Short alanine-based peptides may form 3₁₀-helices and not α-helices in aqueous solution

Abstract

SHORT alanine peptides, containing 16 or 17 residues, appear to form α-helices in aqueous solution^1–4. But the main spectroscopic analyses used on helical peptides (circular dichroism⁵ and nuclear magnetic resonance^6–8) cannot distinguish between an α-helix (in which the ith residue is hydrogen-bonded to residue i+4; ref. 9) and the next most common peptide helix, the 3₁₀-helix¹⁰ (i → i + 3 hydrogen-bonding). To address this problem we have designed single and doubly spin-labelled analogues of alanine-based peptides in which the nitroxide spin label forms an unbranched side chain extending from the sulphur atom of a cysteine residue. Here we report the circular dichroism, Fourier-transform infrared and electron-spin resonance spectra of these peptides under helix-forming conditions. The infrared absorbance gives an amide I' band with a frequency that is substantially different from that observed for α-helices. The electron-spin resonance spectra of doubly labelled helices show that the ranking of distances between side chains, around a single turn (residues 4–8), is inconsistent with an α-helical structure. Our experiments suggest that the more likely peptide geometry is a 3₁₀-helix.