Classification and peptide class. Ipamorelin (CAS 170851-70-4) is a synthetic growth-hormone secretagogue of the ghrelin-mimetic / GHRP structural family — a compound studied as a ligand at the growth-hormone secretagogue receptor (GHS-R). Its sequence, H-Aib-His-D-2-Nal-D-Phe-Lys-NH2, is a short, C-terminally amidated peptide built from non-canonical building blocks: alpha-aminoisobutyric acid (Aib), D-2-naphthylalanine (D-2-Nal), and D-phenylalanine (D-Phe). These D-amino acids and the terminal amide are hallmarks of a designed secretagogue scaffold rather than a native peptide fragment. Sermorelin (CAS 86168-78-7), by contrast, is a GHRH analogue designated GRF(1-29) — it corresponds to the first 29 residues of the endogenous growth-hormone-releasing factor and is therefore a truncated fragment of a native hormone rather than a de novo secretagogue. In short, the two sit on opposite sides of the GH-axis signaling model: one is a ghrelin-receptor-class ligand, the other a GHRH-receptor-class analogue.
Molecular profile. The size contrast between the two is substantial. Ipamorelin has the molecular formula C38H49N9O5 and a molecular weight of 711.85 g/mol, making it a compact small-peptide amide with no sulfur atom in its composition. Sermorelin carries the formula C149H246N44O42S with a molecular weight of 3357.93 g/mol — roughly 4.7 times the mass of Ipamorelin — and includes a single sulfur atom, consistent with a methionine residue within a 29-residue polypeptide chain. The larger residue count and higher nitrogen and oxygen content of Sermorelin reflect its identity as a near-full GHRH fragment, whereas Ipamorelin's minimal five-residue framework reflects a rationally minimized secretagogue pharmacophore.
Structural framing. Ipamorelin's short, amidated, D-amino-acid-containing scaffold is characteristic of engineered stability against exopeptidase recognition, and its aromatic D-2-Nal/D-Phe motif is typical of the GHRP secretagogue class. Sermorelin's structure is a linear polypeptide replicating the N-terminal domain of GHRH; catalog synonyms list it explicitly as GRF(1-29) and a GHRH analogue. This is why analytical identity testing differs in practice — a five-residue amide is well suited to straightforward mass-spec and HPLC characterization, while a ~3.4 kDa polypeptide is typically profiled with peptide-mapping and higher-resolution mass workflows.
Physical format. As research materials, both compounds are typically supplied as sterile lyophilized (freeze-dried) white-to-off-white powders intended for reconstitution with a compatible sterile diluent prior to in-vitro handling. Reconstitution chemistry is comparable in principle for both — dissolution of the lyophilate followed by cold storage of the resulting solution — with lyophilized material generally stored frozen and reconstituted solutions kept refrigerated and protected from repeated freeze-thaw. The practical difference in format is one of molecular robustness rather than form factor: Sermorelin's larger polypeptide chain presents more sequence surface for handling-related degradation pathways than Ipamorelin's small amidated pentapeptide, which is why analytical stability monitoring is a routine part of characterizing either material.