232 research outputs found
Impact of Porphyrin Binding to GENOMES UNCOUPLED 4 on Tetrapyrrole Biosynthesis in planta
Plant tetrapyrrole biosynthesis (TPS) provides the indispensable chlorophyll (Chl) and heme molecules in photosynthetic organisms. Post-translational mechanisms control the enzymes to ensure a balanced flow of intermediates in the pathway and synthesis of appropriate amounts of both endproducts. One of the critical regulators of TPS is GENOMES UNCOUPLED 4 (GUN4). GUN4 interacts with magnesium chelatase (MgCh), and its binding of the catalytic substrate and product of the MgCh reaction stimulates the insertion of Mg2+ into protoporphyrin IX. Despite numerous in vitro studies, knowledge about the in vivo function of the GUN4:porphyrin interaction for the whole TPS pathway, particularly in plants, is still limited. To address this, we focused on two highly conserved amino acids crucial for porphyrin-binding to GUN4 and analyzed GUN4-F191A, R211A, and R211E substitution mutants in vitro and in vivo. Our analysis confirmed the importance of these amino acids for porphyrin-binding and the stimulation of plant MgCh by GUN4 in vitro. Expression of porphyrin-binding deficient F191A, R211A, and R211E in the Arabidopsis gun4-2 knockout mutant background revealed that, unlike in cyanobacteria and green algae, GUN4:porphyrin interactions did not affect the stability of GUN4 or other Arabidopsis TPS pathway enzymes in vivo. In addition, although they shared diminished porphyrin-binding and MgCh activation in vitro, expression of the different GUN4 mutants in gun4-2 had divergent effects on the TPS and the accumulation of Chl and Chl-binding proteins. For instance, expression of R211E, but not R211A, induced a substantial decrease of ALA synthesis rate, lower TPS intermediate and Chl level, and strongly impaired accumulation of photosynthetic complexes compared to wild-type plants. Furthermore, the presence of R211E led to significant growth retardation and paler leaves compared to GUN4 knockdown mutants, indicating that the exchange of R211 to glutamate compromised TPS and Chl accumulation more substantially than the almost complete lack of GUN4. Extensive in vivo analysis of GUN4 point mutants suggested that F191 and R211 might also play a role beyond porphyrin-binding.Peer Reviewe
A particle-field representation unifies paradigms in active matter
Active matter research focuses on the emergent behavior among interacting
self-propelled particles. Unification of seemingly disconnected paradigms --
active phase-separation of repulsive discs and collective motion of
self-propelled rods -- is a major challenge in contemporary active matter.
Inspired by the quanto-mechanical wave-particle duality, we develop an approach
based on the representation of active particles by smoothed continuum fields.
On the basis of the collision kinetics, we demonstrate analytically and
numerically how nonequilibrium stresses acting among self-driven, anisotropic
objects hinder the formation of phase-separated states as observed for
self-propelled discs and facilitate the emergence of orientational order.
Besides particle shape, the rigidity of self-propelled objects controlling the
symmetry of emergent ordered states is as a crucial parameter: impenetrable,
anisotropic rods are found to form polar, moving clusters, whereas large-scale
nematic structures emerge for soft rods, notably separated by a bistable
coexistence regime. These results indicate that the symmetry of the ordered
state is not dictated by the symmetry of the interaction potential but is
rather a dynamical, emergent property of active systems. This unifying
theoretical framework can represent a variety of active systems: living cell
tissues, bacterial colonies, cytoskeletal extracts as well as shaken granular
media.Comment: 14 pages, 8 figure
Semiclassical time--dependent propagation in three dimensions: How accurate is it for a Coulomb potential?
A unified semiclassical time propagator is used to calculate the
semiclassical time-correlation function in three cartesian dimensions for a
particle moving in an attractive Coulomb potential. It is demonstrated that
under these conditions the singularity of the potential does not cause any
difficulties and the Coulomb interaction can be treated as any other
non-singular potential. Moreover, by virtue of our three-dimensional
calculation, we can explain the discrepancies between previous semiclassical
and quantum results obtained for the one-dimensional radial Coulomb problem.Comment: 8 pages, 4 figures (EPS
Coulomb blockade effects in driven electron transport
We study numerically the influence of strong Coulomb repulsion on the current
through molecular wires that are driven by external electromagnetic fields. The
molecule is described by a tight-binding model whose first and last site is
coupled to a respective lead. The leads are eliminated within a perturbation
theory yielding a master equation for the wire. The decomposition into a
Floquet basis enables an efficient treatment of the driving field. For the
electronic excitations in bridged molecular wires, we find that strong Coulomb
repulsion significantly sharpens resonance peaks which broaden again with
increasing temperature. By contrast, Coulomb blockade has only a small
influence on effects like non-adiabatic electron pumping and coherent current
suppression.Comment: 9 pages, 7 figures. Added a plot for temperature dependence of
resonance peaks. Published versio
Spin correlations in spin blockade
We investigate spin currents and spin-current correlations for double quantum
dots in the spin blockade regime. By analysing the time evolution of the
density matrix, we obtain the spin resolved currents and derive from a
generating function an expression for the fluctuations and correlations. Both
the charge current and the spin current turn out to be generally
super-Poissonian. Moreover, we study the influence of ac fields acting upon the
transported electrons. In particular, we focus on fields that cause spin
rotation or photon-assisted tunnelling.Comment: 14 pages, 9 figure
Global impacts of tropospheric halogens (Cl, Br, I) on oxidants and composition in GEOS-Chem [Discussion paper]
We present a simulation of the global present-day composition of the troposphere which includes the chemistry of halogens (Cl, Br, I). Building on previous work within the GEOS-Chem model we include emissions of inorganic iodine from the oceans, anthropogenic and biogenic sources of halogenated gases, gas phase chemistry, and a parameterised approach to heterogeneous halogen chemistry. Consistent with Schmidt et al. (2016) we do not include sea-salt debromination. Observations of halogen radicals (BrO, IO) are sparse but the model has some skill in reproducing these. Modelled IO shows both high and low biases when compared to different datasets, but BrO concentrations appear to be modelled low. Comparisons to the very sparse observations dataset of reactive Cl species suggest the model represents a lower limit of the impacts of these species, likely due to underestimates in emissions and therefore burdens. Inclusion of Cl, Br, and I results in a general improvement in simulation of ozone (O3) concentrations, except in polar regions where the model now underestimates O3 concentrations. Halogen chemistry reduces the global tropospheric O3 burden by 18.6 %, with the O3 lifetime reducing from 26 to 22 days. Global mean OH concentrations of 1.28 × 106 molecules cm−3 are 8.2 % lower than in a simulation without halogens, leading to an increase in the CH4 lifetime (10.8 %) due to OH oxidation from 7.47 to 8.28 years. Oxidation of CH4 by Cl is small (∼ 2 %) but Cl oxidation of other VOCs (ethane, acetone, and propane) can be significant (∼ 15–27 %). Oxidation of VOCs by Br is smaller, representing 3.9 % of the loss of acetaldehyde and 0.9 % of the loss of formaldehyde
Iodine monoxide in the Western Pacific marine boundary layer
A latitudinal cross-section and vertical profiles of iodine monoxide (IO) are reported from the marine boundary layer of the Western Pacific. The measurements were taken using Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) during the TransBrom cruise of the German research vessel Sonne, which led from Tomakomai, Japan (42° N, 141° E) through the Western Pacific to Townsville, Australia (19° S, 146° E) in October 2009. In the marine boundary layer within the tropics (between 20° N and 5° S), IO mixing ratios ranged between 1 and 2.2 ppt, whereas in the subtropics and at mid-latitudes typical IO mixing ratios were around 1 ppt in the daytime. The profile retrieval reveals that the bulk of the IO was located in the lower part of the marine boundary layer. Photochemical simulations indicate that the organic iodine precursors observed during the cruise (CH3I, CH2I2, CH2ClI, CH2BrI) are not sufficient to explain the measured IO mixing ratios. Reasonable agreement between measured and modelled IO can only be achieved, if an additional sea-air flux of inorganic iodine (e.g. I2) is assumed in the model. Our observations add further evidence to previous studies that reactive iodine is an important oxidant in the marine boundary layer
Semiclassical description of multiphoton processes
We analyze strong field atomic dynamics semiclassically, based on a full
time-dependent description with the Hermann-Kluk propagator. From the
properties of the exact classical trajectories, in particular the accumulation
of action in time, the prominent features of above threshold ionization (ATI)
and higher harmonic generation (HHG) are proven to be interference phenomena.
They are reproduced quantitatively in the semiclassical approximation.
Moreover, the behavior of the action of the classical trajectories supports the
so called strong field approximation which has been devised and postulated for
strong field dynamics.Comment: 10 pages, 11 figure
Atomic-scale confinement of optical fields
In the presence of matter there is no fundamental limit preventing
confinement of visible light even down to atomic scales. Achieving such
confinement and the corresponding intensity enhancement inevitably requires
simultaneous control over atomic-scale details of material structures and over
the optical modes that such structures support. By means of self-assembly we
have obtained side-by-side aligned gold nanorod dimers with robust
atomically-defined gaps reaching below 0.5 nm. The existence of
atomically-confined light fields in these gaps is demonstrated by observing
extreme Coulomb splitting of corresponding symmetric and anti-symmetric dimer
eigenmodes of more than 800 meV in white-light scattering experiments. Our
results open new perspectives for atomically-resolved spectroscopic imaging,
deeply nonlinear optics, ultra-sensing, cavity optomechanics as well as for the
realization of novel quantum-optical devices
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