Understanding Peptide Sequences: One-Letter and Three-Letter Amino Acid Codes

Every synthetic peptide is defined by its amino acid sequence — the linear order of amino acid residues from the N-terminus (amino end) to the C-terminus (carboxyl end). The sequence determines the molecular weight, physicochemical properties, three-dimensional structure, and biological activity of the peptide. When verifying a peptide product, the first step is confirming that the stated sequence matches the expected compound.

Certificates of Analysis (CoAs) typically report sequences in one or both notation systems. Mass spectrometry data on the CoA should correspond to the theoretical molecular weight calculated from the stated sequence. A mismatch between the sequence, the expected molecular weight, and the observed mass is a definitive red flag for product misidentification.

The one-letter code assigns a single uppercase letter to each of the 20 standard amino acids. This system was proposed by Margaret Dayhoff in the 1960s and later adopted by the IUPAC-IUB Joint Commission on Biochemical Nomenclature. The one-letter code is compact and widely used in databases (UniProt, NCBI), sequence alignment tools (BLAST, ClustalW), and bioinformatics applications.

The assignment logic follows certain patterns: amino acids with unique starting letters use that letter (G for Glycine, L for Leucine, V for Valine). Where multiple amino acids share a starting letter, the most common or earliest-discovered receives it (A for Alanine), and others receive phonetically or mnemonically related letters (F for Phenylalanine from its phenyl group, W for Tryptophan from its double-ring structure).

The three-letter code uses the first three letters of each amino acid name (with exceptions: Ile for Isoleucine, Trp for Tryptophan, Asn for Asparagine, Gln for Glutamine). The first letter is capitalized, and the remaining two are lowercase. This system is more readable than the one-letter code, especially for non-specialists, and is the preferred format on most vendor CoAs and in peptide synthesis reports.

Three-letter codes are separated by hyphens in peptide sequences (e.g., Gly-Glu-Pro-Pro-Pro for the first five residues of BPC-157). This format makes it easy to count residues and identify individual amino acids at a glance. The trade-off is that three-letter sequences take significantly more space than one-letter notation, making them impractical for long protein sequences.

The 20 standard amino acids encoded by the genetic code, with their one-letter codes, three-letter codes, molecular weights (as residue in a peptide chain), and key properties:

• G / Gly — Glycine (57.02 Da) — Simplest amino acid, no side chain, high conformational flexibility
• A / Ala — Alanine (71.04 Da) — Small hydrophobic residue, methyl side chain
• V / Val — Valine (99.07 Da) — Branched-chain hydrophobic, isopropyl side chain
• L / Leu — Leucine (113.08 Da) — Branched-chain hydrophobic, isobutyl side chain
• I / Ile — Isoleucine (113.08 Da) — Branched-chain hydrophobic, sec-butyl side chain
• P / Pro — Proline (97.05 Da) — Cyclic, imparts rigidity to peptide backbone
• F / Phe — Phenylalanine (147.07 Da) — Aromatic hydrophobic, benzyl side chain
• W / Trp — Tryptophan (186.08 Da) — Largest amino acid, indole aromatic ring, UV absorbance at 280 nm
• M / Met — Methionine (131.04 Da) — Sulfur-containing, susceptible to oxidation
• S / Ser — Serine (87.03 Da) — Hydroxyl group, polar uncharged, phosphorylation site
• T / Thr — Threonine (101.05 Da) — Hydroxyl group, polar uncharged, two chiral centers
• C / Cys — Cysteine (103.01 Da) — Thiol group, forms disulfide bonds, redox-sensitive
• Y / Tyr — Tyrosine (163.06 Da) — Phenol aromatic ring, UV absorbance at 280 nm, phosphorylation site
• H / His — Histidine (137.06 Da) — Imidazole ring, pKa near physiological pH, catalytic residue
• D / Asp — Aspartic acid (115.03 Da) — Carboxyl side chain, negatively charged at pH 7
• E / Glu — Glutamic acid (129.04 Da) — Carboxyl side chain, negatively charged at pH 7
• N / Asn — Asparagine (114.04 Da) — Amide group, susceptible to deamidation
• Q / Gln — Glutamine (128.06 Da) — Amide group, susceptible to deamidation and pyroglutamate formation
• K / Lys — Lysine (128.09 Da) — Amino side chain, positively charged at pH 7, common modification site
• R / Arg — Arginine (156.10 Da) — Guanidinium group, strongly positively charged at pH 7

Peptide sequences are written from N-terminus (left) to C-terminus (right) by convention. The N-terminus is the end with the free amino group, and the C-terminus has the free carboxyl group. For example, BPC-157 is written as GEPPPGKPADDAGLV (one-letter) or Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val (three-letter). This is a 15-residue peptide (pentadecapeptide).

The molecular weight of a peptide is calculated by summing the residue masses of all amino acids and adding 18.02 Da for the water molecule that completes the terminal groups. For BPC-157: the sum of all 15 residue masses plus 18.02 Da equals the theoretical monoisotopic mass of 1419.53 Da, which should match the mass spectrum on the CoA.

Many research peptides contain non-standard amino acids or post-translational modifications that fall outside the standard 20-residue code. Common examples include D-amino acids (mirror images of the natural L-forms, denoted with a lowercase d- prefix: d-Phe, d-Ala), N-methyl amino acids (MeLeu, MeAla), acetylated N-termini (Ac-), amidated C-termini (-NH2), and unnatural amino acids such as Aib (alpha-aminoisobutyric acid) or Nal (naphthylalanine).

These modifications are critical for peptide identity and function. D-amino acid substitutions confer protease resistance. N-methylation alters backbone flexibility and membrane permeability. Terminal modifications affect charge and stability. When verifying a peptide sequence, ensure that all modifications are accounted for in the molecular weight calculation.

Several online tools calculate peptide molecular weight from a sequence input. ExPASy ProtParam and PepCalc.com accept both one-letter and three-letter input and return monoisotopic mass, average mass, isoelectric point, and extinction coefficient. These calculators handle standard amino acids automatically; modified residues may need to be entered manually with their specific mass increments.

When using MW calculators to verify CoA data, ensure you select the correct mass type. Monoisotopic mass (based on the most abundant isotope of each element) is reported by high-resolution MS instruments, while average mass (weighted average of all isotopes) is more common on lower-resolution instruments. The difference between monoisotopic and average mass increases with peptide size.

• Confusing I (Isoleucine) and L (Leucine): These isomeric amino acids have identical molecular weights (113.08 Da) and cannot be distinguished by standard mass spectrometry. Sequence verification for I/L positions requires MS/MS fragmentation or Edman degradation.
• Ignoring terminal modifications: An Ac- prefix adds 42.04 Da and a -NH2 suffix subtracts 0.98 Da compared to the free acid form. Omitting these from MW calculations produces mismatches with MS data.
• Forgetting counter-ions: Peptides are supplied as salts (typically TFA or acetate). The counter-ion mass is not part of the peptide molecular weight but affects the gross weight of the lyophilized material.
• Misreading D vs L configuration: D-amino acids have the same mass as their L counterparts but different biological properties. The sequence notation (d-Phe vs Phe) is the only way to distinguish them from CoA documentation.

Why are there two notation systems? The three-letter code was developed first for readability. The one-letter code was introduced later for compactness in computational applications and database storage. Both are maintained by IUPAC-IUB as parallel standards.

How do I calculate the molecular weight from a peptide sequence? Sum the residue masses of all amino acids in the sequence, add 18.02 Da for the terminal water molecule, and add or subtract masses for any modifications (acetylation, amidation, etc.). Online tools like ExPASy ProtParam automate this calculation.