Investigation of protonated and sodiated leucine-enkephalin by hydrogen–deuterium exchange and theoretical calculations

Abstract

In this work, protonated and sodiated leucine-enkephalin (LE) were investigated by gas-phase hydrogen–deuterium exchange (HDX) performed on a linear ion trap time-of-flight mass spectrometer. It is found that more hydrogen atoms are exchanged in protonated LE than in sodiated LE, indicating the different conformations of the two peptide ions. To clarify further the experimental results, the conformations were calculated by using density functional theory, which shows that the terminal amino group is the most thermodynamically stable protonation site, while the sodium ion coordinated to four carbonyl oxygen atoms forms the most favourable sodium adduct. Limited HDX reactions of sodiated LE are explained by the rigid conformation and fewer exchangeable acidic hydrogen atoms from sodium coordination.