Studies on peptides. CLXVIII. Syntheses of three peptides isolated from horseshoe crab hemocytes, tachyplesin I, tachyplesin II, and polyphemusin I.

Tachyplesin I, a 17-residue peptide amide with two disulfide bridges isolated from an acid extract of horseshoe crab hemocyte debris, was synthesized by the 9-fluorenylmethyloxycarbonyl (Fmoc)-based solid-phase method followed by two steps of deprotection and subsequent air-oxidation. The thioanisole-mediated deprotection with 1 M trifluoromethanesulfonic acid was first employed to cleave the peptide amide from the resin and, at the same time, to deprotect the side chain protecting groups employed, except for the S-Acm (acetamidomethyl) group. The 4 Acm groups attached were next cleaved by silver trifluoromethanesulfonate. In addition, two related peptides, tachyplesin II and polyphemusin I, were similarly synthesized. Synthetic tachyplesin I inhibited the lipopolysaccharide-mediated activation of clotting factor C to the same extent as did the corresponding natural peptide. The relative potencies of synthetic tachyplesin II and synthetic polyphemusin I with respect to natural tachyplesin I (taken as 1) were 2.1 and 0.61, respectively.