Identification of the aspartimide structure in a previously-reported peptide

The octapeptide Leu-Ser-Arg-Leu-Phe-Asp-Asn-Ala (hGH 6-13) has been reported to show insulin-potentiating properties, and its synthesis by conventional solid-phase synthesis was previously described. It is now shown that peptide synthesized with diisopropylethylamine or N-methylmorpholine as neutralizing base in each cycle does have the above structure. When triethylamine was used as neutralizing base however, the Asp residue was converted to its imide, the presence of which has been demonstrated by means of infrared, 1H-NMR and fast-atom bombardment mass spectra, and by reactivity studies, electrophoresis at several pH values, and enzymic hydrolysis. Only the imide form of the peptide possesses the previously reported biological properties. A study of imide formation from protected and unprotected peptides showed that cyclization occurred during a wide range of acid and base treatments, but 10% triethylamine in CH2Cl2 was most effective, producing over 40% of imide in 90 min. The results of the investigation are compared with others in the literature, including those for the peptide hormone secretin.