# Question d8740

Apr 23, 2017

An amino acid can act as a buffer because it can react with added acids and bases to keep the pH nearly constant.

#### Explanation:

The general formula of an amino acid is $\text{H"_2"NCHRCOOH}$, where $\text{R}$ is a side chain characteristic of each amino acid.

An amino acid has both an acidic group ($\text{COOH}$) and a basic group (${\text{NH}}_{2}$).

Thus, it can act as both an acid and a base.

In very acidic media, the ${\text{NH}}_{2}$ group will be protonated, and in very basic media, the $\text{COOH}$ group will be deprotonated.

At an intermediate pH (the isoelectric point, pI), both ends are in their ionic form.

In glycine, the $\text{COOH}$ group has $\text{p} {K}_{\textrm{a 1}} = 2.34$, and the $\text{H"_3"N"^"+}$ group has ""K_text(a2) = 9.60.

At the isoelectric point, "p"I = ("p"K_text(a1) + "p"K_text(a2))/2 = 5.97#

$\underbrace{\text{H"_3 stackrelcolor(blue)(+)("N")"CHRCOOH")_color(red)("At low pH") ⇌ underbrace("H"_3 stackrelcolor(blue)(+)("N")"CHRCOO"^"-")_color(red)("At pI") ⇌ underbrace("H"_2"NCHRCOO"^"-")_color(red)("At high pH}}$

Amino acids have characteristic titration curves. For example

At $\text{pH 2.34}$, we have equal amounts of the weak base ${\text{RNH}}_{2}$ and its conjugate acid $\text{RNH"_3^"+}$.

A $\text{pH 9.60}$, we have equal amounts of the weak acid $\text{RCOOH}$ and its conjugate base $\text{RCOO"^"-}$.

A buffer consists of a conjugate acid-base pair, so an amino acid has two $\text{pH}$ regions where it can act as a buffer.