Arginine
O
2.01
H2N---CH--C----OH
9.02 CH2
CH2
CH2
NH
12.48 C==NH
NH2
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9.78 O 2.35
H2N----CH---C----OH
CH3
Glycine
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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.
(b) Calculate the isoelectric point (pI) of the dipeptide Arginine-Glycine.
a) 11.13
b) 10.76
a) 11.13
b) 10.76
To calculate the isoelectric point (pI) of a dipeptide, we need to determine the average of the pKa values of all the ionizable functional groups in the peptide.
(a) Glycine-Arginine:
1. Glycine has one ionizable functional group, which is the amino group (pKa = 9.78).
2. Arginine has three ionizable functional groups: the amino group (pKa = 9.78), the carboxyl group (pKa = 2.35), and the guanidino group (pKa = 12.48).
To calculate the pI, we need to consider the charges of each functional group at different pH values. At low pH, the amino group and guanidino group are protonated (positive charge), while the carboxyl group is deprotonated (negative charge). As the pH increases, the amino group and guanidino group will become neutral, and eventually, the carboxyl group will become protonated (positive charge).
So, to find the pI of Glycine-Arginine, we need to find the pH at which the net charge is zero.
- At low pH: Glycine has a positive charge (+1), and Arginine has a positive charge (+2) (from the guanidino group). The net charge is +3.
- As pH increases: At a certain pH, the guanidino group (pKa = 12.48) will lose its positive charge and become neutral. At that pH, the net charge becomes +1.
- As pH increases further: After the guanidino group loses its positive charge, the amino group of Glycine (pKa = 9.78) will lose its positive charge. At that pH, the net charge becomes zero (neutral).
Therefore, the pI of Glycine-Arginine is approximately 9.78.
(b) Arginine-Glycine:
1. Arginine has three ionizable functional groups: the amino group (pKa = 9.78), the carboxyl group (pKa = 2.35), and the guanidino group (pKa = 12.48).
2. Glycine has one ionizable functional group, which is the amino group (pKa = 9.78).
Using the same logic as before, we can calculate the pI of Arginine-Glycine:
- At low pH: Arginine has a positive charge (+3) (from the guanidino group), and Glycine has a positive charge (+1). The net charge is +4.
- As pH increases: At a certain pH, the guanidino group (pKa = 12.48) will lose its positive charge and become neutral. At that pH, the net charge becomes +1.
- As pH increases further: After the guanidino group loses its positive charge, the carboxyl group of Arginine (pKa = 2.35) will become protonated (positive charge). At that pH, the net charge becomes zero (neutral).
Therefore, the pI of Arginine-Glycine is approximately 2.35.
So, the answers are:
(a) The pI of Glycine-Arginine is approximately 9.78.
(b) The pI of Arginine-Glycine is approximately 2.35.