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Amino Acid Code Converter

Convert between one-letter and three-letter amino acid codes. Includes a complete reference table with chemical properties for all 20 standard amino acids.

Converter

Separate multiple codes with spaces or newlines. Mixed case is accepted.

Conversion Result

Amino Acid Reference Table

One-Letter Three-Letter Name Side Chain Class MW (Da) pKa (R group) Hydropathy
A Ala Alanine Nonpolar, Aliphatic 71.08 — +1.800
C Cys Cysteine Polar, Sulfur 103.14 8.37 +2.500
D Asp Aspartic Acid Acidic 115.09 3.90 -3.500
E Glu Glutamic Acid Acidic 129.12 4.07 -3.500
F Phe Phenylalanine Aromatic 147.18 — +2.800
G Gly Glycine Nonpolar, Aliphatic 57.05 — -0.400
H His Histidine Basic, Aromatic 137.14 6.00 -3.200
I Ile Isoleucine Nonpolar, Aliphatic 113.16 — +4.500
K Lys Lysine Basic 128.17 10.50 -3.900
L Leu Leucine Nonpolar, Aliphatic 113.16 — +3.800
M Met Methionine Nonpolar, Sulfur 131.19 — +1.900
N Asn Asparagine Polar, Amide 114.10 — -3.500
P Pro Proline Cyclic (Imino) 97.12 — -1.600
Q Gln Glutamine Polar, Amide 128.13 — -3.500
R Arg Arginine Basic 156.19 12.48 -4.500
S Ser Serine Polar, Hydroxyl 87.08 — -0.800
T Thr Threonine Polar, Hydroxyl 101.10 — -0.700
V Val Valine Nonpolar, Aliphatic 99.13 — +4.200
W Trp Tryptophan Aromatic 186.21 — -0.900
Y Tyr Tyrosine Aromatic 163.18 10.10 -1.300

Amino Acid Classification Guide

Category Amino Acids Properties
Nonpolar Aliphatic Gly, Ala, Val, Leu, Ile, Pro Hydrophobic, buried in protein cores
Aromatic Phe, Tyr, Trp Absorb UV at 280nm (important for spectroscopy)
Polar Uncharged Ser, Thr, Cys, Asn, Gln Hydrophilic, surface-exposed
Acidic (Negative) Asp, Glu Negatively charged at pH 7; pKa ~4
Basic (Positive) Lys, Arg, His Positively charged at pH 7 (except His, pKa 6)
Special Cys (disulfide), Pro (rigid), Gly (flexible) Unique structural roles

Amino Acid Properties at a Glance

  • Hydrophobicity: Most hydrophobic = Ile, Val, Leu; Most hydrophilic = Arg, Asp, Lys
  • Molecular weight: Smallest = Gly (57 Da); Largest = Trp (186 Da)
  • Charge at pH 7: Positive = Arg, Lys; Negative = Asp, Glu; Neutral = all others

Common Usage Patterns

  • In peptide sequences: single-letter codes are standard for sequences over 5 AA
  • In publications: three-letter codes for individual residues in text
  • In databases: single-letter codes for sequence storage
  • In patents: both formats used interchangeably

Frequently Asked Questions

**Why are there both one-letter and three-letter codes?**

The three-letter system was developed first and is more descriptive, making it useful in written text. The one-letter system was introduced later to support computer sequence analysis and compact representation of long sequences. Both standards are maintained by IUPAC.

**How do I remember the one-letter codes?**

A few mnemonic aids: A = Alanine (first letter), C = Cysteine (first letter), G = Glycine (first letter), H = Histidine (first letter), P = Proline (first letter), S = Serine (first letter), V = Valine (first letter). For others: F = Phenylalanine (ph → F), R = aRginine (letter R in name), Y = tYrosine, K = lysine (sounds like K), D = asparDic acid, E = glutamEic acid, M = M (contains M), T = T (contains T), W = tW (tryptophan, double ring), I = I (contains I), L = L (contains L), N = N (contains N), Q = Q (sounds like "cue" → glutamine).

**What about non-standard amino acids?**

Non-standard amino acids (e.g., selenocysteine Sec/U, pyrrolysine Pyl/O, hydroxyproline Hyp) do not have universally accepted one-letter codes. In sequences, they are often represented by special letters (U for selenocysteine, O for pyrrolysine) or written out in full. The converter above handles only the 20 standard amino acids.

**Why are I and L different if they have the same mass?**

Isoleucine (I) and Leucine (L) are structural isomers — they share the same molecular formula (C₆H₁₃NO₂) and therefore the same molecular weight (113.16 Da). However, they differ in the arrangement of their side chains: Leu has an unbranched isobutyl side chain, while Ile has a branched sec-butyl side chain. This structural difference gives them distinct biochemical properties and roles in proteins.

**What does the U stand for in some sequences?**

The letter U stands for selenocysteine (Sec), the 21st proteinogenic amino acid. It is a cysteine analog where sulfur is replaced by selenium. Selenocysteine is encoded by a special UGA codon (normally a stop codon) in the presence of a selenocysteine insertion sequence (SECIS) element. It is found in several selenoproteins important for antioxidant defense.