Heterocyclic peptide backbone modifications in an alpha-helical coiled coil

Abstract

In this paper, we present 1,2,3-triazole epsilon2-amino acids incorporated as a dipeptide surrogate at three positions in the sequence of a known alpha-helical coiled coil. Biophysical characterization indicates that the modified peptides retain much of the helical structure of the parent sequence, and that the thermodynamic stability of the coiled coil depends on the position of the incorporation of the epsilon-residue. Crystal structures obtained for each peptide give insight into the chemical behavior and conformational preferences of the non-natural amino acid and show that the triazole ring can participate in the backbone hydrogen bonding of the alpha-helix as well as template an interhelical crossing between chains in the bundle.

Figure 1

(a) A native dipeptide and the L-leucine derived triazole-ε- amino acid incorporated as a replacement; (b) Sequences for GCN4-pLI and modified peptides 1-3; X denotes incorporation of the ε-residue.

Figure 2

Schematic representation of the crystal structure of peptides 2 (a) and 3 (b) with atomic positions shown for the triazole residues. Each four-helix bundle superposes a crystallographic 2-fold axis and unique chains in each structure are indicated by different colors.10

Figure 3

(a) Detail from the crystal structure of 2 showing participation of the triazole residue in main chain hydrogen bonding; residues and nitrogen atoms of the triazole ring are numbered. (b) A top down view of the crystal structure of 3 showing the hydrophobic plate formed by the triazole ε residues and inter-chain hydrogen bonds bridged by water. (c) Detail of two chains from the crystal structure of 3 showing the inter-helical crossing. Dashed lines indicate hydrogen bonds. In (a) and (c), H atoms are modeled based on N, C, and O coordinates.