How is a peptide sequence written?
A peptide sequence is written from the N-terminus on the left to the C-terminus on the right, listing each amino acid in order. Chemists use either three-letter codes, such as Gly-Ala-Pro, or one-letter codes, such as GAP. One-letter notation is compact and common in databases, while three-letter notation is easier to read when modified or unnatural residues are included. Terminal groups are often shown explicitly, for example H- at the start and -OH or -NH2 at the end.
What does the N-terminus and C-terminus mean?
Every linear peptide has two ends. The N-terminus is the end with a free amine group from the first amino acid, and the C-terminus is the end with a free carboxyl group from the last amino acid. Chemical synthesis on solid support usually builds the chain from the C-terminus toward the N-terminus. Either end can be chemically modified, and those modifications change charge, stability, and how the peptide behaves in in-vitro research models.
What is C-terminal amidation?
C-terminal amidation replaces the carboxylic acid at the end of the chain with an amide, written as -NH2 in a sequence. Many naturally occurring signaling peptides are amidated. The modification removes a negative charge at the C-terminus and, in preclinical research, is associated with resistance to certain carboxypeptidase enzymes. Synthetic amidated peptides are usually made on a resin designed to release the chain as an amide. The names of some catalog compounds, such as N-Acetyl Semax Amidate, describe these terminal modifications.
What is N-terminal acetylation?
N-terminal acetylation caps the free amine at the start of the chain with an acetyl group, written as Ac- in a sequence. This removes a positive charge and, in laboratory studies, can reduce susceptibility to aminopeptidases that degrade peptides from the N-terminal end. Acetylation is added during synthesis as a final capping step. Combined with C-terminal amidation, it produces a peptide with neither terminal charge, which can change solubility and handling compared with the unmodified sequence.
What are D-amino acids and why are they used?
Most amino acids exist in two mirror-image forms, called L and D. Natural proteins are built almost entirely from L-amino acids. Substituting a D-amino acid at a specific position changes the local shape of the peptide and, in preclinical research models, often makes that site resistant to proteases that recognize only L-residues. D-residues are written with a D- prefix in three-letter notation, or sometimes as a lowercase letter in one-letter notation. Several GH-secretagogue research peptides contain D-residues.
What are unnatural amino acids such as Aib?
Unnatural amino acids are building blocks outside the twenty standard residues. A common example is Aib, or alpha-aminoisobutyric acid, which carries two methyl groups on its alpha carbon. Aib restricts chain flexibility and, in research models, can protect nearby peptide bonds from enzymes such as dipeptidyl peptidase-4. Published sequences of incretin research compounds such as semaglutide at /peptides/semaglutide and tirzepatide at /peptides/tirzepatide include Aib substitutions. These residues are incorporated during synthesis like any other building block.
What does cyclization mean for a peptide?
Cyclization links two parts of a peptide chain to form a ring. Common types include a disulfide bridge between two cysteine residues, a head-to-tail amide bond joining the N-terminus to the C-terminus, and a side-chain lactam bridge between residues such as lysine and aspartic acid. A ring constrains the peptide's shape, which in preclinical research can influence receptor binding and resistance to enzymatic breakdown. Cyclization is usually performed after the linear chain is assembled.
How are disulfide bridges indicated in a sequence?
Disulfide bridges are usually indicated by noting which cysteine positions are linked, for example by writing the bond pattern after the sequence or by drawing a line between the two Cys residues in a structural diagram. Some notations use labels such as Cys1 to Cys6 to show connectivity. When a peptide contains more than one disulfide, the specific pairing matters, because different pairings produce different folded isomers. Mass spectrometry confirms the loss of two hydrogens per disulfide formed.
What is lipidation?
Lipidation attaches a fatty acid or lipid chain to a peptide, usually through a linker on a lysine side chain. In preclinical research, lipidated peptides bind reversibly to albumin, which is associated with a much longer persistence in circulation in animal models compared with the unmodified sequence. The published structure of semaglutide, for example, includes a fatty diacid attached through a linker. More background is in the semaglutide research profile at /learn/semaglutide-complete-research-profile and the peptide half-life explainer at /learn/peptide-half-life-explained.
What is a drug affinity complex, or DAC?
DAC refers to a reactive chemical group attached to a peptide that is designed to form a covalent bond with albumin. In research literature, the version of CJC-1295 described as containing DAC carries a maleimide-based group on an added lysine residue for this purpose. The versions without the DAC group are a different molecule with a different molecular weight. Naming conventions for these variants are covered in peptide nomenclature explained at /learn/peptide-nomenclature-explained and the CJC-1295 research profile at /learn/cjc-1295-complete-research-profile.
What is the difference between average and monoisotopic molecular weight?
Average molecular weight uses the natural abundance-weighted mass of each element and is the figure used for weighing and molarity calculations. Monoisotopic mass uses only the most abundant isotope of each element, typically carbon-12, and is the figure mass spectrometers report for the lightest isotopic peak. For larger peptides the two values can differ by a full mass unit or more. Certificates and catalog listings should state which value they report.
Do terminal modifications change molecular weight?
Yes. Every modification changes the molecular formula and therefore the mass. C-terminal amidation lowers the mass by roughly one mass unit compared with the free acid, because an oxygen-containing hydroxyl is replaced by an amine. N-terminal acetylation adds about 42 mass units. Disulfide formation removes about two units per bridge. Lipidation and linkers add considerably more. Mass spectrometry identity checks compare the observed mass against the value calculated for the fully modified structure.
Does the salt form appear in the molecular weight?
Usually not. Published molecular weights normally refer to the free peptide without counterions. The powder in a vial, however, is a salt that also contains trifluoroacetate, acetate, or chloride counterions plus residual water. Laboratories calculating molar concentrations need to account for this difference, either by correcting for net peptide content or by using the full salt molecular weight consistently. This is explained in peptide counterions and salt forms at /learn/peptide-counterions-and-salt-forms.
How is a peptide's structure confirmed after synthesis?
Identity is usually confirmed by mass spectrometry, which checks that the measured mass matches the value calculated for the intended sequence and modifications. Purity is measured by HPLC, which separates the target from related impurities such as deletion sequences. Some laboratories add tandem mass spectrometry to verify sequence order, or amino acid analysis to confirm composition. These methods are explained in how peptide purity is measured at /learn/how-peptide-purity-is-measured and in the purity and COA questions at /questions/purity-testing-and-coa.
Why do small structural changes matter in research?
A single substitution, terminal cap, or added ring can change a peptide's charge, shape, and stability, which in turn can change how it behaves in in-vitro assays and preclinical models. Two compounds with nearly identical names may differ by one residue or one modification and produce different experimental results. For that reason, researchers record the exact sequence, modifications, salt form, and lot number for every experiment rather than relying on a common name alone.
Where can researchers learn more about peptide structure basics?
An introduction to amino acids, peptide bonds, and how peptides differ from proteins is available at /what-are-peptides. Manufacturing steps are covered in how research peptides are made at /learn/how-research-peptides-are-made, and degradation pathways in peptide stability and degradation at /learn/peptide-stability-and-degradation. Shorter answers are collected in the questions hub at /questions. Laboratories sourcing documented reference material can place research orders through the order page at /order.
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