Illustrated Glossary of Organic Chemistry

α-Amino acid (alpha-amino acid): A molecule containing an amino group and a carboxylic acid group that are separated by one carbon, called the α-carbon. In biochemistry there are twenty standard amino acids which differ in the makeup of the side chain (R group) attached to the α-carbon.


General α-amino acid molecular structure


Standard amino acids categorized according to side chain properties

Hydrophobic nonacidic side chains


   

  

  

  

Glycine (Gly)
Alanine (Ala)
Valine (Val)
Leucine (Leu)
Isoleucine (Ile)
   



Proline (Pro)
 Tryptophan (Trp)
 Phenylalanine (Phe)
 Methionine (Met)

Hydrophobic acidic side chains
An acidic side chain is more acidic than water (pKa 15.7).
Cysteine (Cys)
 Tyrosine (Tyr)

Hydrophilic nonacidic side chains


Serine (Ser)
 Threonine (Thr)
 Asparagine (Asn)
 Glutamine (Gln)

Hydrophilic acidic side chains
An acidic side chain is more acidic than water (pKa 15.7).
Aspartic acid (Asp) Glutamic acid (Glu)

Hydrophilic basic side chains
A basic side chain has a nitrogen atom lone pair not delocalized by resonance.

Lysine (Lys) Arginine (Arg) Histidine (His)

There are also numerous nonstandard amino acids, only a few of which are incorporated into peptides and proteins. (Most other nonstandard amino acids are metabolic intermediates.) Selenocysteine and hydroxyproline are nonstandard amino acids.

Selenocysteine
        
Hydroxyproline