Synthesis of an N-methyldehydroalanine-containing fragment of microcystin by combination of solid phase peptide synthesis and beta-elimination in solution.

A new method for the synthesis of dehydroalanine (delta Ala)-containing peptides has been developed by combining solid phase peptide synthesis (tert-butyloxycarbonyl/HF-chemistry) with solution synthesis. A sequence from cyanobacterial hepatotoxin microcystin, Ac-D-gamma-Glu-[N-Me-delta Ala]-D-Ala-Leu amide was chosen as a model peptide. The precursor for the synthesis of the dehydroalanine-containing peptide, Ac-D-gamma-Glu-[N,S-diMeCys]-D-Ala-Leu, was synthesized on a solid phase followed by sulfonium salt formation on the resin. The resulting S,S-dimethylated peptide was cleaved from the resin with liquid HF. The HPLC-purified S,S-dimethylated cysteine-containing precursor peptide was subjected to beta-elimination in solution catalysed by DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) in methanol. The final product, Ac-D-gamma-Glu-[N-Me-delta Ala]-D-Ala-Leu amide, was purified by HPLC, and analysed by mass spectrometry and 1H NMR spectroscopy. The stability of the model peptide under acidic, neutral and basic conditions has been studied.