Question

(a). Why a buffer is commonly used in biochemical reaction? In protein isolation, protease inhibitors are commonly present in the buffer solution, why?

(b). What is the structure of ammonium sulfate? How does ammonium sulfate work to precipitate and fractionate proteins in cell-free extract?

(c). How to define “% saturation of ammonium sulfate”? Be specific.

Answer 1

a)

This is mainly to control pH of proteins; and if there is any addition of acid/base, the pB won't change, since the buffer counterbalances the change:

A buffer is any type of substance that will resist pH change when H+ or OH- is added.

This is typically achieved with equilibrium equations. Both type of buffer will resist both type of additions.

When a weak acid and its conjugate base are added, they will form a buffer

The equations:

The Weak acid equilibrium:

HA(aq) <-> H+(aq) + A-(aq)

Weak acid = HA(aq)

Conjugate base = A-(aq)

Neutralization of H+ ions:

A-(aq) + H+(aq) <-> HA(aq); in this case, HA is formed, H+ is neutralized as well as A-, the conjugate

Neutralization of OH- ions:

HA(aq) + OH-(aq) <-> H2O(l) + A-(aq) ; in this case; A- is formed, OH- is neutralized as well as HA.

Now,

For the weak base equilibrium:

B(aq) + H2O(l) <-> BH+(aq) + OH-(aq)

Weak base = B;

Conjugate acid = BH+

Neutralization of OH- ions:

BH+(aq) + OH-(aq) <-> B(aq) + H2O(l); in this case, OH- is neutralized by BH+, as well as B is created

Neutralization of H+ ions:

B(aq) + H+(aq) <-> BH+(aq)

b)

Ammonium sulfate --> (NH4)2SO4

ammonium ion can act as a weak acid, or if any base is presnet, as a buffer, since

NH4+ + OH- --> NH3 + H2O

which will act as a buffer

c)

% saturation of ammonium sulfate --> amount require din order to bring to a ceratin % the total vlaue of the ammonium sulfate salt in order to precipitate/fractionate proteins