Structure of Sodiated Polyglycines

The intrinsic folding of peptides about a sodium ion has been investigated in detail by using infrared multiple photon dissociation (IRMPD) spectroscopy and a combination of theoretical methods. IRMPD spectroscopy was carried out on sodiated polyglycines Gn-Na+ (n=28), in both the fingerprint and N-H/O-H stretching regions. Interplay between experimental and computational approaches (classical and quantum) enables us to decipher most structural details. The most stable structures of the small peptides up to G6-Na+ maximize metalpeptide interactions with all peptidic C=O groups bound to sodium. In addition, direct interactions between peptide termini are possible for G6-Na+ and larger polyglycines. The increased flexibility of larger peptides leads to more complex folding and internal peptide structuration through gamma or beta turns. A structural transition is found to occur between G6-Na+ and G7-Na+, leading to a structure with sodium coordination that becomes tri-dimensional for the latter. This transition was confirmed by H/D exchange experiments on Gn-Na+ (n=38). The most favorable hydrogen-bonding pattern in G8-Na+ involves direct interactions between the peptide termini and opens the way to salt-bridge formation; however, there is only good agreement between experimental and computational data over the entire spectral range for the charge solvation isomer.