Question

What is the approximate net charge of carnosine (a) at pH 3, (b) pH 5,

(c) pH 7, and (d) pH 9?

Short answer

a) At $\text{pH}$3 = +1

b At $\text{pH}$5 = +1

c) At $\text{pH}$7 = 0

d) At $\text{pH}$9 = -1/2

Step-by-step solution

Carnosine-amino acid

Carnosine is a powerful antioxidant commonly found in proteins. It is composed of two amino acids, beta-alanine, and histidine. Thus, it is a dipeptide molecule because of these two amino acids combination.

Net charge at different

To determine the net charge on amino acid, three conditions is given below:

$\begin{array}{l}{\text{1)pH<pK}}_{\text{a}}\to \text{protonated}\\ {\text{2)pH>pK}}_{\text{a}}\to \text{deprotonated}\\ {\text{3)pH=pK}}_{\text{a}}\to \frac{\text{1}}{\text{2}}\text{protonated,}\frac{\text{1}}{\text{2}}\text{deprotonated}\end{array}$

Carnosine structure with its${\text{pK}}_{\text{a}}$values are given below:

From the given pK_a values, net charge of carnosine can be calculated.

Each ionizing group adds all charges at a given pH to get a net charge.

$\begin{array}{l}\text{pHAminoCarboxylicimidazoleNetcharge}\\ \text{groupgroupring}\\ \text{3+1-1+1+1}\\ \text{5+1-1+1+1}\\ \text{70-1+10}\\ \text{90-1+}\frac{1}{2}\text{-}\frac{\text{1}}{\text{2}}\end{array}$

Hence, the net charge of carnosine at different$\text{pH}$is calculated.