Melanotan II

Melanotan II is a cyclic heptapeptide analogue of alpha-melanocyte stimulating hormone, closed by a lactam bridge between aspartate and lysine side chains. The cyclisation constrains the molecule into the bioactive conformation and confers resistance to proteolysis that the linear parent lacks. It is a non-selective agonist across MC1R, MC3R, MC4R and MC5R, and that lack of selectivity is precisely why it is useful for mapping which receptor subtype drives which observed effect — usually in combination with selective antagonists.

The quality-critical issue is whether the lactam bridge actually closed. Cyclisation is the last and least reliable step in the synthesis, and linear uncyclised material differs from the cyclic product by only 18 daltons — one water molecule. A mass spectrometer set to normal tolerances on a 1024 Da peptide can miss that. We resolve cyclic from linear chromatographically, and the certificate reports linear content as its own line.