Scientific lookup tool Peptide Sequence Database

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Terminal modifications

Ribosomally synthesized peptides begin with a free α-amine and end with a free α-carboxylate unless they are processed or chemically redesigned. Many hormones and nearly all GnRH and somatostatin analogues in this database are terminally modified. Those changes alter mass, charge, protease stability, and receptor recognition.

C-terminal amidation (-NH2)

In biology, C-terminal amides often arise from a glycine-extended precursor processed by peptidylglycine α-amidating monooxygenase (PAM). Chemically, amidation replaces the terminal carboxylic OH with NH2.

  • Formula change versus free acid: −O +N +H
  • Approximate average mass change: ≈ −0.984 Da
  • Notation on this site: -NH2 at the end of display sequences

Examples commonly amidated in literature and in this database: oxytocin, vasopressin, substance P, calcitonin, and exenatide. Filtering by the amidation modification tag is more reliable than inspecting the 1-letter string alone, because -NH2 is not encoded in the twenty-letter alphabet.

C-terminal ethylamide and related amides

Some analogues replace a native amidated glycine with a simpler alkyl amide. Leuprolide, for example, ends in Pro-NHEt (proline ethylamide) rather than Gly-NH2. Ethylamides are called out in display notation and tagged under modifications.

C-terminal amino alcohols

Reduction of a C-terminal residue to an amino alcohol (for example threoninol in octreotide) removes the carboxylic acid and changes both formula and polarity. Display notation often uses an -ol suffix. Treat these as distinct from standard Thr residues when comparing mass or CAS identity.

N-terminal pyroglutamate (pGlu / pE)

N-terminal glutamine (and sometimes glutamate) can cyclize to pyroglutamate, a five-membered lactam. The primary amine is lost, which reduces N-terminal charge and can increase stability toward aminopeptidases.

  • Versus Gln: loss of water (≈ −18.015 Da) upon cyclization
  • 1-letter fields may still use E or Q as a stand-in
  • Display notation is authoritative: look for pGlu or pE

Examples: TRH, GnRH, leuprolide, and goserelin.

1-Deamino / mercaptopropionyl (Mpr) termini

Desmopressin, atosiban, and related analogues replace the N-terminal cysteine’s α-amine with a 3-mercaptopropionyl (Mpr) unit. The thiol still participates in disulfide chemistry, but the N-terminus is no longer a standard amino acid. This site tags that chemistry under deamination-related modifications even though the atom replacement is more specific than simple loss of NH2.

Acetylation and other N-acyl groups

N-terminal acetylation (Ac-) caps the amine with an acetyl group. It appears in some synthetic and histone-related peptides. When present, it is shown in display notation and should be included in mass accounting (+42.037 Da average for acetyl versus free amine, depending on reference state).

Why termini are catalogued separately

Terminal chemistry is easy to miss if you only read a 1-letter core. Two peptides can share the same residue letters and differ only by -OH versus -NH2, yet have different CAS numbers, formulas, and biological properties. Use modification filters when you need all amidated, pyroglutamyl, or deamino entries in the directory.