Question #0a09e

Mar 25, 2017

Methionine is a nonpolar amino acid because it has a nonpolar side-chain.

Explanation:

We usually write the general structure of an amino acid as $\text{H"_2"NCHRCOOH}$.

However, the $\text{H"_2"N}$ end is basic, while the $\text{COOH}$ end is acidic.

The molecule therefore exists as a "zwitterion", $\text{H"_3stackrel("+")("N")"CHRCOO"^"-}$.

Thus, it usually has a charge at one end or the other and is a polar species.

However, the term polar amino acid usually refers to the side-chains that will impart a more polar character to the amino acid.

Side chains with functional groups such as acids, amides, alcohols, and amines do impart a more polar character to an amino acid. They form polar amino acids.

The structure of methionine is

We see that the ${\text{-CH"_2"CH"_2"-S-CH}}_{3}$ side-chain contains only nonpolar $\text{C-C, C-H}$, and $\text{C-S}$ bonds.

Thus, methionine is a nonpolar amino acid.