Kinetically Trapped Liquid-State Conformers of a Sodiated Model Peptide Observed in the Gas Phase

We
investigate the peptide AcPheAla5LysH+, a model
system for studying helix formation in the gas phase, in
order to fully understand the forces that stabilize the helical structure.
In particular, we address the question of whether the local fixation
of the positive charge at the peptide’s C-terminus is a prerequisite
for forming helices by replacing the protonated C-terminal Lys residue
by Ala and a sodium cation. The combination of gas-phase vibrational
spectroscopy of cryogenically cooled ions with molecular simulations
based on density-functional theory (DFT) allows for detailed structure
elucidation. For sodiated AcPheAla6, we find globular rather
than helical structures, as the mobile positive charge strongly interacts
with the peptide backbone and disrupts secondary structure formation.
Interestingly, the global minimum structure from simulation is not
present in the experiment. We interpret that this is due to high barriers
involved in rearranging the peptide–cation interaction that
ultimately result in kinetically trapped structures being observed
in the experiment.