Conformation Changes of Enkephalin in Coordination with Pb^(2+) Investigated by Gas Phase Hydrogen/Deuterium Exchange Mass Spectrometry Combined with Theoretical Calculations

In this study, the effects of lead ions(Pb^(2+)) on the conformations of leucine encephalin(LE) and methionine encephalin(ME) in gas phase were studied using hydrogen/deuterium exchange mass spectrometry(HDX-MS) and quantum chemistry theoretical calculations at the molecular level. The HDX-MS result revealed that the complexes with the monovalent compounds [LE+Pb–H]+ and [ME+Pb–H]+had a 1:1 stoichiometric ratio, and different HDX reactivates were observed in a follow of [ME+H]+>[LE+H]+>[LE+Pb–H]+> [ME+Pb–H]+. Combining the collision-induced dissociation energies of the complexes and their HDX results, it was found that the more stable the complex, the harder it was for HDX. In addition, the favo-rable conformations of the complexes were obtained by theoretical calculations, revealing that the similar coordination type with diffe-rent bond lengths was obtained. Then, the proton affinity(PA) values of the optimized complexes were calculated to interpret the HDX observations, indicating that the higher the PA values, the more difficult it was for HDX. Overall, the experiments and theoretical calculations revealed that Pb^(2+) could induce conformational changes of LE and ME, and generate ME into a more rigid conformation than LE. The results will prompt further fundamental investigations on the conformational properties of LE/ME in coordination with Pb^(2+).