The sodium ion affinity of glycylglycine

The sodium ion affinity of the simplest peptide, glycylglycine (GlyGly), is determined by the kinetic method, from the dissociation kinetics of Na⁺-bound dimers of GlyGly and amino acid reference bases. The dimer ions are formed by electrospray ionization and their competitive dissociations to the metalated monomers are probed by collisionally activated dissociation in a quadrupole ion trap mass spectrometer. The experimental value derived is 203±8 kJ mol^-1 (ΔH_Na at 298 K). Concurrent high-level ab initio calculations predict a GlyGly-Na⁺ bond enthalpy of 200 kJ mol^-1, in excellent agreement with the experimental result. The most stable structure of the GlyGly-Na⁺ complex found by the ab initio calculations involves solvation of the metal charge by the multidentate GlyGly ligand. The Na⁺ affinity reported here lies substantially above older literature values (177-181 kJ mol^-1). Plausible reasons for the earlier underestimations are briefly discussed.