Melanotan I and Melanotan II are both synthetic analogues of alpha-melanocyte-stimulating hormone, and their names invite confusion. Structurally they are quite different molecules, and a laboratory needs to know which one is in the vial. This guide compares them as chemical entities for in vitro research use and makes no claims about use in people.
Melanotan I
Melanotan I, also known as afamelanotide, is a linear peptide of thirteen amino acids. It follows the sequence of natural alpha-MSH with two substitutions, norleucine at position four and D-phenylalanine at position seven, which make it more resistant to breakdown. Its molecular weight is about 1647 Da.
Melanotan II
Melanotan II is shorter and cyclic. It has seven residues, with a lactam bridge between an aspartic acid and a lysine side chain closing part of the sequence into a ring. Its molecular weight is about 1024 Da. In the literature it is described as a less selective melanocortin receptor agonist than the linear peptide, which is a consequence of that compact, constrained structure.
Telling them apart on a certificate
The two are easy to distinguish analytically:
- Mass spectrometry separates them at once: about 1647 Da against about 1024 Da.
- They elute at different retention times under the same HPLC method, so a chromatogram for one cannot stand in for the other.
- The certificate should name the compound in full, not just as Melanotan, and carry the batch number printed on the vial.
Why the distinction matters in a protocol
Because the masses differ by more than half, the same weight of powder contains very different numbers of molecules. A protocol written in molar terms for one compound cannot be carried over to the other by weight. Record the full compound name, batch number and molecular weight used in your calculations, so anyone repeating the work starts from the same figures.
Handling
Both are normally supplied as lyophilised powder and handled like other research peptides: stored cold and dry as the batch label directs, reconstituted with a suitable sterile diluent, and used in solution within a short, recorded period. Both contain tryptophan, which is sensitive to light and oxidation, so protecting solutions from light is sensible practice.