The disulfide bond pairing of the pheromones Er-1 and Er-2 of the ciliated protozoan Euplotes raikovi.

The disulfide pairings of the two Euplotes raikovi pheromones Er-1 and Er-2 have been determined by chemical and mass spectrometric analyses. Cystine-linked peptides from thermolytic digestions of the native molecules were purified by reverse-phase high performance liquid chromatography and identified in the known sequences to make the assignments. The same pairing, Cys(I)-Cys(IV), Cys(II)-Cys(VI), and Cys(III)-Cys(V), was found in both pheromones, suggesting that this pattern occurs commonly throughout this family of molecules. This arrangement of disulfides indicates that the three-dimensional structure is defined by three loops, which can vary in size and charge distribution from one pheromone to another.