A SIMPLE METHOD FOR DIFFERENTIATING LEU AND ILE IN PEPTIDES - THE NEGATIVE-ION MASS-SPECTRA OF [M-H](-) IONS OF PHENYLTHIOHYDANTOIN LEU ANDILE

Phenylthiohydantoin (PTH) amino acid derivatives are formed during seq uential Edman degradation of peptides and proteins. Isomeric Leu and I le (which differ only in respectively having iso-butyl and sec-butyl c hains, and are often difficult to distinguish by conventional mass spe ctrometric techniques) may he readily identified by the characteristic decompositions of the [M-H](-) ions of their PTH derivatives. The Leu spectrum shows major loss of propane, while that of the Ile derivativ e shows elimination of both methane and ethane. This method may be use d routinely with 10 mu g quantities of peptide material.