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The new voices of science fiction
\"Your Future Is Bright! After all, your mother is a robot, your father has joined the alien hive mind, and your dinner will be counterfeit 3D-printed steak. Even though your worker bots have staged a mutiny, and your tour guide speaks only in memes, you can always sell your native language if you need some extra cash.\" -- From publisher's description.
Book
Attosecond band-gap dynamics in silicon
Electron transfer from valence to conduction band states in semiconductors is the basis of modern electronics. Here, attosecond extreme ultraviolet (XUV) spectroscopy is used to resolve this process in silicon in real time. Electrons injected into the conduction band by few-cycle laser pulses alter the silicon XUV absorption spectrum in sharp steps synchronized with the laser electric field oscillations. The observed ∼450-attosecond step rise time provides an upper limit for the carrier-induced band-gap reduction and the electron-electron scattering time in the conduction band. This electronic response is separated from the subsequent band-gap modifications due to lattice motion, which occurs on a time scale of 60 ± 10 femtoseconds, characteristic of the fastest optical phonon. Quantum dynamical simulations interpret the carrier injection step as light-field–induced electron tunneling.
Journal Article
Direct and simultaneous observation of ultrafast electron and hole dynamics in germanium
Understanding excited carrier dynamics in semiconductors is crucial for the development of photovoltaics and efficient photonic devices. However, overlapping spectral features in optical pump-probe spectroscopy often render assignments of separate electron and hole carrier dynamics ambiguous. Here, ultrafast electron and hole dynamics in germanium nanocrystalline thin films are directly and simultaneously observed by ultrafast transient absorption spectroscopy in the extreme ultraviolet at the germanium M
4,5
edge. We decompose the spectra into contributions of electronic state blocking and photo-induced band shifts at a carrier density of 8 × 10
20
cm
−3
. Separate electron and hole relaxation times are observed as a function of hot carrier energies. A first-order electron and hole decay of ∼1 ps suggests a Shockley–Read–Hall recombination mechanism. The simultaneous observation of electrons and holes with extreme ultraviolet transient absorption spectroscopy paves the way for investigating few- to sub-femtosecond dynamics of both holes and electrons in complex semiconductor materials and across junctions.
Understanding excited carrier dynamics in semiconductors is central to the continued development of optoelectronic devices. Using extreme ultraviolet transient absorption spectroscopy, Zürch
et al
. directly and simultaneously observe ultrafast electron and hole dynamics in germanium thin films.
Journal Article
The complete assembly of human LAT1-4F2hc complex provides insights into its regulation, function and localisation
The LAT1-4F2hc complex (SLC7A5-SLC3A2) facilitates uptake of essential amino acids, hormones and drugs. Its dysfunction is associated with many cancers and immune/neurological disorders. Here, we apply native mass spectrometry (MS)-based approaches to provide evidence of super-dimer formation (LAT1-4F2hc)
2
. When combined with lipidomics, and site-directed mutagenesis, we discover four endogenous phosphatidylethanolamine (PE) molecules at the interface and C-terminus of both LAT1 subunits. We find that interfacial PE binding is regulated by 4F2hc-R183 and is critical for regulation of palmitoylation on neighbouring LAT1-C187. Combining native MS with mass photometry (MP), we reveal that super-dimerization is sensitive to pH, and modulated by complex N-glycans on the 4F2hc subunit. We further validate the dynamic assemblies of LAT1-4F2hc on plasma membrane and in the lysosome. Together our results link PTM and lipid binding with regulation and localisation of the LAT1-4F2hc super-dimer.
The amino acid transporter complex LAT1-4F2hc is considered a major drug target for many cancers. Here, the authors apply native mass spectrometry-based approaches to decode a complete LAT1-4F2hc assembly. To do this, they connect post-translational modification and endogenous phospholipid binding to super-dimerization, function and localisation of LAT1-4F2hc.
Journal Article
Protein shape sampled by ion mobility mass spectrometry consistently improves protein structure prediction
Ion mobility (IM) mass spectrometry provides structural information about protein shape and size in the form of an orientationally-averaged collision cross-section (CCS
IM
). While IM data have been used with various computational methods, they have not yet been utilized to predict monomeric protein structure from sequence. Here, we show that IM data can significantly improve protein structure determination using the modelling suite Rosetta. We develop the Rosetta Projection Approximation using Rough Circular Shapes (PARCS) algorithm that allows for fast and accurate prediction of CCS
IM
from structure. Following successful testing of the PARCS algorithm, we use an integrative modelling approach to utilize IM data for protein structure prediction. Additionally, we propose a confidence metric that identifies near native models in the absence of a known structure. The results of this study demonstrate the ability of IM data to consistently improve protein structure prediction.
Collision cross sections (CCS) from ion mobility mass spectrometry provide information about protein shape and size. Here, the authors develop an algorithm to predict CCS and integrate experimental ion mobility data into Rosetta-based molecular modelling to predict protein structures from sequence.
Journal Article
Multiple Evolutionary Origins of Ubiquitous Cu2+ and Zn2+ Binding in the S100 Protein Family
The S100 proteins are a large family of signaling proteins that play critical roles in biology and disease. Many S100 proteins bind Zn2+, Cu2+, and/or Mn2+ as part of their biological functions; however, the evolutionary origins of binding remain obscure. One key question is whether divalent transition metal binding is ancestral, or instead arose independently on multiple lineages. To tackle this question, we combined phylogenetics with biophysical characterization of modern S100 proteins. We demonstrate an earlier origin for established S100 subfamilies than previously believed, and reveal that transition metal binding is widely distributed across the tree. Using isothermal titration calorimetry, we found that Cu2+ and Zn2+ binding are common features of the family: the full breadth of human S100 paralogs-as well as two early-branching S100 proteins found in the tunicate Oikopleura dioica-bind these metals with μM affinity and stoichiometries ranging from 1:1 to 3:1 (metal:protein). While binding is consistent across the tree, structural responses to binding are quite variable. Further, mutational analysis and structural modeling revealed that transition metal binding occurs at different sites in different S100 proteins. This is consistent with multiple origins of transition metal binding over the evolution of this protein family. Our work reveals an evolutionary pattern in which the overall phenotype of binding is a constant feature of S100 proteins, even while the site and mechanism of binding is evolutionarily labile.
Journal Article
Monte Carlo study of particle identification at the CEPC using TPC dE / dx information
The kaon identification is crucial for the flavor physics, and also benefits the flavor and charge reconstruction of the jets. We explore the particle identification capability for tracks with momenta ranging from 2–20 GeV/c using the dE / dx measurements in the Time Projection Chamber at the future Circular Electron-Positron Collider. Based on Monte Carlo simulation, we anticipate that an average 3.2σ (2.6σ) K/π separation can be achieved based on dE / dx information for an optimistic (conservative) extrapolation of the simulated performance to the final system. Time-of-flight (TOF) information from the Electromagnetic Calorimeter can provide K/π separation around 1 GeV/c and reduce the K / p mis-identification rate. By combining the dE / dx and TOF information, we estimate that in the optimistic scenario a kaon selection in inclusive hadronic Z decays with both the average efficiency and purity approaching 95% can be achieved.
Journal Article
Search for lepton-flavor-violating tau decays to ℓα at Belle
A
bstract
We report a search for the lepton-flavor-violating decays
τ
±
→
ℓ
±
α
(
ℓ
=
e
,
μ
), where
α
is an undetected spin-0 particle, such as an axion-like particle using 736 × 10
6
tau lepton pairs collected by the Belle detector at the KEKB asymmetric-energy
e
+
e
−
collider. We find no evidence of signal and obtain the most stringent upper limits on the branching fractions at 95% confidence level:
(
τ
±
→ e
±
α
) < (0.4–6.4)
×
10
−
4
and
(
τ
±
→ μ
±
α
) < (0.2–3.5)
×
10
−
4
at 95% confidence level for an
α
mass in the range 0.0
≤ m
α
≤
1.6 GeV/
c
2
.
Journal Article
Linker Length Drives Heterogeneity of Multivalent Complexes of Hub Protein LC8 and Transcription Factor ASCIZ
LC8, a ubiquitous and highly conserved hub protein, binds over 100 proteins involved in numerous cellular functions, including cell death, signaling, tumor suppression, and viral infection. LC8 binds intrinsically disordered proteins (IDPs), and although several of these contain multiple LC8 binding motifs, the effects of multivalency on complex formation are unclear. Drosophila ASCIZ has seven motifs that vary in sequence and inter-motif linker lengths, especially within subdomain QT2–4 containing the second, third, and fourth LC8 motifs. Using isothermal-titration calorimetry, analytical-ultracentrifugation, and native mass-spectrometry of QT2–4 variants, with methodically deactivated motifs, we show that inter-motif spacing and specific motif sequences combine to control binding affinity and compositional heterogeneity of multivalent duplexes. A short linker separating strong and weak motifs results in stable duplexes but forms off-register structures at high LC8 concentrations. Contrastingly, long linkers engender lower cooperativity and heterogeneous complexation at low LC8 concentrations. Accordingly, two-mers, rather than the expected three-mers, dominate negative-stain electron-microscopy images of QT2–4. Comparing variants containing weak-strong and strong-strong motif combinations demonstrates sequence also regulates IDP/LC8 assembly. The observed trends persist for trivalent ASCIZ subdomains: QT2–4, with long and short linkers, forms heterogeneous complexes, whereas QT4–6, with similar mid-length linkers, forms homogeneous complexes. Implications of linker length variations for function are discussed.
Journal Article
Ultrafast carrier thermalization and trapping in silicon-germanium alloy probed by extreme ultraviolet transient absorption spectroscopy
Semiconductor
alloys containing silicon and germanium are of growing importance for compact and
highly efficient photonic devices due to their favorable properties for direct integration
into silicon
platforms and wide tunability of optical parameters. Here, we report the simultaneous
direct and energy-resolved probing of ultrafast electron and hole dynamics in a
silicon-germanium alloy with the stoichiometry Si0.25Ge0.75 by
extreme ultraviolet transient absorption spectroscopy. Probing the photoinduced dynamics of charge
carriers at the germanium M4,5-edge (∼30 eV) allows the germanium atoms to be
used as reporter atoms for carrier dynamics in the alloy. The photoexcitation of electrons
across the direct and indirect band gap into conduction band (CB) valleys and their subsequent hot carrier
relaxation are observed and compared to pure germanium, where the Ge direct
(
Δ
E
gap
,
Ge
,
direct
=
0.8
eV
)
and Si0.25Ge0.75
indirect gaps (
Δ
E
gap
,
Si
0.25
Ge
0.75
,
indirect
=
0.95
eV
) are comparable in energy. In the alloy,
comparable carrier lifetimes are observed for the X, L, and Γ valleys in the
conduction band.
A midgap feature associated with electrons accumulating in trap states near the CB edge
following intraband thermalization is observed in the Si0.25Ge0.75 alloy.
The successful implementation of the reporter atom concept for capturing the dynamics of
the electronic bands by site-specific probing in solids opens a route to study carrier
dynamics in more complex materials with femtosecond and sub-femtosecond temporal
resolution.
Journal Article