Glutamic acid [side chain R = (CH2)2CO2H] has pKa values of 2.19, 4.25 and 9.67. What would be the predominant species in solution at pH 8?
a) Carboxyl groups in neutral form.
b) Carboxyl groups in deprotonated form, amino group in deprotonated form.
c) Carboxyl groups in protonated form, amino group in deprotonated form.
d) Carboxyl groups in deprotonated form, amino group in protonated form.
The pH of 8 is closer to the pKa value of the carboxyl group's dissociation (pKa = 4.25) than to the amino group's dissociation (pKa = 9.67). Therefore, the carboxyl groups are still partially protonated and in the neutral form, while the amino group is in its deprotonated form.
The answer is (a) carboxyl groups in neutral form.
To determine the predominant species in solution at pH 8, we need to compare the pH with the pKa values of the side chain functional groups of glutamic acid.
The pKa values given are:
pKa1 = 2.19 (carboxyl group)
pKa2 = 4.25 (amino group)
pKa3 = 9.67 (carboxyl group)
At a pH lower than the pKa of a functional group, the species will be predominantly in its protonated form. At a pH higher than the pKa of a functional group, the species will be predominantly in its deprotonated form.
In this case, the pH is 8, which is higher than the first pKa (2.19) but lower than the third pKa (9.67). Therefore, we can determine the predominant species as follows:
a) Carboxyl groups in neutral form. (pH < pKa1)
b) Carboxyl groups in deprotonated form, amino group in deprotonated form. (pKa1 < pH < pKa2)
c) Carboxyl groups in protonated form, amino group in deprotonated form. (pKa2 < pH < pKa3)
d) Carboxyl groups in deprotonated form, amino group in protonated form. (pH > pKa3)
Based on the pH value of 8, we can conclude that:
The correct answer is:
c) Carboxyl groups in protonated form, amino group in deprotonated form.