Calculate the pI (isoelectric point) of the dipeptide, Lys-Gly, assuming that there is negligible shift in the values of acid dissociation constants from the values of the constituent amino acids. Gly pkA 9.87 an pka 2.35; Lys pka 9.18, 2.16 y 10.79
9.985 is the answer
To calculate the isoelectric point (pI) of a dipeptide, you need to determine the pH at which the dipeptide has a net charge of zero. In other words, it is the pH at which the positive and negative charges on the dipeptide balance each other out.
The pKa values of the amino acids in the dipeptide are provided. The pKa is the pH at which half of the molecules of the species are in their deprotonated form (negatively charged) and the other half are in their protonated form (positively charged).
In this case, we have a dipeptide consisting of Lysine (Lys) and Glycine (Gly).
Lysine (Lys) has three ionizable groups: its alpha-amino group, the side chain amino group, and the side chain carboxyl group. The pKa values for Lysine are 9.18, 2.16, and 10.79, respectively.
Glycine (Gly) has two ionizable groups: its alpha-amino group and the side chain carboxyl group. The pKa values for Glycine are 9.87 and 2.35, respectively.
To calculate the pI, we need to consider the protonation state of each ionizable group at different pH values.
1. Start by considering the first pKa value, which represents the pH at which half of the molecules are deprotonated and half are protonated. For Glycine, the alpha-amino group starts to lose its proton at a pH close to 9.87. For Lysine, the alpha-amino group starts to lose its proton at a pH close to 9.18.
2. Next, consider the second pKa value, which represents the pH at which the other half of the molecules are deprotonated. For Glycine, the side chain carboxyl group starts to lose its proton at a pH close to 2.35. For Lysine, the side chain amino group starts to lose its proton at a pH close to 2.16.
3. Finally, consider the third pKa value, which represents the pH at which the remaining half of the molecules are deprotonated. For Lysine, the side chain carboxyl group starts to lose its proton at a pH close to 10.79. Since Glycine does not have a third pKa value, we only need to consider Lysine.
From the information above, we can determine the pH range over which each ionizable group is deprotonated.
- Glycine's alpha-amino group: Deprotonated at pH > 9.87.
- Glycine's side chain carboxyl group: Deprotonated at pH > 2.35.
- Lysine's alpha-amino group: Deprotonated at pH > 9.18.
- Lysine's side chain amino group: Deprotonated at pH > 2.16.
- Lysine's side chain carboxyl group: Deprotonated at pH > 10.79.
To find the pI, we need to determine the pH at which all the ionizable groups are deprotonated.
In this case, since we have the same pH range for the alpha-amino and side chain amino groups, we can assume that they lose their protons simultaneously. Thus, the pH range over which all the ionizable groups are deprotonated is approximately 9.18 to 10.79.
The pI of the dipeptide is the average of these two pH values:
pI = (9.18 + 10.79) / 2 = 9.985
Therefore, the pI of the dipeptide Lys-Gly is approximately 9.985.