Detection of cis-trans isomers of a synthetic peptide fragment of erythropoietin

The synthesis of the protected fragment t-butoxycarbonyl-alanine-isoleucine-serine(benzyl)-proline (Pro)-Pro-OH derived from the hormone erythropoietin is described. The analysis of the peptide by high-pressure liquid chromatography (HPLC) and thin-layer chromatography (TLC) yields apparently inconsistent results. Although HPLC consistently indicates the presence of only one component, TLC reveals a number of distinct species. Because satisfactory amino acid analysis and fast atom bombardment-mass spectrometry results are obtained, we think it possible that the distinct components arise from the cis-trans isomerization of the peptide bonds to the prolyl residues. An analysis using capillary electrophoresis under basic conditions identifies four components in the final product. Also, under similar conditions proton nuclear magnetic resonance spectroscopy is able to confirm the presence of cis and trans isomers. The results from this study demonstrate the usefulness of each of the four techniques in identifying the isomerism of the standard amino acid-Pro bond with respect to the peptide's ionic state.