Alkali metal ion-induced conformation changes of methionine- and leucine enkephalin investigated by gas-phase hydrogen/deuterium exchange combined with theoretical calculations

Abstract

We have investigated the effects of alkali-metal ions (G+ = K+, Rb+, or Cs+) on the conformation of leucine enkephalin (LE) and methionine enkephalin (ME) using hydrogen/deuterium exchange mass spectrometry and theoretical calculations at the molecular level. The MS result revealed a 1:1 stoichiometric ratio of the G+ coordination with LE/ME. We used [LE/ME+H]+ as the native conformation for comparison purposes, we observed different HDX behaviors following the reactivity order [LE/ME+H]+>[LE/ME+K]+>[LE/ME+Rb]+>[LE/ME+Cs]+ and [LE+G]+>[ME+G]+. Eight hydrogen atoms were HD-exchanged for [LE/ME+H]+, whereas only 4, 3, and 2 hydrogen atoms underwent HDX for [LE/ME+K]+, [LE/ME+Rb]+, and [LE/ME+Cs]+, respectively. Additionally, we obtained different dissociation processes and different fragmentations for the various complexes, and their varied HDX behaviors suggest that the alkali-metal ions K+, Rb+, and Cs+ produce effects on the LE/ME conformations that differ from those of H+. Further, we investigated the favorable conformations of the complexes by theoretical calculations, and found that K+, Rb+, and Cs+ are coordinated with three carbonyl oxygens and one deprotonated carboxylic acid of LE/ME, whereas the terminal amino group is the most stable site for protonation. We calculated the proton affinity (PA) for each active hydrogen atom on the optimized complexes, which reasonably explained the HDX results that a high PA value corresponds to low HDX reactivity. Overall, the calculations showed that alkali-metal ions can induce ME into a more rigid conformation than LE. Even the native conformation of ME is more exposed than LE, and the conformation of LE/ME is stabilized and rigid owing to the increase in the size of the complexed alkali metals.