New degradation product of des-Gly10-NH2-LH-RH-ethylamide (fertirelin) in aqueous solution.

A new degradation product was found in an ampule preparation of des-Gly10-NH2-LH-RH-ethylamide (fertirelin) stored at room temperatures. The same product, produced at a higher temperature, was identified to be a diastereomeric mixture of cyclo(His-Trp) by proton nuclear magnetic resonance spectroscopy, mass spectrometry, and comparison with authentic samples. Preferred conformations regarding substituents on the diketopiperazine ring of cyclo(His-Trp) were calculated using coupling constants between vicinal protons.