Alanine, isoelectric point

At pH 9.7, alanine (isoelectric point (lEP) 6.0) has a charge of -1 and will migrate to the anode. Lysine (lEP 9.7) is at its isoelectric point and will not move. Aspartic acid (lEP 2.8) has a charge of -2 and will also migrate to the anode, faster than alanine. 

Thus if a mixture containing alanine aspartic acid and lysine is subjected to electrophoresis m a buffer that matches the isoelectric point of alanine (pH 6 0) aspartic acid (pi = 2 8) migrates toward the positive electrode alanine remains at the origin and lysine (pi =9 7) migrates toward the negative elec trode (Figure 27 3b)... 

FIGURE 27 3 Application of electrophoresis to the separation of aspartic acid alanine and lysine according to their charge type at a pH corresponding to the isoelectric point (pi) of alanine... 

I 21.10 Predict whether the isoelectric points for the following a-amino acids are considerably acidic, slightly acidic, or basic (a) alanine, (b) lysine, (c) aspartic acid, (d) cystine, (e) tyrosine. (See Table 21-1 and Problem 21.6.) ... 

Problem 21.43 What occurs when an electric current is passed through an aqueous solution, buffered at pH = 6.0, of alanine (6.0), glutamic acid (3.2). and arginine (10.7) The isoelectric points are shown in parentheses. ... 

How Much Alanine Is Present as the Completely Uncharged Species At a pH equal to the isoelectric point of alanine, the net charge on alanine is zero. Two structures can be drawn that have a net charge of zero, but the predominant form of alanine at its pi is zwitterionic. 

The isoelectric point is the pH at which [H2A+] = [A-], and, therefore, the average charge of alanine is 0. To go from the isoionic solution (pure HA in water) to the isoelectric solution, we could add just enough strong acid to decrease [A ] and increase [H2A 11 until they are equal. Adding acid necessarily lowers the pH. For alanine, the isoelectric pH must be lower than the isoionic pH. 

Yields vary with pH, and for the production of ammonia from glycine (37) and alanine (33) the point of lowest ammonia yield is the isoelectric point of the amino acid. 

You will obtain a titration curve of an amino acid with a neutral side chain such as glycine, alanine, phenylalanine, leucine, or valine. If pH meters are available, you read the pH directly from the instrument after each addition of the base. If a pH meter is not available, you can obtain the pH with the aid of indicator papers. From the titration curve obtained, you can determine the pK values and the isoelectric point. 

If in a solution of alanine, the number of negatively charged carboxylate groups, —COO-, is 1 X 1020 at the isoelectric point, what is the number of positively charged amino groups, NH3+ ... 

Figure 8.8. Resolution by isoelectric focusing in an immobilized pH gradient (pH range 7.35-7.55 over 20 cm path) of two human fetal hemoglobins, one with glycine (lower) instead of alanine (upper) in position 136 of the gamma chains. The isoelectric points of the hemoglobins differ by only 0.003 pH units. (Courtesy of P. G. Righetti, University of Milano.)...

N-alkyl (3-alanine derivatives, the monopropionates , have distinct isoelectric points while most other amphoteric surfactants do not. The isoelectric point is the pH at which the molecule is internally neutralized, existing as a zwitterion. Aqueous solubility and the propensity to foam are lowest at the isoelectric point. 

As an example, consider a mixture of alanine, lysine, and aspartic acid in a buffer solution at pH 6. Alanine is at its isoelectric point, in its dipolar zwitterionic form with a net charge of zero. A pH of 6 is more acidic than the isoelectric pH for lysine (9.7), so lysine is in the cationic form. Aspartic acid has an isoelectric pH of 2.8, so it is in the anionic form. 

What is the structure of each amino acid at its isoelectric point (a) alanine (b) methionine (c) aspartic acid (d) lysine ... 

Notice that the isoelectric points of the 13 amino acids without an acidic or basic side chain are simply the average of the two dissociation constants, pifai and pifa2. Alanine, for instance, has pJEgi = 2.34 and 9-69, so the p/ of alanine is (2.34 + 9.69)/2, or 6.01. For the four amino acids with either a strongly or weakly acidic side chain, the pi is the average of the two lowest pi a values, and for the three amino acids with a basic side chain, the pZ is the average of the two highest pK values. 

Each leg of the titration curve is calculated separately. The first leg, from pH 1 to 6, corresponds to the dissociation of protonated alanine, H2A the second leg, from pH 6 to 11, corresponds to the dissociation of zwitterionic alanine, HA. Exactly halfway between the two legs is the isoelectric point at 6.01. In essence, it s as if we started with H2A at low pH and then titrated with NaOH. When 0.5 equiv of NaOH is added, the deprotonation of H2A is 50% done when 1.0 equiv of NaOH is added, the deprotonation of HgA" is complete and HA predominates (the isoelectric point) when... 

Notice that the isoelectric points of the 13 amino acids without an acidic or basic side chain are simply the average of the two dissociation constants, p ai and pK. 2- Alanine, for instance, has pSr i = 2.34 and p/ a2 9.69, so... 

Table 5 presents the amino acid compositions of a number of apoferritins together with their isoelectric point (taken from the literature) where available. There are a number of features whihc are of quite considerable interest. In the first place, although there are clear cut differences between the proteins of different species, a number of very considerable similarities exist. Thus, for all of the apoferritins for which data is available the content of non-polar amino acids (proline, glycine, alanine, valine, methionine, leucine, isoleucine, phenylalanine and tryptophan) remains constant at about 45%. What is even more remarkable is that the content... 

There is a pH at which the net charge on a molecule is zero, that is, if we add all the charges on the molecule together, the result is zero. This pH is known as the isoelectric point, pi. The pi of alanine is about 6.0. The pi of a protein can be used to separate closely related proteins by isoelectric focusing. If the pi is above 7, the protein is said to be basic if the pi is below, 7, the protein is said to be acidic, although all proteins have both acidic and basic groups. 

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