Classification. Both compounds belong to the same broad class: synthetic melanocortin peptide analogues derived from the alpha-MSH pharmacophore, prepared by solid-phase peptide synthesis (SPPS) and purified by HPLC. Melanotan-1 is also known as afamelanotide and is formally described as [Nle4-D-Phe7]-alpha-MSH, indicating that it retains the full alpha-MSH-length backbone with two residue substitutions. Melanotan-2 (MT-2) is described as a cyclic alpha-MSH analogue — a truncated, conformationally constrained construct rather than a full-length sequence. In short, both sit in the melanocortin-analogue family, but Melanotan-1 is a linear full-length analogue while Melanotan-2 is a shortened, cyclized one.
Peptide structure and class. The structural contrast is the defining difference between the two. Melanotan-1 is a linear tridecapeptide (13 amino acids), with the reported sequence Ac-Ser-Tyr-Ser-Nle-Glu-His-D-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2 — an N-terminal acetyl group, a C-terminal amide, the non-proteinogenic residue norleucine (Nle) at position 4, and a D-phenylalanine (D-Phe) at position 7. Melanotan-2 is a cyclic heptapeptide (7 amino acids), reported as Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2, in which a lactam bridge between the aspartate and lysine side chains closes a ring around the conserved His-D-Phe-Arg-Trp core. Both share the acetylated N-terminus, the C-terminal amide, the Nle substitution, and the D-Phe residue; the essential divergence is that Melanotan-1 presents its core sequence on an open linear chain, whereas Melanotan-2 constrains a shorter version of that core into a covalently closed macrocyclic ring.
Molecular profile. These architectural differences are reflected directly in the molecular data. Melanotan-1 has the molecular formula C78H111N21O19 and a molecular weight of approximately 1646.87 g/mol, consistent with its 13-residue length. Melanotan-2 has the molecular formula C50H69N15O9 and a molecular weight of approximately 1024.18 g/mol, consistent with its 7-residue cyclic construction. The two compounds therefore differ by roughly 620 g/mol and carry distinct, non-overlapping CAS registry numbers (75921-69-6 for Melanotan-1; 121062-08-6 for Melanotan-2), allowing unambiguous identity confirmation by mass spectrometry and HPLC retention. The larger residue count and greater heteroatom content of Melanotan-1 are the expected signature of its longer linear backbone.
Physical format. As supplied for research, the two materials are closely matched in presentation. Both are offered as lyophilized (freeze-dried) powders at a stated purity of at least 99% by HPLC with a certificate of analysis (COA) available, and both are cataloged in a 10 mg unit at the research grade. Reported reconstitution chemistry is likewise parallel: each is described as soluble in bacteriostatic water, with the specific solvent selection to be confirmed against the product COA. Recommended storage for both is the lyophilized powder at -20°C, protected from light and moisture, with any reconstituted solution held refrigerated at 2-8°C. In practical laboratory handling terms the two are near-identical; their meaningful differences are structural and molecular, not formatting.