46 research outputs found
Hybrid Approaches to Structural Characterization of Conformational Ensembles of Complex Macromolecular Systems Combining NMR Residual Dipolar Couplings and Solution Xâray Scattering
Solving structures or structural ensembles of large macromolecular systems in solution poses a challenging problem. While NMR provides structural information at atomic resolution, increased spectral complexity, chemical shift overlap, and short transverse relaxation times (associated with slow tumbling) render application of the usual techniques that have been so successful for medium sized systems (\u3c50 \u3ekDa) difficult. Solution X-ray scattering, on the other hand, is not limited by molecular weight but only provides low resolution structural information related to the overall shape and size of the system under investigation. Here we review how combining atomic resolution structures of smaller domains with sparse experimental data afforded by NMR residual dipolar couplings (which yield both orientational and shape information) and solution X-ray scattering data in rigid-body simulated annealing calculations provides a powerful approach for investigating the structural aspects of conformational dynamics in large multidomain proteins. The application of this hybrid methodology is illustrated for the 128 kDa dimer of bacterial Enzyme I which exists in a variety of open and closed states that are sampled at various points in the catalytic cycles, and for the capsid protein of the human immunodeficiency virus
Mechanistic Insight into Nanoparticle Surface Adsorption by Solution NMR Spectroscopy in an Aqueous Gel
Engineering nanoparticle (NP) functions at the molecular level requires a detailed understanding of the dynamic processes occurring at the NP surface. Herein we show that a combination of darkâstate exchange saturation transfer (DEST) and relaxation dispersion (RD) NMR experiments on gelâstabilized NP samples enables the accurate determination of the kinetics and thermodynamics of adsorption. We used the former approach to describe the interaction of cholic acid (CA) and phenol (PhOH) with ceria NPs with a diameter of approximately 200â
nm. Whereas CA formed weak interactions with the NPs, PhOH was tightly bound to the NP surface. Interestingly, we found that the adsorption of PhOH proceeds via an intermediate, weakly bound state in which the small molecule has residual degrees of rotational diffusion. We believe the use of aqueous gels for stabilizing NP samples will increase the applicability of solution NMR methods to the characterization of nanomaterials
âSurface Contrastâ NMR reveals nonâinnocent role of support in Pd/CeO2 catalyzed phenol hydrogenation
Ceria (CeO 2 )âsupported metals are widely used as catalysts because of their exceptional redox properties. Here, we use surface contrast NMR methods to investigate the hydrogenation of phenol by Pd supported on ceria nanoparticles. We show that the rigid and planar binding of phenol to Pd is mediated by a weak and highly mobile association of the small molecule to ceria. Interestingly, while addition of phosphate to the mixture does not perturb the adsorption of phenol on Pd, it destabilizes its interaction with ceria and proportionally decreases the rate of catalytic conversion. Our data provide strong experimental evidence that weak interactions between adsorbate and ceria are catalytically competent, and explain the exceptional performance of Pd/CeO 2 for reductive conversions under mild reaction conditions
The Structure of Pre-transitional Protoplanetary Disks I: Radiative Transfer Modeling of the Disk+Cavity in the PDS 70 system
Through detailed radiative transfer modeling, we present a disk+cavity model
to simultaneously explain both the SED and Subaru H-band polarized light
imaging for the pre-transitional protoplanetary disk PDS 70. Particularly, we
are able to match not only the radial dependence, but also the absolute scale,
of the surface brightness of the scattered light. Our disk model has a cavity
65 AU in radius, which is heavily depleted of sub-micron-sized dust grains, and
a small residual inner disk which produces a weak but still optically thick NIR
excess in the SED. To explain the contrast of the cavity edge in the Subaru
image, a factor of ~1000 depletion for the sub-micron-sized dust inside the
cavity is required. The total dust mass of the disk may be on the order of 1e-4
M_sun, only weakly constrained due to the lack of long wavelength observations
and the uncertainties in the dust model. The scale height of the
sub-micron-sized dust is ~6 AU at the cavity edge, and the cavity wall is
optically thick in the vertical direction at H-band. PDS 70 is not a member of
the class of (pre-)transitional disks identified by Dong et al. (2012), whose
members only show evidence of the cavity in the millimeter-sized dust but not
the sub-micron-sized dust in resolved images. The two classes of
(pre-)transitional disks may form through different mechanisms, or they may
just be at different evolution stages in the disk clearing process.Comment: 28 pages (single column), 7 figures, 1 table, ApJ accepte
New Techniques for High-Contrast Imaging with ADI: the ACORNS-ADI SEEDS Data Reduction Pipeline
We describe Algorithms for Calibration, Optimized Registration, and Nulling
the Star in Angular Differential Imaging (ACORNS-ADI), a new, parallelized
software package to reduce high-contrast imaging data, and its application to
data from the SEEDS survey. We implement several new algorithms, including a
method to register saturated images, a trimmed mean for combining an image
sequence that reduces noise by up to ~20%, and a robust and computationally
fast method to compute the sensitivity of a high-contrast observation
everywhere on the field-of-view without introducing artificial sources. We also
include a description of image processing steps to remove electronic artifacts
specific to Hawaii2-RG detectors like the one used for SEEDS, and a detailed
analysis of the Locally Optimized Combination of Images (LOCI) algorithm
commonly used to reduce high-contrast imaging data. ACORNS-ADI is written in
python. It is efficient and open-source, and includes several optional features
which may improve performance on data from other instruments. ACORNS-ADI
requires minimal modification to reduce data from instruments other than
HiCIAO. It is freely available for download at
www.github.com/t-brandt/acorns-adi under a BSD license.Comment: 15 pages, 9 figures, accepted to ApJ. Replaced with accepted version;
mostly minor changes. Software update
Constraining the Movement of the Spiral Features and the Locations of Planetary Bodies within the AB Aur System
We present new analysis of multi-epoch, H-band, scattered light images of the
AB Aur system. We used a Monte Carlo, radiative transfer code to simultaneously
model the system's SED and H-band polarized intensity imagery. We find that a
disk-dominated model, as opposed to one that is envelope dominated, can
plausibly reproduce AB Aur's SED and near-IR imagery. This is consistent with
previous modeling attempts presented in the literature and supports the idea
that at least a subset of AB Aur's spirals originate within the disk. In light
of this, we also analyzed the movement of spiral structures in multi-epoch
H-band total light and polarized intensity imagery of the disk. We detect no
significant rotation or change in spatial location of the spiral structures in
these data, which span a 5.8 year baseline. If such structures are caused by
disk-planet interactions, the lack of observed rotation constrains the location
of the orbit of planetary perturbers to be >47 AU.Comment: 8 pages, 3 figures, 1 table, Accepted to Ap