Using quantum chemical calculations and infrared multiphoton dissociation (IRMPD) spectroscopy in the fingerprint and X-H stretching regions, we demonstrate here that the all-Ala (b6) fragment ion features a macrocyclic structure with C(2) symmetry. For this structure, the ionizing proton is equally shared by the Ala(1) and Ala(4) amide oxygens in a Zundel-type symmetric (X…H(+)…X) H-bond.