Characterization of the multiple forms of mast cell degranulating peptide by NMR spectroscopy.

MCD peptide is a toxic component of bee venom that contains 22 amino acid residues and two disulfide bridges. The peptide was prepared synthetically and purified by HPLC. The proton NMR spectrum of aqueous MCD peptide was assigned at 300 MHz. The NMR and HPLC results showed that the peptide is an approximately 2:1 mixture of two slowly interconverting species, most likely conformers. Proline-12 is proposed as the locus of the conformational equilibrium. Measurements of the proton-proton nuclear Overhauser effect confirmed that the connectivity of the major form is C3-C15 and C5-C19.