Introduction to Peptides

What a peptide is

A peptide is a short chain of amino acids linked end to end by peptide bonds. Each amino acid contributes a backbone segment and a variable side chain, and it is the order of those side chains — the sequence — that gives a particular peptide its identity.

Peptides versus proteins

The boundary between "peptide" and "protein" is a convention rather than a hard rule. Short chains are normally called peptides, longer folded chains with a stable three-dimensional structure are normally called proteins. Because there is no universally fixed cutoff, the practical approach is to work from the sequence and the documented specification rather than from the label alone.

Sequence notation

By convention a sequence is written from the N-terminus (free amine end) to the C-terminus (carboxyl end). This direction matters: reversing it describes a different molecule. Sequences appear in documents either as three-letter codes (Ala-Gly-Ser) or as single-letter codes (AGS), and both notations are common in certificates of analysis and in literature.

Why length and composition matter in practice

As chains get longer, synthesis becomes more demanding, purification becomes more selective, and solubility becomes less predictable. Charge distribution and hydrophobicity are driven by the side chains present, so two peptides of identical length can behave very differently in solution. In practice this is why each batch is documented with its own analytical results rather than assumed to behave like a previous lot.

Where you see this on documentation

A typical certificate of analysis states the sequence, the molecular weight, and the analytical results obtained for that specific batch. Reading that document alongside the sequence is the reliable way to confirm you have received what you ordered.