6 research outputs found
Structural Characterization of Nanoscale Meshworks within a Nucleoporin FG Hydrogel
The permeability barrier of nuclear pore complexes (NPCs)
controls
all exchange of macromolecules between the cytoplasm and the cell
nucleus. It consists of phenylalanine–glycine (FG) repeat domains
apparently organized as an FG hydrogel. It has previously been demonstrated
that an FG hydrogel derived from the yeast nucleoporin Nsp1p reproduces
the selectivity of authentic NPCs. Here we combined time-resolved
optical spectroscopy and X-ray scattering techniques to characterize
such a gel. The data suggest a hierarchy of structures that form during
gelation at the expense of unstructured elements. On the largest scale,
protein-rich domains with a correlation length of ∼16.5 nm
are evident. On a smaller length scale, aqueous channels with an average
diameter of ∼3 nm have been found, which possibly represent
the physical structures accounting for the passive sieving effect
of nuclear pores. The protein-rich domains contain characteristic
β-structures with typical inter-β-strand and inter-β-sheet
distances of 1.3 and 0.47 nm, respectively. During gelation, the formation
of oligomeric associates is accompanied by the transfer of phenylalanines
into a hydrophobic microenvironment, supporting the view that this
process is driven by a hydrophobic collapse
Ultrafast Dynamical Study of Pyrene-<i>N,N</i>-dimethylaniline (PyDMA) as an Organic Molecular Diode in Solid State
Femtosecond optical pump–probe
spectroscopy has been employed
for studying the directly linked electron donor–acceptor system
pyrene-<i>N,N</i>-dimethylaniline (PyDMA) in solid state.
This DMA-pyrene derivative discussed is being applied as a molecular
diode system switching on an ultrafast time scale. Our ultrafast solid-state
studies reveal a complex photochemistry of this molecular crystal
system. Strong couplings of the optically induced charge-transfer
state with the radical ion pair state allow a femtosecond transition
of the latter. One could see on the highest occupied molecular orbital–lowest
unoccupied molecular orbital (HOMO–LUMO) description that a
pure optical transition switches the system from a conducting to a
blocked system because the molecular orbitals (MOs) of DMA moiety
lie in a node plane of the LUMO. Within 800 fs the system relaxes
back to the ground state and/or forms a radical ion pair, which is
the surprising result of our study; when the system was probed further,
the system underwent vibrational cooling and enhanced population inversion
of the radical ion pair
Evidence for Point Transformations in Photoactive Molecular Crystals by the Photoinduced Creation of Diffuse Diffraction Patterns
Time-resolved diffuse X-ray scattering is one powerful
method for
monitoring the progression from the creation of local structural changes
inside a crystalline material up to the transformation of the whole
crystalline bulk. In this work, we study the mechanism of phototransformation
of a molecular crystal by time-resolved diffuse X-ray scattering.
Here, an optical excitation source, like a pulsed laser, initiates
structural transformations which are monitored by X-ray scattering
techniques. We have studied the dimerization process of the molecular
switch α-styrylpyrylium (trifluoromethanesulfonate) TFMS, in
particular for understanding whether cooperative effects influence
the changes of the structure in the bulk and its periodicity. Upon
illumination with optical light, α-styrylpyrylium TFMS instantaneously
photoswitches. Depending on the optical fluence, X-ray diffuse planes
are observed prior to phototransformation of the bulk. In the early
stages of transformation, the analysis reveals systems of randomly
distributed islands of product clusters with gradual growth in size
and amount. The degree of transformation follows the optical excitation
profile, i.e., the spatial absorption of the laser beam. In the present
studies, no influence of cooperativity on the photodimerization process
has been found
Dissecting Local Atomic and Intermolecular Interactions of Transition-Metal Ions in Solution with Selective X‑ray Spectroscopy
Determining covalent and charge-transfer contributions
to bonding in solution has remained an experimental challenge. Here,
the quenching of fluorescence decay channels as expressed in
dips in the L-edge X-ray spectra of solvated 3d transition-metal ions
and complexes was reported as a probe. With a full set of experimental
and theoretical ab initio L-edge X-ray spectra of aqueous Cr<sup>3+</sup>, including resonant inelastic X-ray scattering, we address covalency
and charge transfer for this prototypical transition-metal ion in
solution. We dissect local atomic effects from intermolecular interactions
and quantify X-ray optical effects. We find no evidence for the asserted
ultrafast charge transfer to the solvent and show that the dips are
readily explained by X-ray optical effects and local atomic state
dependence of the fluorescence yield. Instead, we find, besides ionic
interactions, a covalent contribution to the bonding in the aqueous
complex of ligand-to-metal charge-transfer character
From Ligand Fields to Molecular Orbitals: Probing the Local Valence Electronic Structure of Ni<sup>2+</sup> in Aqueous Solution with Resonant Inelastic X‑ray Scattering
Bonding of the Ni<sup>2+</sup>(aq)
complex is investigated with
an unprecedented combination of resonant inelastic X-ray scattering
(RIXS) measurements and ab initio calculations at the Ni L absorption
edge. The spectra directly reflect the relative energies of the ligand-field
and charge-transfer valence-excited states. They give element-specific
access with atomic resolution to the ground-state electronic structure
of the complex and allow quantification of ligand-field strength and
3d–3d electron correlation interactions in the Ni<sup>2+</sup>(aq) complex. The experimentally determined ligand-field strength
is 10<i>Dq</i> = 1.1 eV. This and the Racah parameters characterizing
3d–3d Coulomb interactions <i>B</i> = 0.13 eV and <i>C</i> = 0.42 eV as readily derived from the measured energies
match very well with the results from UV–vis spectroscopy.
Our results demonstrate how L-edge RIXS can be used to complement
existing spectroscopic tools for the investigation of bonding in 3d
transition-metal coordination compounds in solution. The ab initio
RASPT2 calculation is successfully used to simulate the L-edge RIXS
spectra
L‑Edge X‑ray Absorption Spectroscopy of Dilute Systems Relevant to Metalloproteins Using an X‑ray Free-Electron Laser
L-edge spectroscopy of 3d transition
metals provides important
electronic structure information and has been used in many fields.
However, the use of this method for studying dilute aqueous systems,
such as metalloenzymes, has not been prevalent because of severe radiation
damage and the lack of suitable detection systems. Here we present
spectra from a dilute Mn aqueous solution using a high-transmission
zone-plate spectrometer at the Linac Coherent Light Source (LCLS).
The spectrometer has been optimized for discriminating the Mn L-edge
signal from the overwhelming O K-edge background that arises from
water and protein itself, and the ultrashort LCLS X-ray pulses can
outrun X-ray induced damage. We show that the deviations of the partial-fluorescence
yield-detected spectra from the true absorption can be well modeled
using the state-dependence of the fluorescence yield, and discuss
implications for the application of our concept to biological samples