Effect of Transition-Metal Ions on the Conformation of Encephalin Investigated by Hydrogen/Deuterium Exchange and Theoretical Calculations

We have studied the effects of different 3d orbitals in divalent transition-metal ions [G(2+) = Mn2+ (d(5)), Fe2+ (d(6)), Co2+ (d(7)), Ni2+ (d(8)), Cu2+ (d(9)), or Zn2+ (d(10))] on the conformations of leucine encephalin (LE) and methionine encephalin (ME) in the gas phase using hydrogen/deuterium exchange mass spectrometry (HDX-MS) and theoretical calculations at the molecular level. The HDX-MS reveals a 1:1 stoichiometric monovalent complex of [LE/ME + G - H](+) and observed that the different HDX reactivities follow the trend Fe2+ < Co2+ < Ni2+ < Mn2+ < Cu2+ approximate to Zn2+ and that [ME + Mn/Cu/Zn - H](+) > [LE + Mn/Cu/Zn - H](+), while [LE + Fe/Co/Ni - H](+) > [ME + Fe/Co/Ni - H](+). We cross-correlated the collision-induced dissociation energies of the complexes with the HDX results and found that the more stable the complex, the harder it is for it to undergo HDX. Furthermore, we used theoretical calculations to optimize the favorable conformations of the complexes and found the same interaction structure of G(2+) coordination with the five carbonyl oxygens of LE/ME that have different bond lengths. Finally, we calculated the proton affinity (PA) values of the optimized complexes in order to interpret the HDX observations that the higher the PA values, the more difficult it is for the complex to undergo HDX. Overall, both the experiments and the theoretical calculations show that the six metal ions have different effects on the LE/ME conformation, with the low-energy stability of the G(2+) 3d orbitals corresponding to more dramatic effects on the LE/ME conformation. In addition, the hardness of the ionic acid corresponding to the fully filled Mn2+ and half-filled Zn2+ orbitals also contributes strongly to the coordination effect; the conformation effect of Fe2+/Co2+/Ni2+ on LE is greater than that on ME, whereas the conformation effect of Mn2+/Cu2+/Zn2+ on ME is greater than that on LE.