How it works
Unlike the site’s other two classes, this group has a recognizable structural through-line. The melanocortin analogs — PT-141 and the two Melanotan sequences — share a cyclic lactam bridge (an Asp2–Lys7 amide) that constrains the backbone into a ring, the clearest example on the site of deliberate structural constraint.
The same analogs carry non-standard residues — norleucine (Nle) in place of methionine, and D-phenylalanine in place of the L-form — and defined terminal chemistry: N-acetylation at one end, a C-terminal amide rather than a free acid at the other. These are compositional facts, visible on the compound pages, not statements about what the molecules do.
Against those constrained cyclic peptides sit short linear neuropeptides — Semax, Selank, DSIP, Epithalon — with no ring and ordinary termini. Placing the two side by side is a lesson in structure: the same class holds a rigid, heavily modified cyclic peptide and a flexible linear tetrapeptide.
Because the grouping is contextual, each member is best understood through its own record — sequence, molecular formula, mass, and the per-lot certificate — not through the class label.
Why it matters
A research classification is a navigational and comparative tool. Grouping by the context in which compounds are studied lets a researcher move between related molecules and see structural patterns — cyclic constraint, non-standard residues, terminal modification — that a single compound page would not reveal.
This class is a particularly good teaching set for conformational constraint: comparing a lactam-bridged melanocortin analog with a linear neuropeptide shows, in two clicks, why a ring changes how a peptide is characterized and handled.
Classifying by context keeps the framing honest: the name marks where these compounds appear in the literature, not an assertion that they share an effect. Each compound page states that no representation is made as to any biological effect.
At Lineará
The Signaling & Neuroactive class page lists every characterized member and links to its compound page, where identity (ESI-MS) and purity (RP-HPLC) are reported per lot.
The cyclic melanocortin analogs make the class the site’s clearest illustration of terminal chemistry and non-standard residues — details drawn residue-by-residue on the compound pages, and reconciled against a computed formula and mass in the identifier work.