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The chemical properties of the R-groups obviously dominate the primary chemical differences between AAs. The chemical properties of the AA R-groups also dominate the chemical contribution an AA residue makes to the overall chemistry of a polypeptide or protein assembled from a given sequence of AAs. Thus, it is useful to consider the fundamental physical or chemical properties of the R-groups themselves. Classifying AAs according to the fundamental physical characteristics of their R-groups results in three basic physical categories -- nonpolar, neutral polar, and charged polar. The individual AAs representing each of these groupings are found in the tables below and their R-groups can be spacefilled as follows:

Locate an AA: Click the 3-letter (for lower left frame) or 1-letter (for lower right frame) abbreviation.

AA classification according to generic R-group chemical properties

aliphatic A I L M P V
aromatic F H W Y
acidic C (weak, pKthiol > 7) D E Y (weak, pKphenol > 7)
basic H (weak, pKimidazole < 7) K R

AA classification according to specific R-group chemical properties

chemical R-groupamino acid

alcohol T,S
phenol Y
thiol C
thioether M
amide Q, N
amino K
carboxyl E,D
guanidinium R
phenyl F
imidazole H
indole W
methyl A
isopropyl V
isobutyl L
sec-butyl I
imino group P (Pro is an imino acid)