Proton Path Research Articles

The Mean Free Path of 13–64 MeV Protons Derived from Statistical Results of Solar Energetic Particle Events

A recent study by Wang et al. investigated gradual solar proton events with energies >10 MeV, as observed by STEREO-A, STEREO-B, and the Solar and Heliospheric Observatory spacecraft. For each event, the spacecraft with the best magnetic connection to the source region among the three spacecraft was identified, and energetic proton intensities observed by the spacecraft were analyzed through fitting. The fitting process produced two parameters, b and c, for four energy channels (13–16 MeV, 20–25 MeV, 32–40 MeV, and 40–64 MeV) in each event. Parameters b and c govern the rise and decay of particle intensities, respectively. Statistical analysis revealed a power-law correlation between b and c, expressed as c ∼ b −γ . In this study, in order to explain the relation between the two parameters, we investigate the model of particle diffusion coefficients in the interplanetary space. In our simulations, the radial mean free path is modeled as a power function of radial distance, successfully reproducing the b–c relation. Consequently, the observations demonstrate that the radial mean free path varies with radial distance in a power law. In future research, the model of diffusion coefficients holds promise in determining the mean free path of energetic protons.

A Detailed Survey of the Parallel Mean Free Path of Solar Energetic Particle Protons and Electrons

In this work, more than a dozen solar energetic particle (SEP) events are identified where the source region is magnetically well connected to at least one spacecraft at 1 au. The observed intensity–time profiles, for all available proton and electron energy channels, are compared to results computed using a numerical one-dimensional SEP transport model in order to derive the parallel mean free paths (pMFPs) as a function of energy (or rigidity) at 1 au. These inversion results are then compared to theoretical estimates of the pMFP, using observed turbulence quantities with observationally motivated variations as input. For protons, a very good comparison between inversion and theoretical results is obtained. It is shown that the observed inter-event variations in the inversion pMFP values can be explained by natural variations in the background turbulence values. For electrons, there is relatively good agreement with pMFPs derived assuming the damping model of dynamical turbulence, although the theoretical values are extremely sensitive to the details of the turbulence dissipation range, which themselves display a high level of variation.

In situ fabrication of atomically adjacent dual-vacancy sites for nearly 100% selective CH4 production

The photocatalytic CO2-to-CH4 conversion involves multiple consecutive proton-electron coupling transfer processes. Achieving high CH4 selectivity with satisfactory conversion efficiency remains challenging since the inefficient proton and electron delivery path results in sluggish proton-electron transfer kinetics. Herein, we propose the fabrication of atomically adjacent anion-cation vacancy as paired redox active sites that could maximally promote the proton- and electron-donating efficiency to simultaneously enhance the oxidation and reduction half-reactions, achieving higher photocatalytic CO2 reduction activity and CH4 selectivity. Taking TiO2 as a photocatalyst prototype, the operando electron paramagnetic resonance spectra, quasi in situ X-ray photoelectron spectroscopy measurements, and high-angle annular dark-field-scanning transmission electron microscopy image analysis prove that the VTi on TiO2 as initial sites can induce electron redistribution and facilitate the escape of the adjacent oxygen atom, thereby triggering the dynamic creation of atomically adjacent dual-vacancy sites during photocatalytic reactions. The dual-vacancy sites not only promote the proton- and electron-donating efficiency for CO2 activation and protonation but also modulate the coordination modes of surface-bound intermediate species, thus converting the endoergic protonation step to an exoergic reaction process and steering the CO2 reduction pathway toward CH4 production. As a result, these in situ created dual active sites enable nearly 100% CH4 selectivity and evolution rate of 19.4 μmol g-1 h-1, about 80 times higher than that of pristine TiO2. Thus, these insights into vacancy dynamics and structure-function relationship are valuable to atomic understanding and catalyst design for achieving highly selective catalysis.

Dynamic acceleration of energetic protons by an interplanetary collisionless shock

Context. Interplanetary collisionless shocks are known to be capable of accelerating charged particles up to hundreds of MeV. However, the underlying acceleration mechanisms are still under debate. Aims. We present the dynamic behaviors of energetic protons that are accelerated by an interplanetary shock that was observed with unprecedented high-resolution measurements by the Electron-Proton Telescope sensor of the Energetic Particle Detector suite on board the Solar Orbiter spacecraft on 2021 November 3. We constrain the potential acceleration mechanisms and processes. Methods. We first reconstructed the proton pitch-angle distributions (PADs) in the solar wind frame. Then, we examined the temporal flux profile, PAD, and the velocity distribution function of energetic protons close to the shock, and we qualitatively compared the observations with theoretical predictions. Moreover, we applied a velocity dispersion analysis (VDA) to an observed velocity dispersion event and derived the proton path length and release time at the shock. Then, we tested this derivation by comparing it with the shock motion and the magnetic field configuration. Results. We find that ∼1000–4000 keV protons exhibit a rapid-rise, rapid-decay temporal flux profile with a clear velocity dispersion ∼2 min before the shock, similar to impulsive solar energetic particle events. The proton path length based on the VDA of this event is consistent with the length derived from the shock motion and magnetic field configuration. The peak spectrum in this event appears to be steeper than the spectrum at the shock. Furthermore, we find that ∼50–200 keV proton fluxes peak between ∼10 and ∼20 s before the shock, with an inverse velocity dispersion. The velocity dispersion event and the inverse velocity dispersion event are both accompanied by magnetic kinks or switchbacks. In addition, two distinct proton populations appear near the shock. The first population at energies below ∼300 keV is characterized by a power-law spectrum with an index of ∼6–7 and a flux profile that increases before and decreases after the shock. The other population at energies above ∼300 keV shows a long-lasting, anti-sunward-beamed PAD across the shock and a flux profile that remains relatively constant before and increases slightly after the shock. Conclusions. These results suggest that the shock acceleration of energetic protons is highly dynamic due to temporal and/or spatial variations at the shock front. The observation of the velocity dispersion event further suggests that shock acceleration can be impulsive and efficient, which may be due to the interaction between the shock and magnetic kinks or switchbacks. Moreover, these results may support shock-drift acceleration and diffusive shock acceleration as candidate acceleration mechanisms at interplanetary shocks.

Structure and non-reactive dynamics of the dimeric catalytic domain of human carbonic anhydrase IX

Despite the availability of high resolution crystal structure of the catalytic domain of human carbonic anhydrase IX (denoted as HCA IX-c), its solution structure and functional dynamics are mostly unknown. In this article, we report classical molecular dynamics (MD) simulation studies on the structure and non-reactive dynamics of dimeric HCA IX-c (d-HCA IX-c, with chains A and B) in water and compare them to each chain simulated in water as a monomeric solute (mA/mB). While the average solution structures appear to be largely similar in all systems, d-HCA IX-c shows significant deviations from mA and mB in terms of non-reactive reorganization of the active site needed to initiate the rate determining proton transfer step between a zinc-bound water and one of the sidechain N-atoms of a distal histidine residue (His-64). First, 2−6 water molecules are found to form fluctuating hydrogen bonded networks that would serve as proton paths across the active site. A minimally frustrated core of each monomer in the dimeric protein is found to be associated with a significantly higher likelihood of forming complete proton transfer pathways mediated by 2 water molecules synchronously at the two active sites hosted by the monomeric chains. The relative stability and kinetics of transition between two major sidechain rotamers of His-64 are also affected by dimerization. The sidechain rotation of His-64 is found to involve at least one transition as slow as 105s−1 in chain A of d-HCA IX-c thereby rendering it slower than the actual rate determining proton transfer step. This observation indicates an important deviation from the widely accepted mechanism of catalysis by HCAs.

Exploration of differentiability in a proton computed tomography simulation framework

Objective. Gradient-based optimization using algorithmic derivatives can be a useful technique to improve engineering designs with respect to a computer-implemented objective function. Likewise, uncertainty quantification through computer simulations can be carried out by means of derivatives of the computer simulation. However, the effectiveness of these techniques depends on how ‘well-linearizable’ the software is. In this study, we assess how promising derivative information of a typical proton computed tomography (pCT) scan computer simulation is for the aforementioned applications. Approach. This study is mainly based on numerical experiments, in which we repeatedly evaluate three representative computational steps with perturbed input values. We support our observations with a review of the algorithmic steps and arithmetic operations performed by the software, using debugging techniques. Main results. The model-based iterative reconstruction (MBIR) subprocedure (at the end of the software pipeline) and the Monte Carlo (MC) simulation (at the beginning) were piecewise differentiable. However, the observed high density and magnitude of jumps was likely to preclude most meaningful uses of the derivatives. Jumps in the MBIR function arose from the discrete computation of the set of voxels intersected by a proton path, and could be reduced in magnitude by a ‘fuzzy voxels’ approach. The investigated jumps in the MC function arose from local changes in the control flow that affected the amount of consumed random numbers. The tracking algorithm solves an inherently non-differentiable problem. Significance. Besides the technical challenges of merely applying AD to existing software projects, the MC and MBIR codes must be adapted to compute smoother functions. For the MBIR code, we presented one possible approach for this while for the MC code, this will be subject to further research. For the tracking subprocedure, further research on surrogate models is necessary.

Ultra-thin nanohoneycomb porous CoMoO4 with excellent catalytic performance for water splitting at large current densities

Electrocatalytic water splitting for hydrogen production is considered the most effective and cleanest strategy to make full use of renewable energy. It still lacks the highly efficient and stable bifunctional catalyst to deliver high current density for large scale application of water splitting. Here, we innovatively developed a facile method to synthesize a three-dimensional (3D) honeycomb porous CoMoO4@pNi (porous nickel) catalyst with ultra-thin nanosheet, the generated oxygen vacancies and mesoporous structures with defects as active sites. We also precisely tuned the thickness and array structure to investigate its catalytic performance. The as-prepared CoMoO4@pNi catalyst delivers the overpotential of 377 mV for HER and 410 mV for OER, respectively, and only demands 1.99 V to reach a large current density of 1000 mA cm−2 for overall water splitting. CoMoO4@pNi catalyst is also operated for 200 h under 500 and 1000 mA cm−2, respectively, and both showed an ignorable current decrease. The excellent activity and stability are ascribed to the high surface area, short transport path for electrons and protons, the full contact between the exposed active sites and electrolyte, and the inherent stable nanohoneycomb porous structure of the ultra-thin nanosheet.

Identification of hydrogen bonding network for proton transfer at the quinol oxidation site of Rhodobacter capsulatus cytochrome bc1

Cytochrome bc1 catalyzes electron transfer from quinol (QH2) to cytochrome c in reactions coupled to proton translocation across the energy-conserving membrane. Energetic efficiency of the catalytic cycle is secured by a two-electron, two-proton bifurcation reaction leading to oxidation of QH2 and reduction of the Rieske cluster and heme bL. The proton paths associated with this reaction remain elusive. Here we used site-directed mutagenesis and quantum mechanical (QM) calculations to analyze the contribution of protonable side chains located at the heme bL side of the quinol oxidation site (Qo) in Rhodobacter capsulatus cytochrome bc1. We observe that the proton path is effectively switched off when H276 and E295 are simultaneously mutated to the non-protonable residues in the H276F/E295V double mutant. The two single mutants, H276F or E295V, are less efficient, but still transfer protons at functionally-relevant rates. Natural selection exposed two single mutations, N279S and M154T, that restored the functional proton transfers in H276F/E295V. QM calculations indicated that H276F/E295V traps the side chain of Y147 in a position distant from QH2, while either N279S or M154T induce local changes releasing Y147 from that position. This shortens the distance between the protonable groups of Y147 and D278 and/or increases mobility of the Y147 side chain, which makes Y147 efficient in transferring protons from QH2 toward D278 in H276F/E295V. Overall, our study identified an extended hydrogen bonding network, build up by E295, H276, D278 and Y147, involved in efficient removal of the proton from QH2 at the heme bL side of Qo.

The Ancestral Shape of the Access Proton Path of Mitochondrial ATP Synthases Revealed by a Split Subunit-a.

The passage of protons across membranes through F1Fo-ATP synthases spins their rotors and drives the synthesis of ATP. While the principle of torque generation by proton transfer is known, the mechanisms and routes of proton access and release and their evolution are not fully understood. Here, we show that the entry site and path of protons in the lumenal half channel of mitochondrial ATP synthases are largely defined by a short N-terminal α-helix of subunit-a. In Trypanosoma brucei and other Euglenozoa, the α-helix is part of another polypeptide chain that is a product of subunit-a gene fragmentation. This α-helix and other elements forming the proton pathway are widely conserved across eukaryotes and in Alphaproteobacteria, the closest extant relatives of mitochondria, but not in other bacteria. The α-helix blocks one of two proton routes found in Escherichia coli, resulting in a single proton entry site in mitochondrial and alphaproteobacterial ATP synthases. Thus, the shape of the access half channel predates eukaryotes and originated in the lineage from which mitochondria evolved by endosymbiosis.

The ESS monolith vessel design and possibilities to introduce a UCN/VCN source

The ESS ERIC neutron source design includes a helium cooled tungsten spallation target and a liquid hydrogen moderator, enclosed in a vacuum vessel (the monolith vessel). The neutron source and moderator are shielded by stainless steel and casted iron to block neutron and gamma radiation emitted in directions other than along the neutron beam ports. After the ESS concept design was approved, questions were raised if it was possible to introduce a UCN/VCN (Ultra Cold Neutron/Very Cold Neutron) source into the ESS design. Since the shielding concept of the monolith vessel is to 100% fill the monolith void except for the proton and neutron beam paths, there are no cavities in the monolith vessel left, where one could easily place a UCN/VCN source. An additional challenge is the pipe routing for the UCN/VCN cooling media. However, some of the shielding blocks and monolith components are removable and could possibly be redesigned, enabling implementation of a UCN/VCN source. This article presents a feasibility study how to physically introduce a UCN/VCN source in the present ESS design, focusing on the geometrical possibilities. Four possible locations for the UCN/VCN source were identified, which are considered to be feasible but will require some redesign of the monolith vessel shielding.

Projection Deconvolution for Proton CT Using the Spatially Variant Path Uncertainty

Proton computed tomography (pCT) suffers from a lower spatial resolution compared to X-ray CT due to the stochastic nonlinear proton paths. The most likely path (MLP) formalism provides an estimate for the proton path as well as the uncertainty around this estimate. Using the MLP estimate for the image reconstruction instead of straight integration lines has been shown to improve the spatial resolution of pCT images. In this work, the aim is to further increase the spatial resolution by also including the path uncertainty in the reconstruction algorithm. We proposed a projection-based deconvolution method, applied within the framework of a direct reconstruction algorithm based on distance-driven binning. We used an MLP formalism accounting for tracker resolution in addition to multiple Coulomb scattering (MCS). We investigated deconvolution artifacts and proposed a method to mitigate them via spatial regularization. Our method was tested on Monte Carlo simulated data, using a water cylinder with aluminium inserts and two slices of an anthropomorphic phantom. Our results showed an improvement of spatial resolution in all cases (up to 29% or 60% for the cylindrical phantom, depending on whether deconvolution artifacts were corrected for or not). Overshoot artifacts were observed in the case of the cylindrical phantom but were less prominent in the case of the anthropomorphic phantom. In conclusion, we have shown that including the path uncertainty in the reconstruction can notably improve the spatial resolution.

Tools for analyzing protonation states and for tracing proton transfer pathways with examples from the Rb. sphaeroides photosynthetic reaction centers.

Protons participate in many reactions. In proteins, protons need paths to move in and out of buried active sites. The vectorial movement of protons coupled to electron transfer reactions establishes the transmembrane electrochemical gradient used for many reactions, including ATP synthesis. Protons move through hydrogen bonded chains of waters and hydroxy side chains via the Grotthuss mechanism and by proton binding and release from acidic and basic residues. MCCE analysis shows that proteins exist in a large number of protonation states. Knowledge of the equilibrium ensemble can provide a rational basis for setting protonation states in simulations that fix them, such as molecular dynamics (MD). The proton path into the QB site in the bacterial reaction centers (RCs) of Rb. sphaeroides is analyzed by MD to provide an example of the benefits of using protonation states found by the MCCE program. A tangled web of side chains and waters link the cytoplasm to QB. MCCE analysis of snapshots from multiple trajectories shows that changing the input protonation state of a residue in MD biases the trajectory shifting the proton affinity of that residue. However, the proton affinity of some residues is more sensitive to the input structure. The proton transfer networks derived from different trajectories are quite robust. There are some changes in connectivity that are largely restricted to the specific residues whose protonation state is changed. Trajectories with QB•- are compared with earlier results obtained with QB [Wei et. al Photosynthesis Researchvolume152,pages153-165 (2022)] showing only modest changes. While introducing new methods the study highlights the difficulty of establishing the connections between protein conformation.

Demonstrative measurement of proton-nuclear reaction by deconvolving the prompt gamma-ray spectra

In proton therapy, there has been an increasing demand for measuring the employed dose distribution to improve treatment accuracy and quality. One method of estimating radiation dose in real time is to visualize the prompt gamma rays emitted during nuclear reactions between protons and nuclei in the patient’s body. However, this is difficult without accurate measurements of nuclear cross sections, that cover a 10–100 MeV energy range. Herein, we propose a novel method for determining cross section of low-energy protons with 12C and 16O by measuring the prompt gamma rays emitted from stacked slab phantoms placed along the proton path. These emitted gamma rays provide direct information on the nuclear reactions occurring at different proton energies. By applying an iterative inference using the response matrix, the contamination of various prompt gamma rays can be resolved; thus, the cross sections of various reactions were obtained at arbitrary proton energies. As a first step to this study, we applied this method to measure the nuclear reaction of protons with 12C and 16O nuclei, which are the major elements constituting the human body. The cross section of the 12C(p,p)12C∗ reaction, which produces a prominent gamma-ray peak at 4.4 MeV, reproduced the one in literature well, supporting the validity of the proposed method. Furthermore, we show that even weak prompt gamma rays can be resolved for a wide range of proton energies below 70 MeV, for which no cross sectional data have been reported so far.

Extension of the open-source TIGRE toolbox for proton imaging

Proton irradiation is a well-established method to treat deep-seated tumors in radio oncology. Usually, an X-ray computed tomography (CT) scan is used for treatment planning. Since proton therapy is based on the precise knowledge of the stopping power describing the energy loss of protons in the patient tissues, the Hounsfield units of the planning CT have to be converted. This conversion introduces range errors in the treatment plan, which could be reduced, if the stopping power values were extracted directly from an image obtained using protons instead of X-rays. Since protons are affected by multiple Coulomb scattering, reconstruction of the 3D stopping power map results in limited image quality if the curved proton path is not considered. This work presents a substantial code extension of the open-source toolbox TIGRE for proton CT (pCT) image reconstruction based on proton radiographs including a curved proton path estimate. The code extension and the reconstruction algorithms are GPU-based, allowing to achieve reconstruction results within minutes. The performance of the pCT code extension was tested with Monte Carlo simulated data using three phantoms (Catphan® high resolution and sensitometry modules and a CIRS patient phantom). In the simulations, ideal and non-ideal conditions for a pCT setup were assumed. The obtained mean absolute percentage error was found to be below 1% and up to 8 lp/cm could be resolved using an idealized setup. These findings demonstrate that the presented code extension to the TIGRE toolbox offers the possibility for other research groups to use a fast and accurate open-source pCT reconstruction.

A measurement of the effective mean free path of solar wind protons

Weakly collisional plasmas are subject to nonlinear relaxation processes, which can operate at rates much faster than the particle collision frequencies. This causes the plasma to respond like a magnetised fluid despite having long particle mean free paths. In this Letter the effective collisional mechanisms are modelled in the plasma kinetic equation to produce density, pressure and magnetic-field responses to compare with spacecraft measurements of the solar wind compressive fluctuations at 1 AU. This enables a measurement of the effective mean free path of the solar wind protons, found to be ${\approx }4 \times 10^{5}$ km, which is approximately $10^{3}$ times shorter than the collisional mean free path. These measurements are shown to support the effective fluid behaviour of the solar wind at scales above the proton gyroradius and demonstrate that effective collision processes alter the thermodynamics and transport of weakly collisional plasmas.

Protons in Gating the Kv1.2 Channel: A Calculated Set of Protonation States in Response to Polarization/Depolarization of the Channel, with the Complete Proposed Proton Path from Voltage Sensing Domain to Gate.

We have in the past proposed that proton motion constitutes the gating current in the potassium channel Kv1.2 and is responsible for the gating mechanism. For this to happen, there must be a proton path between the voltage-sensing domain (VSD) and the channel gate, and here we present quantum calculations that lead to a specific pair of proton paths, defined at the molecular level, with well-defined water molecule linkages, and with hydrogen bonding between residues; there is also at least one interpath crossover, where protons can switch paths. Quantum calculations on the entire 563-atom system give the complete geometry, the energy, and atomic charges. Calculations show that three specific residues (in the pdb 3Lut numbering, H418, E327, R326), and the T1 intracellular moiety, all of which have been shown experimentally to be involved in gating, would necessarily be protonated or deprotonated in the path between the VSD and the gate. Hydroxyl reorientation of serine and threonine residues are shown to provide a means of adjusting proton directions of motion. In the deprotonated state for K312, a low energy state, our calculations come close to reproducing the X-ray structure. The demonstration of the existence of a double proton path between VSD and gate supports the proposed proton gating mechanism; when combined with our earlier demonstration of proton generation in the VSD, and comparison with other systems that are known to move protons, we are close to achieving the definition of a complete gating mechanism in molecular detail. The coupling of the paths to the VSD, and to the PVPV section that essentially forms the gate, can be easily seen from the results of the calculation. The gate itself remains for further computations.

Tuning Electrode-Membrane Interface for Highly Efficient Polymer Electrolyte Membrane Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFCs) are typically comprised of porous electrodes with randomly distributed mixtures of Pt/C catalyst and ionomer formed during the ink deposition process. The randomness of catalyst particles, ionomer films, and voids, create a non-ideal disordered electrode structure with high tortuosity. These intrinsic characteristics of conventional electrodes create significant challenges during PEMFC operation. These challenges include: 1) low catalyst utilization; 2) high O2 transport resistance to catalyst through the ionomer and pores of carbon support; and 3) limited electronic/protonic conductivities. To improve the performance of the PEMFC electrode, we have previously shown that structured electrodes based on a patterned membrane surface can increase the interfacial area between the electrode and the membrane, enhancing mass transport at high current densities, and augmenting its ionic conductivity.[1][2] In this work, we will present further optimization of these type of structured electrodes using free-standing architectures, reduced ionomer content, and new catalyst deposition processes. More importantly, these newly tuned electrode structures can improve the performance of PEMFCs by maximizing three-phase boundaries and optimizing the electron, proton, gas, and water transport paths. Acknowledgement: We gratefully acknowledge support from the Laboratory Directed Research & Development Program at Los Alamos National Laboratory. Reference: [1] S. K. Babu, R. Mukundan, D. A. Cullen, and J. S. Spendelow, “Co-Axial Nafion Nanowire Electrode.” ECS, Oct. 15, 2019.[2] S. K. Babu, J. S. Spendelow, and R. L. Borup, “Meso-Structured Array Electrode for Polymer Electrolyte Fuel Cells.” ECS, Oct. 03, 2017.

In vivo production of fluorine-18 in a chicken egg tumor model of breast cancer for proton therapy range verification

Range verification of clinical protontherapy systems via positron-emission tomography (PET) is not a mature technology, suffering from two major issues: insufficient signal from low-energy protons in the Bragg peak area and biological washout of PET emitters. The use of contrast agents including 18O, 68Zn or 63Cu, isotopes with a high cross section for low-energy protons in nuclear reactions producing PET emitters, has been proposed to enhance the PET signal in the last millimeters of the proton path. However, it remains a challenge to achieve sufficient concentrations of these isotopes in the target volume. Here we investigate the possibilities of 18O-enriched water (18-W), a potential contrast agent that could be incorporated in large proportions in live tissues by replacing regular water. We hypothesize that 18-W could also mitigate the problem of biological washout, as PET (18F) isotopes created inside live cells would remain trapped in the form of fluoride anions (F-), allowing its signal to be detected even hours after irradiation. To test our hypothesis, we designed an experiment with two main goals: first, prove that 18-W can incorporate enough 18O into a living organism to produce a detectable signal from 18F after proton irradiation, and second, determine the amount of activity that remains trapped inside the cells. The experiment was performed on a chicken embryo chorioallantoic membrane tumor model of head and neck cancer. Seven eggs with visible tumors were infused with 18-W and irradiated with 8-MeV protons (range in water: 0.74 mm), equivalent to clinical protons at the end of particle range. The activity produced after irradiation was detected and quantified in a small-animal PET-CT scanner, and further studied by placing ex-vivo tumours in a gamma radiation detector. In the acquired images, specific activity of 18F (originating from 18-W) could be detected in the tumour area of the alive chicken embryo up to 9 h after irradiation, which confirms that low-energy protons can indeed produce a detectable PET signal if a suitable contrast agent is employed. Moreover, dynamic PET studies in two of the eggs evidenced a minimal effect of biological washout, with 68% retained specific 18F activity at 8 h after irradiation. Furthermore, ex-vivo analysis of 4 irradiated tumours showed that up to 3% of oxygen atoms in the targets were replaced by 18O from infused 18-W, and evidenced an entrapment of 59% for specific activity of 18F after washing, supporting our hypothesis that F- ions remain trapped within the cells. An infusion of 18-W can incorporate 18O in animal tissues by replacing regular water inside cells, producing a PET signal when irradiated with low-energy protons that could be used for range verification in protontherapy. 18F produced inside cells remains entrapped and suffers from minimal biological washout, allowing for a sharper localization with longer PET acquisitions. Further studies must evaluate the feasibility of this technique in dosimetric conditions closer to clinical practice, in order to define potential protocols for its use in patients.

Comparison of proton transfer paths to the QA and QB sites of the Rb. sphaeroides photosynthetic reaction centers.

The photosynthetic bacterial reaction centers from purple non-sulfur bacteria use light energy to drive the transfer of electrons from cytochrome c to ubiquinone. Ubiquinone bound in the QA site cycles between quinone, QA, and anionic semiquinone, QA·-, being reduced once and never binding protons. In the QB site, ubiquinone is reduced twice by QA·-, binds two protons and is released into the membrane as the quinol, QH2. The network of hydrogen bonds formed in a molecular dynamics trajectory was drawn to investigate proton transfer pathways from the cytoplasm to each quinone binding site. QA is isolated with no path for protons to enter from the surface. In contrast, there is a complex and tangled network requiring residues and waters that can bring protons to QB. There are three entries from clusters of surface residues centered around HisH126, GluH224, and HisH68. The network is in good agreement with earlier studies, Mutation of key nodes in the network, such as SerL223, were previously shown to slow proton delivery. Mutational studies had also shown that double mutations of residues such as AspM17 and AspL210 along multiple paths in the network presented here slow the reaction, while single mutations do not. Likewise, mutation of both HisH126 and HisH128, which are at the entry to two paths reduce the rate of proton uptake.

Fast Magnetosonic Waves in a Dipolarizing Flux Bundle Inside the Geosynchronous Orbit

AbstractAlthough commonly observed in the inner magnetosphere, fast magnetosonic (MS) waves are often owe to be excited by those proton rings resulting from the so‐called energy‐dependent drift path of ring current protons. In this letter, we report an interesting MS wave event in a dipolarizing flux bundle (DFB) observed by the Van Allen Probe A. In the DFB, perpendicular proton fluxes are found depleted at low and intermediate energies, while enhanced at high energies, which should be attributed to proton accelerations by DFB. As a result, proton ring could be locally formed. Using the observed plasma environment parameters and the modeled proton distributions, we have calculated the linear growth rates for MS waves and found that these MS waves might be locally excited. Our results provide insight into the generation of MS waves in a DFB, and provide a new channel to excite MS waves in the inner magnetosphere.