Gas-phase analytical techniques provide very valuable tools for tackling the
structural complexity of macromolecular structures such as those encountered in
biological systems. Conformational dynamics of polypeptides and polypeptide
assemblies underlie most biological functionalities, yet great difficulties arise when
investigating such phenomena with the well-established techniques of X-ray
crystallography and NMR. In areas such as these ion mobility interfaced with mass
spectrometry (IMMS) and molecular modelling can make a significant contribution.
During an IMMS experiment analyte ions drift in a chamber filled with an inert gas;
measurement of the transport properties of analyte ions under the influence of a weak
electric field can lead to determination of the orientationally-averaged collision
cross-section of all resolved ionic species. A comparison with cross-sections
estimated for model molecular geometries can lead to structural assignments. Thus
IMMS can be used effectively to separate gas-phase ions based on their
conformation. The drift tube employed in the experiments described herein is
thermally regulated, which also enables the determination of collision cross-sections
over a range of temperatures, and can provide a view of temperature-dependent
conformational dynamics over the experimental (low microsecond) timescale.
Studies described herein employ IMMS and a gamut of other MS-based techniques,
solution spectroscopy and – importantly – molecular mechanics simulations to assess
a) conformational stability of isolated peptide ions, with a focus on small model
peptides and proteins, especially the Trp cage miniprotein; and b) structural
characteristics of oligomeric aggregates of an amyloidogenic peptide.
The results obtained serve to clarify the factors which dominate the intrinsic stability
of non-covalent structure in isolated peptides and peptide assemblies. Strong
electrostatic interactions are found to play a pivotal role in determining the
conformations of isolated proteins. Secondary structures held together by hydrogen
bonding, such as helices, are stable in the absence of solvent, however gas-phase
protein structures display loss of their hydrophobic cores. The absence of a polar
solvent, “self-solvation” is by far the most potent force influencing the gas-phase configuration of these systems. Geometries that are more compact than the folded
state observed in solution are routinely detected, indicating the existence of
intrinsically stable compact non-native states in globular proteins, illuminating the
nature of proteins’ ‘unfolded’ states
