[The synthesis of motilin, III: Purification and characterization of (13-norleucine) motilin and (13-leucine) motilin (author's transl)].

The preparation of the pure docosapeptide H-Phe-Val-Pro-I1e-Phe-Thr-Tyr-Gly-Glu-Leu-Gln-Arg-Nle-Glu-Glu-Lys-Glu-Arg-Asn-Lys-Gly-Gln-OH ([Nle13]motilin) and of the analogous [Leu13]motilin from the crude synthetic materials obtained after deblocking of the overall protected docosapeptide derivatives by means of trifluoroacetic acid is described. In a preliminary experiment the separation of crude [Nle13]-motilin into several components, some of which being indistinguishable by thin layer chromatography, was achieved by repeated ion-exchange chromatography on QAE-Sephadex A-25 and SP-Sephadex C-25. Subsequent characterization of some of the isolated side-products, using amino acid analysis as well as spectroscopic (UV, CD) and enzymatic methods, in comparison to the major product enabled conclusions on the reasons of their formation. The undesired formation of des-(Thr6 -Tyr7 Gly8)[Nle13] motilin and of [D-Phe5-Nle13]motilin could be avoided during the subsequent major synthesis of [Nle13] motilin and during the preparation of [Leu13] motilin by changing certain synthetic conditions and, respectively, by using a specially purified protected fragment of the sequence 1-8, being free of diastereomers. Single ion-exchange chromatography on SP-Sephadex C-25 was sufficient for the purification of the so obtained crude synthetic end-products.