is added to milk, these side chains are protonated, and casein precipitates. Casein is used in making cheese, and it is obtained by adding acid to milk or by adding bacteria that make lactic acid, which has the same effect. 

The isoionic point of hen egg white lysozyme is 10.8. What does this value indicate about its amino acid composition ... 

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