2,949 research outputs found
Spectral mixture analysis of EELS spectrum-images
Recent advances in detectors and computer science have enabled the
acquisition and the processing of multidimensional datasets, in particular in
the field of spectral imaging. Benefiting from these new developments, earth
scientists try to recover the reflectance spectra of macroscopic materials
(e.g., water, grass, mineral types...) present in an observed scene and to
estimate their respective proportions in each mixed pixel of the acquired
image. This task is usually referred to as spectral mixture analysis or
spectral unmixing (SU). SU aims at decomposing the measured pixel spectrum into
a collection of constituent spectra, called endmembers, and a set of
corresponding fractions (abundances) that indicate the proportion of each
endmember present in the pixel. Similarly, when processing spectrum-images,
microscopists usually try to map elemental, physical and chemical state
information of a given material. This paper reports how a SU algorithm
dedicated to remote sensing hyperspectral images can be successfully applied to
analyze spectrum-image resulting from electron energy-loss spectroscopy (EELS).
SU generally overcomes standard limitations inherent to other multivariate
statistical analysis methods, such as principal component analysis (PCA) or
independent component analysis (ICA), that have been previously used to analyze
EELS maps. Indeed, ICA and PCA may perform poorly for linear spectral mixture
analysis due to the strong dependence between the abundances of the different
materials. One example is presented here to demonstrate the potential of this
technique for EELS analysis.Comment: Manuscript accepted for publication in Ultramicroscop
Modulation of calmodulin lobes by different targets: an allosteric model with hemiconcerted conformational transitions
Calmodulin, the ubiquitous calcium-activated second messenger in eukaryotes,
is an extremely versatile molecule involved in many biological processes:
muscular contraction, synaptic plasticity, circadian rhythm, and cell cycle,
among others. The protein is structurally organised into two globular lobes,
joined by a flexible linker. Calcium modulates calmodulin activity by favoring
a conformational transition of each lobe from a closed conformation to an open
conformation. Most targets have a strong preference for one conformation over
the other, and depending on the free calcium concentration in a cell,
particular sets of targets will preferentially interact with calmodulin. In
turn, targets can increase or decrease the calcium affinity of the calmodulin
molecules to which they bind. Interestingly, experiments with the tryptic
fragments showed that most targets have a much lower affinity for the N-lobe
than for the C-lobe. Hence, the latter predominates in the formation of most
calmodulin-target complexes. We showed that a relatively simple allosteric
mechanism, based the classic MWC model, can capture the observed modulation of
both the isolated C-lobe, and intact calmodulin, by individual targets.
Moreover, our model can be naturally extended to study how the calcium affinity
of a single pool of calmodulin is modulated by a mixture of competing targets
in vivo
Differentiated, promoter-specific response of [4Fe-4S] NsrR DNA-binding to reaction with nitric oxide
NsrR is an iron-sulfur cluster protein that regulates the nitric oxide (NO) stress response of many bacteria. NsrR from Streptomyces coelicolor regulates its own expression and that of only two other genes, hmpA1 and hmpA2, which encode HmpA enzymes predicted to detoxify NO. NsrR binds promoter DNA with high affinity only when coordinating a [4Fe-4S] cluster. Here we show that reaction of [4Fe-4S] NsrR with NO affects DNA-binding differently depending on the gene promoter. Binding to the hmpA2 promoter was abolished at ~2 NO per cluster, while for the hmpA1 and nsrR promoters, ~4 and ~8 NO molecules, respectively, were required to abolish DNA binding. Spectroscopic and kinetic studies of the NO reaction revealed a rapid, multi-phase, non-concerted process involving up to 8 â 10 NO molecules per cluster, leading to the formation of several iron-nitrosyl species. A distinct intermediate was observed at ~2 NO per cluster, along with two further intermediates at ~4 and ~6 NO. The NsrR nitrosylation reaction was not significantly affected by DNA-binding. These results show that NsrR regulates different promoters in response to different concentrations of NO. Spectroscopic evidence indicates that this is achieved by different NO-FeS complexes
Mass spectrometric identification of [4Fe-4S](NO)x intermediates of nitric oxide sensing by regulatory iron-sulfur cluster proteins
Nitric oxide (NO) can function as both a cytotoxin and a signalling molecule. In both cases, reaction with ironâsulfur (FeâS) cluster proteins plays an important role because FeâS clusters are reactive towards NO and so are a primary site of general NO-induced damage (toxicity). This sensitivity to nitrosylation is harnessed in the growing group of regulatory proteins that function in sensing of NO via an FeâS cluster. Although information about the products of cluster nitrosylation is now emerging, detection and identification of intermediates remains a major challenge, due to their transient nature and the difficulty in distinguishing spectroscopically similar iron-NO species. Here we report studies of the NO-sensing FeâS cluster regulators NsrR and WhiD using non-denaturing mass spectrometry, in which non-covalent interactions between the protein and Fe/S/NO species are preserved. The data provide remarkable insight into the nitrosylation reactions, permitting identification, for the first time, of protein-bound mono-, di- and tetranitrosyl [4Feâ4S] cluster complexes ([4Feâ4S](NO), [4Feâ4S])(NO) 2 and [4Feâ4S](NO) 4 ) as intermediates along pathways to formation of product Roussin's red ester (RRE) and Roussin's black salt (RBS)-like species. The data allow the nitrosylation mechanisms of NsrR and WhiD to be elucidated and clearly distinguished
A novel three-dimensional macrocellular carbonaceous biofuel cell
Here we report the first membrane-free biofuel cell obtained using three-dimensional carbonaceous foam electrodes. We first developed a new synthetic pathway to produce a new carbonaceous foam electrode material bearing porosity both on the meso and macroporous scales. We proved that by increasing the porosity of our three-dimensional foams we could increase the current density of our modified electrodes. Then, by choosing the right combination of enzyme and mediator, and the right loading of active components, we achieved high current densities for an anodic system. Finally, we combined the improved cathode and anode to build a new membrane-free hybrid enzymatic biofuel cell consisting of a mediated anode and a mediator-free cathode
Quantum State Diffusion and Time Correlation Functions
In computing the spectra of quantum mechanical systems one encounters the
Fourier transforms of time correlation functions, as given by the quantum
regression theorem for systems described by master equations. Quantum state
diffusion (QSD) gives a useful method of solving these problems by unraveling
the master equation into stochastic trajectories; but there is no generally
accepted definition of a time correlation function for a single QSD trajectory.
In this paper we show how QSD can be used to calculate these spectra directly;
by formally solving the equations which arise, we arrive at a natural
definition for a two-time correlation function in QSD, which depends explicitly
on both the stochastic noise of the particular trajectory and the time of
measurement, and which agrees in the mean with the ensemble average definition
of correlation functions.Comment: 16 pages standard LaTeX + 1 figure (uuencoded postscript) Numerous
minor revisions and clarifications. To appear in J. Mod. Optic
Spin Hall effect of light in a random medium
We show that optical beams propagating in transversally disordered materials
exhibit a spin Hall effect and a spin-to-orbital conversion of angular momentum
as they deviate from paraxiality. We theoretically describe these phenomena on
the basis of the microscopic statistical approach to light propagation in
random media, and show that they can be detected via polarimetric measurements
under realistic experimental conditionsComment: 5 pages, 4 figure
Generation of 34S-substituted protein-bound [4Fe-4S] clusters using 34S-L-cysteine
The ability to specifically label the sulphide ions of protein-bound ironâsulphur (FeS) clusters with 34S isotope greatly facilitates structureâfunction studies. In particular, it provides insight when using either spectroscopic techniques that probe cluster-associated vibrations, or non-denaturing mass spectrometry, where the âŒ+2âDa average increase per sulphide enables unambiguous assignment of the FeS cluster and, where relevant, its conversion/degradation products. Here, we employ a thermostable homologue of the O-acetyl-L-serine sulfhydrylase CysK to generate 34S-substituted L-cysteine and subsequently use it as a substrate for the L-cysteine desulfurase NifS to gradually supply 34S2â for in vitro FeS cluster assembly in an otherwise standard cluster reconstitution protocol
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