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Characterizing Intermediates Along the Transition from Polyproline I to Polyproline II Using Ion Mobility Spectrometry-Mass Spectrometry
journal contribution
posted on 2014-09-10, 00:00 authored by Liuqing Shi, Alison
E. Holliday, Huilin Shi, Feifei Zhu, Michael
A. Ewing, David H. Russell, David E. ClemmerPolyproline
exists predominately as the all-cis polyproline I
(PPI) helix in aliphatic alcohols, whereas the all-trans polyproline II (PPII) helix is favored in aqueous
solutions. Previous ion mobility spectrometry-mass spectrometry (IMS-MS)
work demonstrates that the gas-phase conformations of polyproline
ions can be related to the corresponding PPI and PPII helices in solution
[J. Phys. Chem. B 2004, 108, 4885].
Here, we use IMS-MS to examine the detailed intermediate steps associated
with the process of Polyproline-13 (Pro13) conversion from the PPI
helix to the PPII helix upon solvent exchange. Collision cross section
distributions of Pro13 [M + 2H]2+ ions obtained at different
transition times indicate the presence of two major conformers, identified
as the PPI and PPII helices, and six conformers that appear as subpopulations
of polyproline. Further analysis shows a transition mechanism with
sequential cis–trans isomerizations followed
by a parallel process to establish PPII and two smaller subpopulations
at equilibrium. Temperature-dependent studies are used to obtain Arrhenius
activation parameters for each step of the mechanism, and molecular
dynamics simulations provide insight about the structures of the intermediates.
It appears that prolines sequentially flip from cis to trans starting from the N-terminus.
However, after the first few transitions, possible steps take place
at the center of the peptide chain; subsequently, several pathways
appear to be accessible at the same time. Our results reflect the
existence of stable subpopulations in polyprolines and provide new
insight into the structural changes during the transition process
of polyproline peptides converting from PPI to PPII in aqueous solution.