A left‐handed 31 helical conformation in the Alzheimer Aβ(12–28) peptide

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Abstract
We show for the first time that the secondary structure of the Alzheimer β‐peptide is in a temperature‐dependent equilibrium between an extended left‐handed 31 helix and a flexible random coil conformation. Circular dichroism spectra, recorded at 0.03 mM peptide concentration, show that the equilibrium is shifted towards increasing left‐handed 31 helix structure towards lower temperatures. High resolution nuclear magnetic resonance (NMR) spectroscopy has been used to study the Alzheimer peptide fragment Aβ(12–28) in aqueous solution at 0°C and higher temperatures. NMR translation diffusion measurements show that the observed peptide is in monomeric form. The chemical shift dispersion of the amide protons increases towards lower temperatures, in agreement with the increased population of a well‐ordered secondary structure. The solvent exchange rates of the amide protons at 0°C and pH 4.5 vary within at least two orders of magnitude. The lowest exchange rates (0.03–0.04 min−1) imply that the corresponding amide protons may be involved in hydrogen bonding with neighboring side chains.