The effect of non-covalent binding of sodium to a peptide on the H/D exchange reaction

Abstract

The gas phase H/D exchange of some sodium attached peptide ions, (5Ala-<italic>n</italic>H+<italic>n</italic>Na).H⁺, <italic>n</italic> = 2,3,4,5 have been studied with an electrospray ionization linear ion trap time-of-flight mass spectrometry (ESI-LIT-TOFMS) system. The exchange reaction rates of the peptide with different sodium ions have been investigated and the mechanism of the sodium ion effect was discussed. When two Na⁺ attached to the peptide and formed (5Ala-2H+2Na).H⁺, there was no H/D exchange in our experimental conditions. The exchange rates were accelerated if 3, 4 or 5 Na⁺ attached to the peptide and formed (5Ala-3H+3Na).H⁺, (5Ala-4H+4Na).H⁺, and (5Ala-5H+5Na).H⁺ ions. The peptide might have different conformations if different sodium ions were attached. The effect of the <italic>q</italic>-value of the quadrupole field on the H/D exchange rate were also studied in this work. The exchange rate was not changed if the <italic>q</italic> < 0.8, and the rate were certainly increased when the <italic>q</italic> is close to 0.908 because the ions have higher kinetic energy at this point.