Amino Acid Explorer
All 20 standard amino acids with classification, molecular weight, and pKa values.
20 Standard Amino AcidsNonpolar: 9Polar Uncharged: 6Positively Charged: 3Negatively Charged: 2
Nonpolar
Hydrophobic side chains; tend to cluster in protein interiorsGlycineG
Gly|MW 75.03
Hydrogen atom (smallest amino acid)
pKa1=2.34pKa2=9.60
AlanineA
Ala|MW 89.09
Methyl group (–CH₃)
pKa1=2.34pKa2=9.69
ValineV
Val|MW 117.15
Isopropyl group (branched-chain)
pKa1=2.32pKa2=9.62
LeucineL
Leu|MW 131.17
Isobutyl group (branched-chain)
pKa1=2.36pKa2=9.60
IsoleucineI
Ile|MW 131.17
sec-Butyl group (branched-chain)
pKa1=2.36pKa2=9.68
ProlineP
Pro|MW 115.13
Cyclic pyrrolidine (imino acid)
pKa1=1.99pKa2=10.60
PhenylalanineF
Phe|MW 165.19
Benzyl group (aromatic)
pKa1=1.83pKa2=9.13
TryptophanW
Trp|MW 204.23
Indole ring (aromatic, largest)
pKa1=2.83pKa2=9.39
MethionineM
Met|MW 149.21
Thioether group (–CH₂CH₂SCH₃)
pKa1=2.28pKa2=9.21
Polar Uncharged
Hydrophilic side chains with no charge at physiological pHSerineS
Ser|MW 105.09
Hydroxymethyl group (–CH₂OH)
pKa1=2.21pKa2=9.15
ThreonineT
Thr|MW 119.12
Hydroxyl group on β-carbon
pKa1=2.11pKa2=9.62
CysteineC
Cys|MW 121.16
Thiol group (–SH), forms disulfide bonds
pKa1=1.96pKa2=10.28pKaR=8.18
TyrosineY
Tyr|MW 181.19
Phenol group (aromatic hydroxyl)
pKa1=2.20pKa2=9.11pKaR=10.07
AsparagineN
Asn|MW 132.12
Carboxamide group (–CONH₂)
pKa1=2.02pKa2=8.80
GlutamineQ
Gln|MW 146.15
Carboxamide group (–CH₂CONH₂)
pKa1=2.17pKa2=9.13