3 research outputs found
Charge Effect on the Formation of Polyoxometalate-Based Supramolecular Polygons Driven by Metal Coordination
The metal-driven self-assembly of
a Keggin-based hybrid bearing two remote pyridine units was investigated.
The resulting supramolecular species were identified by combination
of 2D diffusion NMR spectroscopy (DOSY) and electrospray ionization
mass spectrometry (ESI-MS) as a mixture of molecular triangles and
squares. This behavior is different from that of the structural analogue
Dawson-based hybrid displaying a higher charge, which only led to
the formation of molecular triangles. This study highlights the decisive
effect of the charge of the POMs in their self-assembly processes
that disfavors the formation of large assemblies. An isothermal titration
calorimetry (ITC) experiment confirmed the stronger binding in the
case of the Keggin hybrids. A correlation between the diffusion coefficient <i>D</i> and the molecular mass <i>M</i> of the POM-based
building block and its coordination oligomers was also observed. We
show that the diffusion coefficient of these compounds is mainly determined
by their occupied volume rather than by their shape
IRMPD Spectroscopy: Evidence of Hydrogen Bonding in the Gas Phase Conformations of Lasso Peptides and their Branched-Cyclic Topoisomers
Lasso peptides are natural products
characterized by a mechanically
interlocked topology. The conformation of lasso peptides has been
probed in the gas phase using ion mobilityâmass spectrometry
(IMâMS) which showed differences in the lasso and their unthreaded
branched-cyclic topoisomers depending on the ion charge states. To
further characterize the evolution of gas phase conformations as a
function of the charge state and to assess associated changes in the
hydrogen bond network, infrared multiple photon dissociation (IRMPD)
action spectroscopy was carried out on two representative lasso peptides,
microcin J25 (MccJ25) and capistruin, and their branched-cyclic topoisomers.
For the branched-cyclic topoisomers, spectroscopic evidence of a disruption
of neutral hydrogen bonds were found when comparing the 3+ and 4+
charge states. In contrast, for the lasso peptides, the IRMPD spectra
were found to be similar for the two charge states, suggesting very
little difference in gas phase conformations upon addition of a proton.
The IRMPD data were thus found consistent and complementary to IMâMS,
confirming the stable and compact structure of lasso peptides in the
gas phase
Ion MobilityâMass Spectrometry of Lasso Peptides: Signature of a Rotaxane Topology
Ion mobility mass spectrometry data
were collected on a set of
five class II lasso peptides and their branched-cyclic topoisomers
prepared in denaturing solvent conditions with and without sulfolane
as a supercharging agent. Sulfolane was shown not to affect ion mobility
results and to allow the formation of highly charged multiply protonated
molecules. Drift time values of low charged multiply protonated molecules
were found to be similar for the two peptide topologies, indicating
the branched-cyclic peptide to be folded in the gas phase into a conformation
as compact as the lasso peptide. Conversely, high charge states enabled
a discrimination between lasso and branched-cyclic topoisomers, as
the former remained compact in the gas phase while the branched-cyclic
topoisomer unfolded. Comparison of the ion mobility mass spectrometry
data of the lasso and branched-cyclic peptides for all charge states,
including the higher charge states obtained with sulfolane, yielded
three trends that allowed differentiation of the lasso form from the
branched-cyclic topology: low intensity of highly charged protonated
molecules, even with the supercharging agent, low change in collision
cross sections with increasing charge state of all multiply protonated
molecules, and narrow ion mobility peak widths associated with the
coexistence of fewer conformations and possible conformational changes