Globule to helix transition in sodiated polyalanines

The structures of sodiated polyalanine peptides containing 8-12 residues are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and classical and quantum modeling. Calculations indicate that the α-helical structure is the most stable conformation for the peptides whatever their size. The IRMPD spectra provide evidence for the coexistence of helical and globular shapes for Ala₈Na⁺, and possibly for Ala₉Na⁺. The turning point from globule to helix is thus found at Ala_8-9Na⁺. The N-H and O-H stretching region allows identifying a new spectroscopic pattern typical for α-helical structures of polyalanines.