Question

Draw the predominant form for glutamate in a solution with the following pH:

a. 0 b. 3 c. 6 d. 11

Short answer

Answer:

(a) At pH = 0

(b) At pH = 3

(c) At pH = 6

(d)At pH = 11

Step-by-step solution

pH scale

The pH scale is used to describe the acidity of a solution, whereas the pK_a indicates the tendency of a compound to lose its proton.

Thus, the pK_a is characteristic of a particular compound, much like a melting point or a boiling point.

The concentration of protons in a solution is indicated by pH. This concentration is written as either [H⁺] or [H₃O⁺].

$\mathrm{pH}=-\log \left[{\mathrm{H}}^{+}\right]$

The acidic form (with the proton) predominates if the pH of the solution is less than the pKa of the ionizable group, and the basic form (without the proton) predominates if the pH of the solution is greater than the pKa of the ionizable group

Acidic strength and pKa values

Acid strengths are normally using ${\mathrm{pK}}_{\mathrm{a}}$values rather than ${\mathrm{K}}_{\mathrm{a}}$values, where the ${\mathrm{pK}}_{\mathrm{a}}$is the negative common logarithm of ${\mathrm{K}}_{\mathrm{a}}$(acid dissociation constant): ${\mathrm{pK}}_{\mathrm{a}}=-{\mathrm{logK}}_{\mathrm{a}}$

The stronger acid has a smaller pK_a, and a weaker acid has a larger pK_a

Structure of Glutamic acid

Glutamic acid structure consists of two carboxyl groups –COOH and one amino group –NH_2. It has one carboxyl-terminal end and an amino-terminal end and a side chain with different pK_a values.

Fully protonated form of glutamic acid

Here, each Hydrogen atom associated with each functional group behaves differently at different pH values.

Draw the predominant form for glutamate with the following pH

(a) At pH = 0, all the groups behaves more basic than the medium as all three functional group pK_a > pH.

(b)At pH = 3, only pK_a1< pH. So this can behaves as more acidic as compare to solution, therefore it gives up proton and exist in the form of zwitter ion.

(c) At pH = 6, both the carboxyl group has pKa values less than pH of the solution, so it behaves more acidic than the solution, therefore they will both deprotonate as shown in the structure given below:

(d) At pH = 11. In a strongly basic solution (pH = 11), all the groups are in the basic medium. So here, pK_a1 ,pK_a2,pK_a3,pK_a3 <pH. So,all the groups i.e. two carboxylic acid group and protonated NH₂ group will be completely deprotonated at this pH value as shown in the structure given below: