Question

Question #0a09e

Answer 1

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 `"H"_2"NCHRCOOH"`.

However, the `"H"_2"N"` end is basic, while the `"COOH"` end is acidic.

The molecule therefore exists as a "zwitterion", `"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 `"-CH"_2"CH"_2"-S-CH"_3` side-chain contains only nonpolar `"C-C, C-H"`, and `"C-S"` bonds.

Thus, methionine is a nonpolar amino acid.