Peptide nomenclature encompasses the standardized naming conventions used to describe amino acid sequences, modifications, and structural features. Proper nomenclature ensures clear communication between researchers and accurate ordering of synthetic peptides.
Amino Acid Abbreviations
Each of the 20 standard amino acids has both a three-letter code and a single-letter code established by IUPAC-IUB conventions. For example, alanine is Ala or A, glycine is Gly or G, and tryptophan is Trp or W. Three-letter codes are used in detailed structural descriptions, while single-letter codes are preferred for representing longer sequences compactly. Sequences are conventionally written from the N-terminus (left) to the C-terminus (right), reflecting the direction of ribosomal translation in biology.
Non-standard amino acids use specialized abbreviations. D-amino acids are prefixed with a lowercase “d” (e.g., dAla for D-alanine). Norleucine is abbreviated Nle, ornithine is Orn, and aminoisobutyric acid is Aib. When ordering custom peptides, using correct abbreviations prevents costly synthesis errors and delays.
Naming Modifications and Terminals
Terminal modifications are indicated by prefixes and suffixes. Ac- denotes N-terminal acetylation, while -NH2 indicates C-terminal amidation. These modifications are commonly added to improve metabolic stability and mimic internal peptide bond geometry. Other modifications include pyroglutamic acid at the N-terminus (pyroGlu or pGlu), biotinylation (Biotin-), and fluorescent labeling (FITC-, Rhodamine-).
Disulfide bonds between cysteine residues are indicated by connecting brackets or by specifying the bonded positions (e.g., Cys3-Cys12). Cyclization is noted by “cyclo-” prefix or by specifying the cyclization chemistry. Phosphorylated residues are indicated as pSer, pThr, or pTyr for phosphoserine, phosphothreonine, and phosphotyrosine respectively.
Systematic vs Common Names
Many well-known peptides have common names that differ from their systematic sequence descriptions. Oxytocin, for instance, is systematically Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2 with a disulfide bridge between the two cysteines. Research literature typically uses common names for well-established peptides while providing full sequences for novel compounds. When ambiguity exists, the full sequence with modifications should always be specified to avoid confusion.
Frequently Asked Questions
What does the dash between amino acids mean?
The dash represents a peptide bond connecting adjacent amino acids. H-Ala-Gly-Phe-OH indicates a tripeptide with a free amino group at the N-terminus (H-) and a free carboxyl group at the C-terminus (-OH). Dashes are optional in single-letter notation, where the same peptide would simply be written AGF.
How do I specify a D-amino acid in a sequence?
D-amino acids are indicated with a lowercase “d” prefix in three-letter notation (e.g., dPhe for D-phenylalanine) or by using the lowercase single-letter code in some conventions (though this can be ambiguous). When ordering peptides, always use three-letter codes for D-amino acids to ensure clarity with the synthesis facility.