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 Ala8−9Na + . The N−H and O− H stretching region allows identifying a new spectroscopic pattern typical for α-helical structures of polyalanines. SECTION: Biophysical Chemistry and Biomolecules