Question #d8740

1 Answer
Apr 23, 2017

Answer:

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 #"H"_2"NCHRCOOH"#, where #"R"# is a side chain characteristic of each amino acid.

An amino acid has both an acidic group (#"COOH"#) and a basic group (#"NH"_2#).

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

In very acidic media, the #"NH"_2# group will be protonated, and in very basic media, the #"COOH"# group will be deprotonated.

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

In glycine, the #"COOH"# group has #"p"K_text(a1) = 2.34#, and the #"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("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

www.bioinfo.org.cn

At #"pH 2.34"#, we have equal amounts of the weak base #"RNH"_2# and its conjugate acid #"RNH"_3^"+"#.

A #"pH 9.60"#, we have equal amounts of the weak acid #"RCOOH"# and its conjugate base #"RCOO"^"-"#.

A buffer consists of a conjugate acid-base pair, so an amino acid has two #"pH"# regions where it can act as a buffer.