Comprehensive
Analysis of Gly-Leu-Gly-Gly-Lys Peptide Dication Structures and Cation-Radical
Dissociations Following Electron Transfer: From Electron Attachment
to Backbone Cleavage, Ion–Molecule Complexes, and Fragment
Separation
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Abstract
Experimental data from ion mobility
measurements and electron transfer dissociation were combined with
extensive computational analysis of ion structures and dissociation
energetics for Gly-Leu-Gly-Gly-Lys cations and cation radicals. Experimental
and computational collision cross sections of (GLGGK + 2H)<sup>2+</sup> ions pointed to a dominant folding motif that is represented in
all low free-energy structures. The local folding motifs were preserved
in several fragment ions produced by electron transfer dissociation.
Gradient optimizations of (GLGGK + 2H)<sup>+•</sup> cation-radicals
revealed local energy minima corresponding to distonic zwitterionic
structures as well as aminoketyl radicals. Both of these structural
types can isomerize to low-energy tautomers that are protonated at
the radical-containing amide group forming a new type of intermediates,
−C<sup>•</sup>O<sup>–</sup>NH<sub>2</sub><sup>+</sup>– and −C<sup>•</sup>(OH)NH<sub>2</sub><sup>+</sup>–, respectively. Extensive mapping with B3LYP,
M06-2X, and MP2(frozen core) calculations of the potential energy
surface of the ground doublet electronic state of (GLGGK + 2H)<sup>+•</sup> provided transition-state and dissociation energies
for backbone cleavages of the N–C<sub>α</sub> and amide
C–N bonds leading to ion–molecule complexes. The complexes
can undergo facile prototropic migrations that are catalyzed by the
Lys ammonium group and isomerize enolimine <i><b>c</b></i>-type fragments to the more stable amide tautomers. In contrast,
interfragment hydrogen atom migrations in the complexes were found
to have relatively high transition energies and did not compete with
fragment separation. The extensive analysis of the intermediate and
transition-state energies led to the conclusion that the observed
dissociations cannot proceed competitively on the same potential energy
surface. The reactive intermediates for the dissociations originate
from distinct electronic states that are accessed by electron transfer