A Designed Zn2+-Binding Amphiphilic Polypeptide: Energetic Consequences of π-Helicity

Abstract

The π-helix is a secondary structure with 4.4 amino acids per helical turn. Although it was proposed in 1952, no experimental support for its existence was obtained until the mid-1980s. While short peptides are unlikely to assume a marginally stable secondary structure spontaneously, they might do so in the presence of appropriate structural constraints. In this paper, we describe a peptide that is designed to assume a π-helical conformation when stabilized by cetyltrimethylammonium bromide (CTAB) micelles and Zn²⁺. In the designed peptide, lipophilic amino acids are placed such that it would be amphiphilic in the π-helical, but not in the α-helical, conformation. Also, two His residues are incorporated with i, i + 5 spacing, designed to allow binding of Zn²⁺ in a π-helical but not an α-helical conformation. The peptide was found to form moderately stable monolayers at the air−water interface, with a collapse pressure that almost doubled when there was Zn²⁺ in the subphase. Also, CTAB micelles induced a marked increase in the helicity of the peptide. In 50% TFE, the peptide had a CD spectrum consistent with an α-helical structure. The addition of 1 mM Zn²⁺ to this solvent caused a saturable decline in ellipticity to approximately half of its original value. The peptide also bound Zn²⁺ when it was bound to CTAB micelles, with Zn²⁺ again inducing a decrease in ellipticity. The peptide had slightly greater affinity for Zn²⁺ in the presence of the CTAB than in a 50% TFE solution (K_d = 3.1 × 10^-4 M in CTAB and 2.3 × 10^-4 M in TFE). van't Hoff analysis indicated that thermal denaturation of the peptide in 50% TFE containing 1 mM Zn²⁺ was associated with both enthalpic and entropic changes that were greater than those in the absence of Zn²⁺. These observations are all consistent with the proposal that the peptide assumed a π-helical conformation in the presence of Zn²⁺ and CTAB micelles, and has allowed the stability of this rare conformation to be assessed.