Shown above are images of two amino acids, along with the pKas of each of their titrateable groups.
(a) Calculate the isoelectric point (pI) of the dipeptide Glycine-Arginine.
unanswered
(b) Calculate the isoelectric point (pI) of the dipeptide Arginine-Glycine.
a) 11.13
b) 10.76
can you tell the formula chemgam please.
Had the answer from someone else and still awaiting the formula to double check.
If you check week 12: Amino Acids and Protein Synthesis, S32E5: SYNTHESIS OF PEPTIDES, you can work your way through this problem...
To calculate the isoelectric point (pI) of a dipeptide, we need to consider the pKa values of the titratable groups of the amino acids involved.
(a) Calculate the isoelectric point (pI) of the dipeptide Glycine-Arginine:
Step 1: Identify the titratable groups in both amino acids.
- Glycine (Gly) has two titratable groups: the amino group (pKa = 9.60) and the carboxyl group (pKa = 2.34).
- Arginine (Arg) has three titratable groups: the α-amino group (pKa = 9.00), the guanidinium group (pKa1 = 12.48, pKa2 = 10.76), and the carboxyl group (pKa = 2.17).
Step 2: Determine the charge of each titratable group at different pH values.
- At a pH below the pKa of a group, it will be protonated (positively charged).
- At a pH above the pKa of a group, it will be deprotonated (negatively charged).
Step 3: Calculate the net charge of the dipeptide at different pH values.
- At low pH values (below the pKa of the carboxyl groups), both amino acids will be protonated, carrying a positive charge.
- As the pH increases and approaches the pKa of a group, it will start to lose its positive charge. At high pH values (above the pKa of the amino groups), both amino acids will be deprotonated, carrying no net charge.
Step 4: Find the pH value where the net charge of the dipeptide is zero.
- The isoelectric point (pI) is the pH at which the net charge of the dipeptide is zero.
Therefore, to calculate the pI of the dipeptide Glycine-Arginine, we need to find the pH at which both the Glycine and Arginine residues are neutral (zero charge).
(b) Calculate the isoelectric point (pI) of the dipeptide Arginine-Glycine:
The same steps mentioned above can be applied to calculate the pI of Arginine-Glycine. Simply consider the pKa values of Arginine and Glycine in the calculations.
Remember to consider all titratable groups in both amino acids and determine the net charge at different pH values, and then find the pH value where the net charge of the dipeptide is zero.