The skeletal structures of the two amino acids, glycine and lysine, are given below along with the values of the relevant acid dissociation constants (pKa).
(b) 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.
answer = 9.985
what's the answer to part a ??
Where's part a) question?
haven't done it yet..am struggling with it..need help...
pKa+log10(10^2)= 2.35+2= 4.35
Thank you, bonjo!
At site 4, name the sidechain interaction.
Hydrophobic interactions
Dipole-dipole interactions
Hydrogen bonds
Ionic bonds
Covalent bonds
To calculate the pI (isoelectric point) of the dipeptide Lys-Gly, you need to determine the charge on the dipeptide at different pH values.
The pI is the pH at which the dipeptide carries no net charge. This happens when the positive and negative charges on the dipeptide balance each other out.
Here's the step-by-step process to calculate the pI:
1. Identify the ionizable groups in each amino acid. In this case, Lysine (Lys) has two ionizable groups: the amino group (NH3+) and the carboxyl group (COO-), while Glycine (Gly) only has the carboxyl group (COO-).
2. Determine the pKa values for the ionizable groups. The pKa values for the amino group of Lysine and the carboxyl group of both Lysine and Glycine are usually given in the question or can be found in a reference table. For example, let's assume the pKa values are:
- pKa1 (amino group of Lysine): 9.0
- pKa2 (carboxyl group of Lysine and Glycine): 2.2
3. Write down the equation for the reaction of each ionizable group with water. For example, for the amino group of Lysine, you have:
NH3+ + H2O ⇄ NH4+ + OH-
4. Construct a table to track the charges on the dipeptide at different pH values. Start with the lowest pH value (0) and go up to the highest pH value (14). Use the pKa values to determine how the charges change with increasing pH.
| pH | Free NH3+ | Free COO- | Dipeptide charge |
|----|-----------|-----------|-----------------|
| 0 | + | - | + |
| 1 | + | - | + |
| 2 | + | - | + |
| ...| ... | ... | ... |
| 9 | ? | ? | ? |
| ...| ... | ... | ... |
|14 | ? | ? | ? |
5. Find a pH value where the dipeptide charge is zero (i.e. where the positive and negative charges balance each other out). In this case, it will occur when the amino group of Lysine (NH3+) is neutralized and becomes NH2.
To find this pH value, compare the two pKa values. The first pKa (pKa1) is for the amino group (NH3+), and the second pKa (pKa2) is for the carboxyl group (COO-). Since the pKa1 is higher than the pKa2, it means that at a pH higher than the pKa1, the N-terminal amino group (NH3+) will be neutralized.
Therefore, the pI of the dipeptide Lys-Gly can be approximated by the pKa1 of Lysine: pI ≈ 9.0
In this case, the calculated pI of the dipeptide Lys-Gly would be 9.0. However, please note that this is an approximate calculation, and the actual pI might differ slightly due to the simplified assumption that there is no shift in the pKa values from the constituent amino acids.