Amino Acids Classification and Structures

Vocabulary practice flashcards defining the five main categories of standard amino acids and each individual amino acid structure based on its R group.

Amino Acid Side Chains Chart

A diagram illustrating the structures and R-group classifications of standard amino acids divided into five major chemical classes.

Nonpolar, aliphatic R groups

Nonpolar, non-aromatic hydrocarbon: Glycine, Ala, Val, Leuc, Meth, Iso

Polar, uncharged R groups

Side chains that are polar but uncharged at neutral pH: Ser, Threo, Cys, Pro, Asp, Glu

Positively charged R groups

Basic side chains that carry a net positive charge at physiological pH: Lys, Arg, Hist

Negatively charged R groups

Acidic side chains that carry a net negative charge at physiological pH: Asp and Glu

Nonpolar, aromatic R groups

Aromatic ring systems in their side chains: Phe, Tyr, Tryp

Glycine

A nonpolar, aliphatic amino acid whose R group consists of a single hydrogen atom ($\text{H}$).

Alanine

A nonpolar, aliphatic amino acid whose R group is a methyl group ($\text{-CH}_3$).

Valine

A nonpolar, aliphatic amino acid whose R group is a branched hydrocarbon group ($\text{-CH(CH}_3)_2$).

Leucine

A nonpolar, aliphatic amino acid whose R group is a branched hydrocarbon chain ($\text{-CH}_2\text{CH(CH}_3)_2$).

Methionine

A nonpolar, aliphatic amino acid whose R group contains a thioether side chain ($\text{-CH}_2\text{-CH}_2\text{-S-CH}_3$).

Isoleucine

A nonpolar, aliphatic amino acid whose R group is a branched sec-butyl side chain ($\text{-CH(CH}_3)\text{CH}_2\text{CH}_3$).

Serine

A polar, uncharged amino acid whose R group contains a primary alcohol group ($\text{-CH}_2\text{OH}$).

Threonine

A polar, uncharged amino acid whose R group contains a secondary alcohol group ($\text{-CH(OH)CH}_3$).

Cysteine

A polar, uncharged amino acid whose R group contains a sulfhydryl/thiol group ($\text{-CH}_2\text{SH}$).

Proline

A polar, uncharged amino acid with a cyclic ring structure where the aliphatic side chain is bonded back to the backbone amino group.

Asparagine

A polar, uncharged amino acid whose R group contains an amide functional group ($\text{-CH}_2\text{CONH}_2$).

Glutamine

A polar, uncharged amino acid whose R group contains an extended amide side chain ($\text{-CH}_2\text{CH}_2\text{CONH}_2$).

Lysine

A positively charged amino acid whose R group terminates in a primary amino group ($\text{-CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{NH}_3^+$).

Arginine

A positively charged amino acid whose R group features a protonated guanidino structure.

Histidine

A positively charged amino acid whose R group features an imidazole ring system.

Aspartate

A negatively charged amino acid whose R group contains a carboxylate group ($\text{-CH}_2\text{COO}^-$).

Glutamate

A negatively charged amino acid whose R group contains an extended carboxylate group ($\text{-CH}_2\text{CH}_2\text{COO}^-$).

Phenylalanine

A nonpolar, aromatic amino acid whose R group consists of a phenyl ring attached to a methylene group ($\text{-CH}_2\text{-C}_6\text{H}_5$).

Tyrosine

A nonpolar, aromatic amino acid whose R group consists of a phenol ring attached to a methylene group.

Tryptophan

A nonpolar, aromatic amino acid whose R group consists of an indole ring system attached to a methylene group.