Peptide identity is often discussed through sequence and mass: which amino acids are present, in what order, and whether mass spectrometry supports the expected molecular weight.

Those are essential questions, but they are not the whole identity story.

Most amino acids used in peptides are chiral. That means the molecule can exist in left- and right-handed arrangements that share the same atomic formula but occupy three-dimensional space differently. In peptide language, this is usually described through L- and D-amino acid configuration.

That difference can matter. A peptide containing an L-amino acid at a given position is not automatically equivalent to the same sequence containing the D-form at that position. Even when the nominal mass looks similar, stereochemistry can affect conformation, enzymatic recognition, receptor interaction, aggregation behavior, analytical separation, and impurity interpretation.