Use of low-and high-energy collision-induced dissociation tandem mass spectrometry in the identification of an unusual amino acid in a semisynthetic polypeptide.

During production of the semisynthetic eicosapeptide Phe-Pro-Arg-Pro-Gly-Gly-Gly-Gly-Asn-Gly-Asp-Phe-Glu-Glu-Ile-Pro-Glu-Glu-Tyr-Leu, two minor impurities were isolated in which Ile-15 had been replaced by other amino acids. The molecular weights of the two peptide minor products were 14 u lower and 14 u higher, respectively, than the major product with the foregoing amino acid sequence. It was determined that the first, lower molecular weight, impurity contained Val instead of Ile at position 15 of the sequence, whereas the second, higher molecular weight, impurity contained an unusual amino acid at the same position. This amino acid, which was characterized from the low-and high-energy collision-induced dissociation product ion tandem mass spectra of the second peptide impurity, is an α-amino acid with an asymmetric side chain branched at the β-carbon.