Preparation of protected peptidyl thioester intermediates for native chemical ligation by Nalpha-9-fluorenylmethoxycarbonyl (Fmoc) chemistry: considerations of side-chain and backbone anchoring strategies, and compatible protection for N-terminal cysteine

J Pept Res. 2005 Mar;65(3):395-410. doi: 10.1111/j.1399-3011.2005.00241.x.

Abstract

Native chemical ligation has proven to be a powerful method for the synthesis of small proteins and the semisynthesis of larger ones. The essential synthetic intermediates, which are C-terminal peptide thioesters, cannot survive the repetitive piperidine deprotection steps of N(alpha)-9-fluorenylmethoxycarbonyl (Fmoc) chemistry. Therefore, peptide scientists who prefer to not use N(alpha)-t-butyloxycarbonyl (Boc) chemistry need to adopt more esoteric strategies and tactics in order to integrate ligation approaches with Fmoc chemistry. In the present work, side-chain and backbone anchoring strategies have been used to prepare the required suitably (partially) protected and/or activated peptide intermediates spanning the length of bovine pancreatic trypsin inhibitor (BPTI). Three separate strategies for managing the critical N-terminal cysteine residue have been developed: (i) incorporation of N(alpha)-9-fluorenylmethoxycarbonyl-S-(N-methyl-N-phenylcarbamoyl)sulfenylcysteine [Fmoc-Cys(Snm)-OH], allowing creation of an otherwise fully protected resin-bound intermediate with N-terminal free Cys; (ii) incorporation of N(alpha)-9-fluorenylmethoxycarbonyl-S-triphenylmethylcysteine [Fmoc-Cys(Trt)-OH], generating a stable Fmoc-Cys(H)-peptide upon acidolytic cleavage; and (iii) incorporation of N(alpha)-t-butyloxycarbonyl-S-fluorenylmethylcysteine [Boc-Cys(Fm)-OH], generating a stable H-Cys(Fm)-peptide upon cleavage. In separate stages of these strategies, thioesters are established at the C-termini by selective deprotection and coupling steps carried out while peptides remain bound to the supports. Pilot native chemical ligations were pursued directly on-resin, as well as in solution after cleavage/purification.

MeSH terms

  • Amino Acid Sequence
  • Amino Acids / chemical synthesis*
  • Amino Acids / chemistry*
  • Aprotinin / chemical synthesis
  • Aprotinin / chemistry
  • Cysteine / chemistry*
  • Fluorenes / chemical synthesis*
  • Fluorenes / chemistry*
  • Molecular Sequence Data
  • Peptide Fragments / chemical synthesis*
  • Peptide Fragments / chemistry*
  • Sulfur Compounds / chemical synthesis
  • Sulfur Compounds / chemistry

Substances

  • Amino Acids
  • Fluorenes
  • N(alpha)-fluorenylmethyloxycarbonylamino acids
  • Peptide Fragments
  • Sulfur Compounds
  • Aprotinin
  • Cysteine