Chemical synthesis of human beta-endorphin(1-27) analogs by peptide segment coupling. Leucine and glycine residues bearing thiocarboxyl functions as junctions for peptide segment coupling.

[Gly8]beta hEP(1-27)NH2 and [L-Leu8]beta hEP(1-27)NH2, two analogs of human beta-endorphin, were synthesized by both all-stepwise solid phase synthesis and peptide segment coupling. For the peptide segment coupling method, two thiocarboxyl peptides. Msc-[Gly8]beta hEP(1-8)SH and Msc-[L-Leu8]beta hEP(1-8)SH, were synthesized by standard solid phase method on 4-[alpha-(Boc-Gly-S)benzyl]phenoxyacetamidomethy-resin and 4-[alpha-(Boc-L-Leu-S)benzyl]phenoxyacetamidomethy-resin. These two thiocarboxyl peptides were coupled to H-[Lys(Cit)9,19,24]-beta hEP(9-27)NH2. [Gly8]beta hEP(1-27)NH2 and [L-Leu8]beta hEP(1-27)NH2 were obtained after removal of Msc groups and citraconyl groups from products of the segment coupling reaction. The yields of both [Gly8]beta hEP(1-27)NH2 and [L-Leu8]beta hEP(1-27)NH2 in the segment coupling reaction were approximately 18%. Less than 1% of racemization of Leu-8 occurred during coupling of Msc-[L-Leu8]beta hEP(1-8)SH to H-[Lys(Cit)9,19,24]-beta hEP(9-27)NH2. Results of amino acid composition analysis, analysis by reverse phase high pressure liquid chromatography and receptor binding activity assays of the analogs showed that peptide analogs prepared by segment coupling method and those prepared by all-stepwise solid phase synthesis were identical. Results of receptor binding activity assays suggested that the molecular charge properties of beta-endorphin(1-27) and its analogs influenced the receptor binding activity.