RESUMEN
Differences of protonated and lithiated leucine-enkephalin(LE) were investigated by hydrogen deuterium exchange-mass spectrometry(HDX-MS) combined with quantum chemistry calculation. The results revealed that the protonated ions possessed very high product yield with all hydrogen atoms being exchanged, while the reaction of lithiated LE stopped after exchanging five hydrogen atoms in the same experimental conditions. The different HDX behaviours probably indicated their conformational differences. To further clarify the experimental results, the most stable conformations of protonated and lithiated leucine-enkephalin were calculated by density functional theory. It was found that terminal amino group was the most thermodynamically stable protonation site,while Li+in coordination of four carbonyl oxygen atoms formed the most favourable lithiated LE. The reaction field reduction of lithium LE was probably due to the less acidity of hydrogen atoms and the increasing rigid conformation change induced by lithium ion.