Guides

Peptide Nomenclature and Conventions: A Complete Reference

Dr. James ChenNovember 19, 20258 min read

Accurate peptide description requires standardized nomenclature. Miscommunication about sequences, modifications, or structures can lead to failed experiments, wasted resources, and irreproducible results. This guide covers the conventions used in peptide science for clear, unambiguous communication.

Amino Acid Nomenclature

Standard Amino Acids

**Three-Letter Codes:**

Amino AcidThree-LetterOne-Letter

|------------|--------------|------------|

AlanineAlaA

ArginineArgR

AsparagineAsnN

Aspartic AcidAspD

CysteineCysC

Glutamic AcidGluE

GlutamineGlnQ

GlycineGlyG

HistidineHisH

IsoleucineIleI

LeucineLeuL

LysineLysK

MethionineMetM

PhenylalaninePheF

ProlineProP

SerineSerS

ThreonineThrT

TryptophanTrpW

TyrosineTyrY

ValineValV

**Ambiguous Codes:**

CodeMeaning

|------|---------|

Asx (B)Asn or Asp

Glx (Z)Gln or Glu

Xaa (X)Any amino acid

Writing Peptide Sequences

**Convention:**

N-terminus on left, C-terminus on right

By default, N-terminus is free amine, C-terminus is free acid

**Three-Letter Format:**

Hyphenated: Ala-Gly-Phe-Leu-Arg

No terminal indication = free termini

**One-Letter Format:**

No separators: AGFLR

Compact for longer sequences

**Indicating Chain Direction:**

H-Ala-Gly-Phe-OH (explicit free termini)

NH2-AGFL-COOH (alternative)

Terminal Modifications

N-Terminal Modifications

**Acetylation:**

Ac-Ala-Gly-Phe or Ac-AGF

CH3CO- group

**Formylation:**

For-Ala-Gly-Phe or fMet (special case)

CHO- group

**Pyroglutamic Acid:**

pGlu- or pE

Cyclized glutamate

**Biotin:**

Biotin-Ahx-Ala-Gly (with spacer)

Bio-Ala-Gly

**Fluorescent Labels:**

FITC-Ala-Gly

Fluorescein-Ahx-Ala-Gly

5-FAM-Ala-Gly

**Fatty Acids:**

Myr-Ala-Gly (myristoyl)

Pal-Ala-Gly (palmitoyl)

C-Terminal Modifications

**Amidation:**

Ala-Gly-Phe-NH2 or AGF-NH2

Very common, mimics natural peptides

**Specific Amides:**

Ala-Gly-Phe-NHMe (N-methylamide)

Ala-Gly-Phe-NEt (ethylamide)

**Esters:**

Ala-Gly-Phe-OMe (methyl ester)

Ala-Gly-Phe-OEt (ethyl ester)

**Aldehyde:**

Ala-Gly-Arg-H or Ala-Gly-Arg-CHO

For protease inhibitors

Side Chain Modifications

Phosphorylation

**Phosphoserine:**

pSer or S(PO3H2) or Sep

Ser(P)

**Phosphothreonine:**

pThr or T(PO3H2) or Thr(P)

**Phosphotyrosine:**

pTyr or Y(PO3H2) or Tyr(P)

**In Sequence:**

Ala-pSer-Gly or Ala-S(PO3H2)-Gly

Methylation

**Lysine Methylation:**

Lys(Me) = monomethyl

Lys(Me2) = dimethyl

Lys(Me3) = trimethyl

**Arginine Methylation:**

Arg(Me) = monomethyl

Arg(Me2-a) = asymmetric dimethyl

Arg(Me2-s) = symmetric dimethyl

Acetylation

**Lysine Acetylation:**

Lys(Ac) or K(Ac)

Glycosylation

**General:**

Ser(GalNAc) or Thr(GalNAc)

Asn(GlcNAc) for N-linked

Other Modifications

Cys(Acm) = acetamidomethyl protected

Lys(Biotin) = biotinylated lysine

Tyr(NO2) = nitrotyrosine

Met(O) = methionine sulfoxide

Non-Standard Amino Acids

D-Amino Acids

**Convention:**

Lowercase: d-Ala or D-Ala

In sequence: D-Ala-Gly-Phe or dA-G-F

Beta-Amino Acids

beta-Ala or bAla

Different backbone connectivity

Unusual Amino Acids

AbbreviationFull Name

|--------------|-----------|

NleNorleucine

OrnOrnithine

CitCitrulline

HseHomoserine

SarSarcosine (N-methylglycine)

Aib2-aminoisobutyric acid

NvaNorvaline

Abu2-aminobutyric acid

Spacers and Linkers

Ahx = 6-aminohexanoic acid

Ava = 5-aminovaleric acid

betaAla = beta-alanine

PEGn = polyethylene glycol (n units)

Cyclic Peptides

Head-to-Tail Cyclization

cyclo(Ala-Gly-Phe-Leu-Arg)

cyclo(-AGFLR-)

c(AGFLR)

Side Chain Cyclization

**Disulfide:**

Cys(1)-Ala-Gly-Cys(1) (bridge notation)

Indicates which Cys residues are paired

**Lactam:**

cyclo(Glu-Ala-Lys) [side chain to side chain]

Specify Glu-Lys connectivity

**Thioether:**

Indicate with appropriate notation

Multiple Bridges

Use superscript numbers:

Cys1-Ala-Cys2-Gly-Cys1-Phe-Cys2

Numbers indicate pairing

Disulfide Bond Notation

Single Disulfide

Parenthetical: C(S-S)C

Superscript: Cys1-Ala-Cys1

Multiple Disulfides

**Sequential Numbering:**

Cys1-Ala-Cys2-Gly-Cys2-Phe-Cys1

Same number = bonded pair

**Explicit Notation:**

[Cys3-Cys15] [Cys8-Cys22]

List of bonded pairs

Position Numbering

Standard Convention

Number from N-terminus

First residue = 1 (or 0 in some systems)

Include position when discussing specific residues

Examples

Ser8 = serine at position 8

K12R = lysine 12 replaced with arginine

pS15 = phosphoserine at position 15

Fragment Notation

For partial sequences from larger proteins:

Peptide(1-20) = first 20 residues

Protein(45-60) = residues 45-60

Mutation Nomenclature

Point Mutations

Original-Position-New: K12R

Position can include protein name: EGFR-K12R

Multiple Mutations

Separate with commas: K12R, S15A, G20V

Or slashes: K12R/S15A/G20V

Deletions

del or delta symbol

deltaK12 or K12del

Insertions

ins: 12insA (alanine inserted after position 12)

Mass and Formula Conventions

Molecular Weight Reporting

Monoisotopic mass: Based on most abundant isotopes

Average mass: Weighted average of isotopes

Specify which is being reported

Chemical Formula

Include all atoms

Account for terminal groups

Standard format: C, H, N, O, S, others

Best Practices

For Ordering Peptides

Include:

Complete sequence with termini specified

All modifications explicitly stated

Disulfide connectivity if applicable

Desired purity and quantity

For Publications

Define non-standard abbreviations

Specify sequence direction

Indicate modifications unambiguously

Provide complete sequence in methods

Common Errors to Avoid

Assuming termini (always specify)

Ambiguous modification placement

Inconsistent D-amino acid notation

Unclear disulfide connectivity

Conclusion

Standardized peptide nomenclature ensures clear communication throughout the research community. Proper notation of sequences, modifications, and structures prevents costly errors and enables reproducibility. When in doubt, err on the side of explicit detail rather than assuming shared understanding.