43833-G4
Chemically Triggered Assembly of Peptide Materials
Stabilized helical peptides show potential as therapeutics capable of modulating protein function
through enhanced helix-protein binding. Helix turn stabilization may be accomplished though
covalent sidechain crosslinking on one face of the helix, in i and i+4 or i+7 positions,or
replacement of a main chain hydrogen bond with a covalent linker.We report herein a new
helix-nucleating i and i+4 crosslinking strategy based on copper catalyzed azide-alkyne [3+2]
“click” cycloaddition and demonstrate the ability of this method and metal complexation to
restore coiled-coil dimerization in a folding-incompetent sequence crippled by 2 helix-breaking
glycine residues. This peptide structure stabilization strategy complements known intramolecular
methods such as ring closing metathesis and allows convergent installation of functional groups
pendant to the bis-alkyne linker that could be used to modulate peptide binding, targeting, and
membrane permeability.
We studied 21 residue sequences derived from the GCN4 leucine zipper in which the central
heptad contains glycine residues in the c and e helix positions and crosslinking residues (X) in
the b and f (i and i+4) positions, leaving the hydrophobic core residues in the a and d positions
intact as isoleucine and leucine, respectively (Figure 1). Metal complexation with i and i+4
X=His residues is known to induce monomeric helix folding, though it has not been previously
demonstrated to restore structure in peptides containing 2 glycine residues. We postulated that
the bis-triazole product of a double [3+2] cycloaddition between i and i+4 azidoalanine (Az)
residues and a bis alkyne could yield a non-labile covalent linker isosteric with i and i+4 His-His
metal complex (Figure 1A). We prepared peptides which may be crosslinked in the central
heptad by Ni2+ complexation with histidine and bis-alkyne cyclization with Az, respectively.
Indeed, side chain crosslinking by these methods restores both folding and dimerization. We
have thus demonstrated a new methodology for helix structural nucleation using double
azide-alkyne [3+2] cycloadditions to form i and i+4 constrained peptide sequences. This
chemistry is sufficiently robust to allow intermolecular cyclization with few sideproducts, setting
the stage for convergent functionalization and structure nucleation. This technology may be
useful as a means of synthesizing new building blocks for peptide-based materials or preparing
stabilized helical peptides capable of binding protein interfaces and altering biological function.
These studies are currently underway in our lab. This study was accepted for publication,
citing PRF support, in ChemBioChem.
