136 research outputs found
DiffHopp: A Graph Diffusion Model for Novel Drug Design via Scaffold Hopping
Scaffold hopping is a drug discovery strategy to generate new chemical
entities by modifying the core structure, the \emph{scaffold}, of a known
active compound. This approach preserves the essential molecular features of
the original scaffold while introducing novel chemical elements or structural
features to enhance potency, selectivity, or bioavailability. However, there is
currently a lack of generative models specifically tailored for this task,
especially in the pocket-conditioned context. In this work, we present
DiffHopp, a conditional E(3)-equivariant graph diffusion model tailored for
scaffold hopping given a known protein-ligand complex
On the Expressive Power of Geometric Graph Neural Networks
The expressive power of Graph Neural Networks (GNNs) has been studied
extensively through the Weisfeiler-Leman (WL) graph isomorphism test. However,
standard GNNs and the WL framework are inapplicable for geometric graphs
embedded in Euclidean space, such as biomolecules, materials, and other
physical systems. In this work, we propose a geometric version of the WL test
(GWL) for discriminating geometric graphs while respecting the underlying
physical symmetries: permutations, rotation, reflection, and translation. We
use GWL to characterise the expressive power of geometric GNNs that are
invariant or equivariant to physical symmetries in terms of distinguishing
geometric graphs. GWL unpacks how key design choices influence geometric GNN
expressivity: (1) Invariant layers have limited expressivity as they cannot
distinguish one-hop identical geometric graphs; (2) Equivariant layers
distinguish a larger class of graphs by propagating geometric information
beyond local neighbourhoods; (3) Higher order tensors and scalarisation enable
maximally powerful geometric GNNs; and (4) GWL's discrimination-based
perspective is equivalent to universal approximation. Synthetic experiments
supplementing our results are available at
https://github.com/chaitjo/geometric-gnn-dojoComment: NeurIPS 2022 Workshop on Symmetry and Geometry in Neural
Representation
Benchmarking Generated Poses: How Rational is Structure-based Drug Design with Generative Models?
Deep generative models for structure-based drug design (SBDD), where molecule
generation is conditioned on a 3D protein pocket, have received considerable
interest in recent years. These methods offer the promise of higher-quality
molecule generation by explicitly modelling the 3D interaction between a
potential drug and a protein receptor. However, previous work has primarily
focused on the quality of the generated molecules themselves, with limited
evaluation of the 3D molecule \emph{poses} that these methods produce, with
most work simply discarding the generated pose and only reporting a "corrected"
pose after redocking with traditional methods. Little is known about whether
generated molecules satisfy known physical constraints for binding and the
extent to which redocking alters the generated interactions. We introduce
PoseCheck, an extensive analysis of multiple state-of-the-art methods and find
that generated molecules have significantly more physical violations and fewer
key interactions compared to baselines, calling into question the implicit
assumption that providing rich 3D structure information improves molecule
complementarity. We make recommendations for future research tackling
identified failure modes and hope our benchmark can serve as a springboard for
future SBDD generative modelling work to have a real-world impact
Scheme for the implementation of a universal quantum cloning machine via cavity-assisted atomic collisions in cavity QED
We propose a scheme to implement the universal quantum cloning
machine of Buzek et.al [Phys. Rev.A 54, 1844(1996)] in the context of cavity
QED. The scheme requires cavity-assisted collision processes between atoms,
which cross through nonresonant cavity fields in the vacuum states. The cavity
fields are only virtually excited to face the decoherence problem. That's why
the requirements on the cavity quality factor can be loosened.Comment: to appear in PR
Oscillations and interactions of dark and dark-bright solitons in Bose-Einstein condensates
Solitons are among the most distinguishing fundamental excitations in a wide
range of non-linear systems such as water in narrow channels, high speed
optical communication, molecular biology and astrophysics. Stabilized by a
balance between spreading and focusing, solitons are wavepackets, which share
some exceptional generic features like form-stability and particle-like
properties. Ultra-cold quantum gases represent very pure and well-controlled
non-linear systems, therefore offering unique possibilities to study soliton
dynamics. Here we report on the first observation of long-lived dark and
dark-bright solitons with lifetimes of up to several seconds as well as their
dynamics in highly stable optically trapped Rb Bose-Einstein
condensates. In particular, our detailed studies of dark and dark-bright
soliton oscillations reveal the particle-like nature of these collective
excitations for the first time. In addition, we discuss the collision between
these two types of solitary excitations in Bose-Einstein condensates.Comment: 9 pages, 4 figure
Label-free segmentation of co-cultured cells on a nanotopographical gradient
The function and fate of cells is influenced by many different factors, one of which is surface topography of the support culture substrate. Systematic studies of nanotopography and cell response have typically been limited to single cell types and a small set of topographical variations. Here, we show a radical expansion of experimental throughput using automated detection, measurement, and classification of co-cultured cells on a nanopillar array where feature height changes continuously from planar to 250 nm over 9 mm. Individual cells are identified and characterized by more than 200 descriptors, which are used to construct a set of rules for label-free segmentation into individual cell types. Using this approach we can achieve label-free segmentation with 84% confidence across large image data sets and suggest optimized surface parameters for nanostructuring of implant devices such as vascular stents
Gas morphology and energetics at the surface of PDRs: new insights with Herschel observations of NGC 7023
We investigate the physics and chemistry of the gas and dust in dense
photon-dominated regions (PDRs), along with their dependence on the
illuminating UV field. Using Herschel-HIFI observations, we study the gas
energetics in NGC 7023 in relation to the morphology of this nebula. NGC 7023
is the prototype of a PDR illuminated by a B2V star and is one of the key
targets of Herschel. Our approach consists in determining the energetics of the
region by combining the information carried by the mid-IR spectrum (extinction
by classical grains, emission from very small dust particles) with that of the
main gas coolant lines. In this letter, we discuss more specifically the
intensity and line profile of the 158 micron (1901 GHz) [CII] line measured by
HIFI and provide information on the emitting gas. We show that both the [CII]
emission and the mid-IR emission from polycyclic aromatic hydrocarbons (PAHs)
arise from the regions located in the transition zone between atomic and
molecular gas. Using the Meudon PDR code and a simple transfer model, we find
good agreement between the calculated and observed [CII] intensities. HIFI
observations of NGC 7023 provide the opportunity to constrain the energetics at
the surface of PDRs. Future work will include analysis of the main coolant line
[OI] and use of a new PDR model that includes PAH-related species.Comment: Accepted for publication in Astronomy and Astrophysics Letters
(Herschel HIFI special issue), 5 pages, 5 figure
The Modelling of InfraRed Dark Clouds
This paper presents results from modelling 450 micron and 850 micron
continuum and HCO+ line observations of three distinct cores of an infrared
dark cloud (IRDC) directed toward the W51 GMC. In the sub-mm continuum these
cores appear as bright, isolated emission features. One of them coincides with
the peak of 8.3 micron extinction as measured by the Midcourse Space Experiment
satellite. Detailed radiative transfer codes are applied to constrain the
cores' physical conditions to address the key question: Do these IRDC-cores
harbour luminous sources? The results of the continuum model, expressed in the
quality-of-fit parameter, are also constrained by the absence of 100
micron emission from IRAS. For the sub-mm emission peaks this shows that
sources of 300 solar luminosities are embedded within the cores. For the
extinction peak, the combination of continuum and HCO+ line modelling indicates
that a heating source is present as well. Furthermore, the line model provides
constraints on the clumpiness of the medium. All three cores have similar
masses of about 70-150 solar masses and similar density structures. The
extinction peak differs from the other two cores by hosting a much weaker
heating source, and the sub-mm emission core at the edge of the IRDC deviates
from the other cores by a higher internal clumpiness.Comment: 13 pages, 13 figures, accepted for publication in A&
Disks around CQ Tau and MWC 758: dense PDR or gas dispersal?
The overall properties of disks surrounding intermediate PMS stars (HAe) are
not yet well constrained by current observations. The disk inclination, which
significantly affect SED modeling, is often unknown. We attempted to resolve
the disks around CQ Tau and MWC 758, to provide accurate constraints on the
disk parameters, in particular the temperature and surface density
distribution. We report arcsecond resolution observations of dust and CO line
emissions with the IRAM array. The disk properties are derived using a standard
disk model. We use the Meudon PDR code to study the chemistry. The two disks
share some common properties. The mean CO abundance is low despite disk
temperatures above the CO condensation temperature. Furthermore, the CO surface
density and dust opacity have different radial dependence. The CQ Tau disk
appears warmer, and perhaps less dense than that of MWC 758. Modeling the
chemistry, we find that photodissociation of CO is a viable mechanism to
explain the low abundance. The photospheric flux is not sufficient for this: a
strong UV excess is required. In CQ Tau, the high temperature is consistent
with expectation for a PDR. The PDR model has difficulty explaining the mild
temperatures obtained in MWC 758, for which a low gas-to-dust ratio is
preferred. A yet unexplored alternative could be that, despite currently high
gas temperatures, CO remains trapped in grains, as the models suggest that
large grains can be cold enough to prevent thermal desorption of CO. The low
inclination of the CQ Tau disk, ~30^\circ, challenges previous interpretations
given for the UX Ori - like luminosity variations of this star. We conclude
that CO cannot be used as a simple tracer of gas-to-dust ratio, the CO
abundance being affected by photodissociation, and grain growth.Comment: Accepted for publication in Astronomy & Astrophysic
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