What is a peptide?

Amino acids join to one another through a covalent bond formed between the carboxyl group of one residue and the amino group of the next. That bond is called a peptide bond, and a chain built from it is a peptide. The definition is structural rather than functional: it describes how the molecule is put together, not what it does.

Where peptides end and proteins begin

There is no sharp boundary. Convention places the division somewhere around fifty amino acids, but the figure is a habit rather than a rule, and different fields use different thresholds. What tends to matter more in practice is behaviour: longer chains generally fold into stable three-dimensional structures whose shape is essential to function, while shorter chains are often flexible in solution and adopt a defined conformation only on binding to something else.

This has a practical consequence for anyone reading the literature. A result obtained with a short synthetic peptide cannot be assumed to carry over to the full-length protein it was derived from, and the reverse is equally true. Fragments are not miniature versions of their parent molecules.

Residue

An individual amino acid unit within a chain. A peptide described as having fifteen residues contains fifteen amino acids joined in sequence.

N-terminus and C-terminus

The two ends of a peptide chain: the N-terminus carries a free amino group, the C-terminus a free carboxyl group. Sequences are written N-terminus first by convention.

Sequence is not the whole specification

Two preparations can share an identical amino acid sequence and still differ in ways that change how they behave in an experiment. Common sources of difference include:

• Terminal modifications. Acetylation at the N-terminus or amidation at the C-terminus alters charge and resistance to enzymatic degradation.
• Counter-ions. A peptide supplied as an acetate salt and the same peptide as a trifluoroacetate salt are not interchangeable in every assay, and the salt contributes to the measured mass.
• Stereochemistry. Substituting a D-amino acid for its L-form leaves the sequence description unchanged in casual writing but produces a different molecule.
• Cyclisation and disulfide bonds. Whether and where a chain is closed determines its shape, and partial or incorrect bonding is a real failure mode in synthesis.
• Water and salt content. Lyophilised material contains residual moisture and salts, so net peptide content is lower than the gross mass.

Why peptides are studied

Peptides sit at a useful midpoint between small molecules and biologics. They can be synthesised chemically, which makes them straightforward to modify systematically, and they can be designed to mimic a specific surface of a larger protein, which makes them useful tools for probing how proteins interact. Endogenous peptides — those the body produces itself — are studied because they are signalling molecules in their own right, acting at receptors that can be mapped and manipulated.

These same properties create difficulties. Peptides are typically broken down rapidly by peptidases, absorbed poorly when taken by mouth, and cleared quickly from circulation. A large part of peptide research is concerned with those obstacles rather than with any biological effect, and a study demonstrating improved stability has not thereby demonstrated that anything useful happens.

Reading claims about peptides carefully

Because peptides are ubiquitous in biology, almost any peptide can be connected to a plausible-sounding mechanism. Plausibility is cheap. The questions that separate a mechanism from a result are whether the effect was measured in a living organism, whether the organism was human, whether the study was controlled, and whether anyone independent has reproduced it.

Our evidence framework sets out those distinctions and is applied consistently across the research library.