Methionine epimerization in cyclic peptides

Abstract

Bioactive flax cyclic octa- and nona-peptides containing single methionine (Met) and its oxidized forms were treated under mild alkaline conditions to perform regio-selective epimerization. Cyclic peptide epimerization at the Met α-proton in a single chemical step has not been reported previously. The epimerization rate varies among Met oxidation states and ring size. These d-amino isomers along with the developed Met alkylation strategy will enable an approach to novel chemical functionalization of biomolecules. The amino acid configurations were confirmed by Marfey derivatizations, and cytotoxicity studies show the difference among the isomers. These d-amino analogs can act as a potential biomarker in plant protein processing and biomedical applications.

Scheme 1. Proposed mechanism of base catalyzed epimerization.

Fig. 1. Crystal structures of LO 4 butanol solvate and LO 2 methanol solvate, with color-coded atoms sulfur – yellow, oxygen – red, nitrogen – blue, carbon – black, α carbon Met/MetO₂ – green, solvent clouds: butanol – brown, water – red, methanol – cyan.

Scheme 2. Alkaline treatment of LO 4.

Fig. 2. HPLC chromatogram of (A) LOs 1, 4 and 7; (B) KOH treated LOs 1, 4 and 7.

Fig. 3. Separation of Marfey derivatized hydrolysate LO 4, LO 7, KOH treated LO 4 and KOH treated LO 7.

Fig. 4. % cytotoxicity of d-Met-LO 8 and l-Met LO 2 against MDA-MB-231 cell lines.