Carboxyl-Catalyzed Prototropic Rearrangements in Histidine Peptide Radicals upon Electron Transfer: Effects of Peptide Sequence and Conformation

Abstract

We report an unusual prototropic rearrangement in gas-phase radicals formed by collisional electron transfer from cesium atoms to protonated peptides HAL, AHL, and ALH at 50 keV. The rearrangement depends on the peptide amino acid sequence and presence or steric accessibility of a free carboxyl group. Upon electron transfer, protonated HAL and ALH rearrange to tautomers that are detected as nondissociated anions in charge-reversal mass spectra. The isomerization is minor in protonated ALH and virtually absent in HAL amide. Electron structure calculations indicate that the gas-phase ions are preferentially protonated in the His imidazole ring and consist of multiple conformers that differ in their hydrogen bonding patterns. Electron transfer to protonated HAL and AHL triggers an exothermic and dynamically barrierless transfer of the carboxyl proton onto the C-2' position of the His ring that occurs on a 120-240 ns time scale. The kinetics of this isomerization are controlled by internal rotations in the radicals to assume conformations favoring the proton transfer. The radical conformations also affect subsequent proton migrations in zwitterionic His imidazoline intermediates that reform the COOH group and result in His ring isomerization. This autocatalytic prototropic rearrangement in gas-phase peptide radicals is analogous to enzyme catalytic reactions involving His and acidic amino acid residues. In contrast to HAL and AHL, the C-2' position is sterically inaccessible in ALH radicals. These radicals undergo proton migrations to the His ring C-5' positions that have moderate energy barriers and are less efficient. RRKM calculations on the combined B3LYP and PMP2/6-311++G(2d,p) potential energy surface of the ground doublet electronic state of the peptide radicals provided rate constants that were quantitatively consistent with the dissociations observed in the gas phase. The formation of minor sequence z(1) and z(2) fragments from AHL was interpreted as occurring in the first excited state of the radical.