New segment-coupling method for peptide synthesis in aqueous solution: application to synthesis of human [Gly17]-beta-endorphin.

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Abstract
The thiolcarboxyl peptide [17-thiolglycine]-.beta.-endorphin-(1-17) (I) was synthesized by the solid-phase method. Reaction of I with citraconic anhydride gave citraconyl-[Lys(Cit)9,GlyS17]-.beta.-endorphin-(1-17) (Ia). Ia was coupled to another synthetic peptide, [Lys(Cit)19,24,28,29]-.beta.-endorphin-(18-31), by reaction with silver nitrate-N-hydroxysuccinimide in water. All citraconyl groups were removed in aqueous acetic acid, and [Gly17]-.beta.-endorphin was isolated in 30-40% yield. The synthetic analog had 10% analgesic potency in mice and 59% rat brain opiate-receptor binding activity when compared with human .beta.-endorphin.