SYNTHESIS OF HUMAN β-ENDORPHIN IN SOLUTION USING BENZYL-TYPE SIDE CHAIN PROTECTIVE GROUPS

Abstract

A solution synthesis of human β-endorphin (β-EP) was carried out by condensation of protected peptide segments bearing N^α-tert.-butyloxycarbonyl groups and benzyl-derived groups for the protection of functionalities in amino acid side chains. Five intermediate segments were assembled in a stepwise manner starting at the carboxyl terminus. Thus, the segment of sequence region (27–31) was coupled to segment (22–26) by the azide method. Segment (19–21) was incorporated into the growing chain by azide coupling, and segment (10–18) by dicyclohexylcarbodiimide coupling in the presence of 1-hydroxybenzotriazole (DDC-HOBt). Solubility problems in condensing the ensuing 22-peptide with segment (1–9) by DDC-HOBt were overcome by using a dimethylformamidephenol mixture as a solvent. Protecting group cleavage by Na in liquid NH₃ was much superior to liquid HF which gave rise to many decomposition products. Homogeneous β_n-EP indistinguishable from authentic material in physiochemical and biological properties, was obtained in a single preparative reversed phase liquid chromatographic step after protecting group cleavage.