posted on 2021-02-24, 22:03authored byAurélien Le Fèvre, Philippe Dugourd, Fabien Chirot
The
dynamics and thermodynamics of structural changes in isolated
glu-fibrinopeptide B (GluFib) were investigated by tandem ion mobility
spectrometry (IMS). Doubly protonated GluFib2+ ions were
first selected by IMS and then stored for a controlled duration in
a thermalized ion trap. Temperature-induced conformational changes
were finally monitored by IMS as a function of trapping time. Based
on this procedure, isomerization rates and equilibrium populations
of the different conformers were determined as a function of temperature.
We demonstrate that the measured thermodynamic quantities can be directly
compared to simulated observables from ensemble molecular modeling
based on appropriate order parameters. We obtained good qualitative
agreement with replica-exchange molecular dynamics simulations based
on the AMOEBA force field and processed using the weighted histogram
analysis method. This suggests that the balance between Coulomb repulsion
and optimal charge solvation is the main source of the observed conformational
bistability. Our results emphasize the differences between the kinetically
driven quasi-equilibrium distributions obtained after collisional
activation and the thermodynamically driven distributions from the
present equilibrium experiments due to entropic effects. As a consequence,
our measurements not only allow straightforward determination of Arrhenius
activation energies but also yield the relative enthalpy and entropy
changes associated to a structural transition.