Two Preparation Methods for Peptide Thioester Containing Tyr(SO₃H) Residue(s) without the Use of Protecting Group for Sulfate Moiety

We report two methods for the preparation of peptide thioesters containing Tyr(SO₃H) residue(s), without use of a protecting group for the sulfate moiety. The first was based on direct thioesterification using carbodiimide on a fully protected peptide acid, prepared on a 2-chlorotrityl (Clt) resin with fluoren-9-ylmethoxycarbonyl (Fmoc)-based solid-phase peptide synthesis (Fmoc-SPPS). Subsequent deprotection of the protecting groups with trifluoroacetic acid (TFA) (0 °C, 4 h) yielded peptide thioesters containing Tyr(SO₃H) residue(s). Peptide thioesters containing one to three Tyr(SO₃H) residue(s), prepared by this method, were used as building blocks for the synthesis of the N^α-Fmoc-protected N-terminal part of P-selectin glycoprotein ligand 1 (PSGL-1) (Fmoc-PSGL-1(43-74)) via silver-ion mediated thioester segment condensation. The other method was based on the thioesterification of peptide azide, derived from a peptide hydrazide prepared on a NH₂NH-Clt-resin with Fmoc-SPPS. Peptide thioester containing two Tyr(SO₃H) residues, prepared via this alternative method, was used as a building block for the one-pot synthesis of the N-terminal extracellular portion of CC-chemokine receptor 5 (CCR5(9-26)) by native chemical ligation (NCL). The two methods for the preparation of peptide thioesters containing Tyr(SO₃H) residue(s) described herein are applicable to the synthesis of various types of sulfopeptides.