Q18.182CP - Chemistry: Molecular Nature Of Matter And Change

Question

Polymers are not very soluble in water, but their solubility increases if they have charged groups. 

(a) Casein, a milk protein, contains many ${\mathbf{\text{COO}}}^{\mathbf{\text{ - }}}$ groups on its side chains. How does the solubility of casein vary with pH?

 (b) Histones are proteins essential to the function of DNA. They are weakly basic due to the presence of side chains with ${\mathbf{\text{ - NH}}}_{\mathbf{\text{2}}}$ and =NH groups. How does the solubility of a histone vary with pH?

Step-by-Step Solution

Answer

a) With increasing pH, the solubility of casein decreases

b) With increasing pH, the solubility of histone decreases

Step 1: (a) To find How does the solubility of casein vary with pH

Casein has ${\text{ - COO}}^{\text{ - }}$. As it dissolve or react with water, the ${\text{ - COO}}^{\text{ - }}$ will produce ${\text{OH}}^{-}$ like in the equation below.

${\text{ - COO}}^{\text{ - }}{\text{ + H}}_{\text{2}}\text{O}\rightleftharpoons {\text{ - COOH + OH}}^{\text{ - }}$

At increasing $\text{pH,the}\left[{\text{OH}}^{\text{ - }}\right]$ of the solution will increase. According to Le Chatelier's principle, the reaction will shift to the left with an increase in ${\text{OH}}^{-}$ .Since the reaction shifted to the reactant side, it will be less soluble. 

Therefore, with increasing pH, the solubility of casein decreases.

Step 2: (b) To find How does the solubility of a histone vary with pH

Histones have ${\text{ - NH}}_{\text{2}}$ ,as it dissolves or react with water, the ${\text{ - NH}}_{\text{2}}$ will produce ${\text{OH}}^{-}$ like in the equation below.

${\text{ - NH}}_{\text{2}}{\text{ + H}}_{\text{2}}\text{O}\rightleftharpoons {\text{ - NH}}_{\text{3}}^{\text{ + }}{\text{ + OH}}^{\text{ - }}$

At increasing pH, the ${\text{OH}}^{-}$ of the solution will increase. According to Le Chatelier's principle, the reaction will shift to the left with an increase ${\text{OH}}^{-}$.Since the reaction shifted to the reactant side, it will be less soluble.

Therefore, with increasing pH, the solubility of histone decreases