calculate the isoelectric point of each of the following amino acids: glutamic acid, serine, histidine, lysine, tyrosine, and arginine
http://en.wikipedia.org/wiki/Isoelectric_point#Calculating_pI_values
calculate the ph at which R-group amino of lysine is 20% dissociated? pka= 10.5
To calculate the isoelectric point of each amino acid, we need to consider the pKa values of its ionizable groups. The isoelectric point (pI) is the pH at which the amino acid has a net charge of zero. Each amino acid has its own unique set of pKa values to consider, which are used to calculate the pI. Here's how you can calculate the pI for each amino acid:
1. Glutamic Acid:
Glutamic acid has two acidic groups: the carboxylic acid group (pKa ≈ 2.2) and the side chain carboxyl group (pKa ≈ 4.3). It also has a basic group: the amino group on the side chain (pKa ≈ 9.7). To calculate the pI, we need to determine which pKa values are the most relevant for each amino acid.
At low pH (below pKa1), both the carboxylic acid group and the side chain carboxyl group will be protonated, while the amino group will be positively charged.
At high pH (above pKa3), both the carboxylic acid group and the side chain carboxyl group will be deprotonated, while the amino group will be uncharged.
To find the pI, average the two pKa values closest to the pH at which the amino acid has a net charge of zero. In this case, it would be (pKa2 + pKa3) / 2 ≈ (4.3 + 9.7) / 2 = 7.0.
Therefore, the isoelectric point of glutamic acid is approximately 7.0.
2. Serine:
Serine has one acidic group: the carboxylic acid group (pKa ≈ 2.2) and one basic group, which is the amino group (pKa ≈ 9.2). Again, we consider the relevant pKa values at low and high pH.
At low pH, the carboxylic acid group will be protonated, while the amino group will be positively charged.
At high pH, the carboxylic acid group will be deprotonated, while the amino group will be uncharged.
To determine the pI, average the two pKa values closest to the pH at which the amino acid is neutrally charged. In this case, it would be (pKa1 + pKa2) / 2 ≈ (2.2 + 9.2) / 2 = 5.7.
Therefore, the isoelectric point of serine is approximately 5.7.
3. Histidine:
Histidine has two acidic groups: the carboxylic acid group (pKa ≈ 1.8) and the imidazole group (pKa ≈ 6.0). It also has one basic group: the amino group (pKa ≈ 9.2). Again, we consider the relevant pKa values at low and high pH.
At low pH, both acidic groups will be protonated, while the amino group will be positively charged.
At high pH, both acidic groups will be deprotonated, while the amino group will be uncharged.
To calculate the pI, average the two pKa values closest to the pH at which the amino acid is neutrally charged. In this case, it would be (pKa2 + pKa3) / 2 ≈ (6.0 + 9.2) / 2 = 7.6.
Therefore, the isoelectric point of histidine is approximately 7.6.
4. Lysine:
Lysine has one acidic group: the carboxylic acid group (pKa ≈ 2.2) and one basic group: the amino group (pKa ≈ 9.2). Consider the relevant pKa values at low and high pH.
At low pH, the carboxylic acid group will be protonated, while the amino group will be positively charged.
At high pH, the carboxylic acid group will be deprotonated, while the amino group will be uncharged.
To calculate the pI, average the two pKa values closest to the pH at which the amino acid is neutrally charged. In this case, it would be (pKa1 + pKa2) / 2 ≈ (2.2 + 9.2) / 2 = 5.7.
Therefore, the isoelectric point of lysine is approximately 5.7.
5. Tyrosine:
Tyrosine has one acidic group: the carboxylic acid group (pKa ≈ 2.2) and one basic group: the amino group (pKa ≈ 9.2). Consider the relevant pKa values at low and high pH.
At low pH, the carboxylic acid group will be protonated, while the amino group will be positively charged.
At high pH, the carboxylic acid group will be deprotonated, while the amino group will be uncharged.
To calculate the pI, average the two pKa values closest to the pH at which the amino acid is neutrally charged. In this case, it would be (pKa1 + pKa2) / 2 ≈ (2.2 + 9.2) / 2 = 5.7.
Therefore, the isoelectric point of tyrosine is approximately 5.7.
6. Arginine:
Arginine has one acidic group: the carboxylic acid group (pKa ≈ 2.2) and two basic groups: the guanidino group (pKa ≈ 12.5) and the amino group (pKa ≈ 9.2). Consider the relevant pKa values at low and high pH.
At low pH, the carboxylic acid group will be protonated, while both basic groups will be positively charged.
At high pH, the carboxylic acid group will be deprotonated, while both basic groups will be uncharged.
To calculate the pI, average the two pKa values closest to the pH at which the amino acid is neutrally charged. In this case, it would be (pKa2 + pKa3) / 2 ≈ (9.2 + 12.5) / 2 = 10.9.
Therefore, the isoelectric point of arginine is approximately 10.9.
By following these steps, you can calculate the isoelectric point of each amino acid.