Complete Fission Research Articles

Completion of mitochondrial division requires the intermembrane space protein Mdi1/Atg44.

Mitochondria are highly dynamic double membrane-bound organelles that maintain their shape in part through fission and fusion. Mitochondrial fission is performed by a dynamin-related protein, Dnm1 (Drp1 in humans), that constricts and divides the mitochondria in a GTP hydrolysis-dependent manner. However, it is unclear whether factors inside mitochondria help coordinate the process and if Dnm1/Drp1 activity is sufficient to complete the fission of both mitochondrial membranes. Here, we identify an intermembrane space protein required for mitochondrial fission in yeast, which we propose to name Mdi1 (also named Atg44). Loss of Mdi1 causes mitochondrial hyperfusion due to defects in fission, but not the lack of Dnm1 recruitment to mitochondria. Mdi1 is conserved in fungal species, and its homologs contain an amphipathic α-helix, mutations of which disrupt mitochondrial morphology. One model is that Mdi1 distorts mitochondrial membranes to enable Dnm1 to robustly complete fission. Our work reveals that Dnm1 cannot efficiently divide mitochondria without the coordinated function of Mdi1 inside mitochondria.

Golgi-derived PI(4)P-containing vesicles drive late steps of mitochondrial division.

Mitochondrial plasticity is a key regulator of cell fate decisions. Mitochondrial division involves Dynamin-related protein-1 (Drp1) oligomerization, which constricts membranes at endoplasmic reticulum (ER) contact sites. The mechanisms driving the final steps of mitochondrial division are still unclear. Here, we found that microdomains of phosphatidylinositol 4-phosphate [PI(4)P] on trans-Golgi network (TGN) vesicles were recruited to mitochondria-ER contact sites and could drive mitochondrial division downstream of Drp1. The loss of the small guanosine triphosphatase ADP-ribosylation factor 1 (Arf1) or its effector, phosphatidylinositol 4-kinase IIIβ [PI(4)KIIIβ], in different mammalian cell lines prevented PI(4)P generation and led to a hyperfused and branched mitochondrial network marked with extended mitochondrial constriction sites. Thus, recruitment of TGN-PI(4)P-containing vesicles at mitochondria-ER contact sites may trigger final events leading to mitochondrial scission.

PI(4)P regulates mitochondrial fission

Cell Biology Mitochondria are dynamic intracellular organelles, the shape and number of which are regulated by various cell-signaling pathways. Mitochondrial division is driven by the recruitment of a constricting guanosine triphosphatase protein at sites of contact with the endoplasmic reticulum, but other factors, including lysosomes, are also involved. Nagashima et al. now document an essential role for Golgi-derived vesicles bearing a specific lipid—phosphatidylinositol 4-phosphate, or PI(4)P—in the final steps of mitochondrial division. Disruption of PI(4)P production results in mitochondrial morphological defects indicative of an inability to complete fission. Science , this issue p. [1366][1] [1]: /lookup/doi/10.1126/science.aax6089

Employing FREYA for fission product yield evaluations

The fast event-by-event fission code FREYA (Fission Reaction Event Yield Algorithm) generates large samples of complete fission events while employing only a few physics-based parameters. Not only is FREYA fast, it is also flexible, able to employ a variety of input formats to test the implications of various fission yield evaluations on neutron and photon observables. We describe how FREYA was applied to the neutron-rich nuclei needed for r-process nucleosynthesis calculations as an example of this flexibility. Finally, we discuss how we plan to make use of this flexibility to extend FREYA to calculations of cumulative fission product yields to aid evaluations of these yields in the future.

Parameter Optimization and Sensitivity Studies of Spontaneous Fission with FREYA

For many years, the state of the art for simulating fission in transport codes amounted to sampling from average distributions. However, such "average" fission models have limited capabilities. Energy is not explicitly conserved and no correlations are available because all particles are emitted independently. However, in a true fission event, the emitted particles are correlated. Recently, Monte Carlo codes generating complete fission events have been developed, thus allowing the use of event-by-event analysis techniques. Such techniques are particularly useful because the complete kinematic information is available for the fission products and the emitted neutrons and photons. It is therefore possible to extract any desired observables, including correlations. The fast event-by-event fission code FREYA (Fission Reaction Event Yield Algorithm) generates large samples of complete fission events, employing only a few physics-based parameters. A recent optimization of these parameters for the isotopes in FREYA that undergo spontaneous fission is described and results are presented. The sensitivity of neutron observables in FREYA to the input yield functions is also discussed and the correlation between the average neutron multiplicity and fragment total kinetic energy is quantified.

Oxygen-catalysed sequential singlet fission

Singlet fission is the photoinduced conversion of a singlet exciton into two triplet states of half-energy. This multiplication mechanism has been successfully applied to improve the efficiency of single-junction solar cells in the visible spectral range. Here we show that singlet fission may also occur via a sequential mechanism, where the two triplet states are generated consecutively by exploiting oxygen as a catalyst. This sequential formation of carriers is demonstrated for two acene-like molecules in solution. First, energy transfer from the excited acene to triplet oxygen yields one triplet acene and singlet oxygen. In the second stage, singlet oxygen combines with a ground-state acene to complete singlet fission. This yields a second triplet molecule. The sequential mechanism accounts for approximately 40% of the triplet quantum yield in the studied molecules; this process occurs in dilute solutions and under atmospheric conditions, where the single-step SF mechanism is inactive.

First record of a-sexual reproduction by fission in Holothuria (lessonothuria) verrucosa (Selenka, 1867) from coastal waters of Karachi, Pakistan

An experiment of a-sexual reproduction by fission was conducted on sea cucumber, Holothuria (Lessonothuria) verrucosa (Selenka, 1867) for 98 days. Anterior fission was observed on Day 1 with the formation of fission on the body of the sea cucumber. Initial length of 14 cm and weight of 21 g was measured at Day 1 in an aquarium under laboratory condition. Complete fission was observed on Day 7 of experiment by formation of 2 specimens; A and P. Initial length and weight for A was 4.5 cm and 3.18 g, while P was 9.5 cm and 17.30 g respectively. Increased weight for specimen A was observed on Day 20 with the first observation of the formation of anal pore. Tentacles started to form on specimen P on Day 34. Complete formation of anus on specimen A was seen on the 98th day of experiment with final weight of 3.84 g and length of 4.6 cm. Large growth of tentacle was also seen on Day 98 of the experiment with a final weight of the specimen P as 17.56 g and length as 9.7 cm.

Fission Reaction Event Yield Algorithm FREYA 2.0.2

FREYA (Fission Reaction Event Yield Algorithm) is a fission event generator which models complete fission events. As such, it automatically includes fluctuations as well as correlations between observables, resulting from conservation of energy and momentum. The purpose of this paper is to present the main differences between FREYA versions 1.0 and 2.0.2 : additional fissionable isotopes, angular momentum conservation, Giant Dipole Resonance form factor for the statistical emission of photons, improved treatment of fission photon emission using RIPL database, and dependence on the incident neutron direction. FREYA 2.0.2 has been integrated into the LLNL Fission Library 2.0.2, which has itself been integrated into MCNP6.2, TRIPOLI-4.10, and can be called from Geant4.10. New version program summaryProgram title:FREYA 2.0.2Program Files doi: ” http://dx.doi.org/10.17632/2mssy7r3gt.1”Licensing provisions: BSD 3-clauseProgramming language: Fortran 90, C++Journal Reference of previous version: J. M Verbeke, J. Randrup, R. Vogt, “Fission Reaction Yield Algorithm FREYA for Event-by-Event Simulation of Fission,” Comp. Phys. Comm. 191, pp. 178–202; doi:10.1016/j.cpc.2015.02.002 (2015).Does the new version supersede the previous version? Yes.Reasons for the new version: New physics and more fissionable isotopes.Summary of revisions: Additional fissionable isotopes, angular momentum conservation, Giant Dipole Resonance form factor for the statistical emission of photons, improved treatment of fission photon emission using RIPL database, and dependence on the incident neutron direction.Nature of problem: Modeling of fission events.Solution method: Simulation of complete fission events, production of secondary fission fragments, fission neutrons and photons.Restrictions: Restricted to spontaneous fission of 238U, 238Pu, 240Pu, 242Pu, 244Cm, 252Cf; neutron-induced fission of 233U, 235U, 238U, 239Pu, 241Pu, for incident neutron energies less than 20 MeV.

Neutron-neutron and neutron-photon correlations withFREYA

For many years, the state of the art for modeling fission in radiation transport codes has involved sampling from average distributions. However, in a true fission event, the energies, momenta and multiplicities of emitted particles are correlated. The FREYA (Fission Reaction Event Yield Algorithm) code generates complete fission events. Event-by-event techniques such as those of FREYA are particularly useful because it is possible to obtain complete kinematic information on the prompt neutrons and photons emitted during the fission process. It is therefore possible to extract any desired correlation observables. We describe FREYA and compare our results with neutron-neutron, neutron-light fragment and neutron-photon correlation data.

Sensitivity of the 252Cf(sf) neutron observables to the FREYA input yield functions Y(A, Z, TKE)

Within the framework of the fission event generator FREYA, we are studying the sensitivity of various neutron observables to the yield distribution Y (A ,Z ,TKE) used as input to the code. Concentrating on spontaneous fission of 252 Cf, we have sampled a large number of different input yield functions based on χ2 fits to the experimental data on Y (A ) and Y (TKE|A ). For each of these input yield distributions, we then use FREYA to generate a large sample of complete fission events from which we extract a variety of neutron observables, including the multiplicity distribution, the associated correlation coefficients, and its factorial moments, the dependence of the mean neutron multiplicity on the total fragment kinetic energy TKE and on the fragment mass number A , the neutron energy spectrum, and the two-neutron angular correlation function. In this way, we can determine the variation of these observables resulting from the uncertainties in the experimental mesurements. The imposition of a constraint on the resulting mean neutron multiplicity reduces the variation of the calculated neutron observables and provides a means for shrinking the uncertainties associated with the measured data.

Theoretical study of effects of the entrance channel on the relative yield of complete fusion and quasifission in heavy-ion collisions within a dinuclear system approach

The relative yield of complete fusion and quasifission components for the $^{12}\mathrm{C}+^{204}\mathrm{Pb}, ^{19}\mathrm{F}+^{197}\mathrm{Au}, ^{30}\mathrm{Si}+^{186}\mathrm{W}$, and $^{48}\mathrm{Ca}+^{168}\mathrm{Er}$ reactions which all lead to the compound nucleus $^{216}\mathrm{Ra}$ are analyzed to calculate the entrance channel effects by comparison of capture, complete fusion, and quasifission cross sections, emission barriers (${B}_{\mathrm{fus}}^{*},{B}_{qf}$), as well as complete fusion probability estimated by statistical method within the framework of the dinuclear system model. The difference among complete fusion probabilities calculated by the dinuclear system model for different entrance channels can be explained by the hindrance to complete fusion due to the larger inner fusion barrier ${B}_{\mathrm{fus}}^{*}$ for the transformation of the dinuclear system into a compound nucleus and the increase of the quasifission contribution due to the decreasing of the emission barrier ${B}_{qf}$ of quasifission as a function of the angular momentum. Although these reactions with different entrance channels populate the same compound nucleus $^{216}\mathrm{Ra}$ at similar excitation energies, the model predicts the negligible quasifission probability for reactions having higher entrance channel mass asymmetry and the dominant decay channel is complete fission. For reactions induced by massive projectiles such as $\mathrm{Si}$ and $\mathrm{Ca}$ having lower entrance channel mass asymmetry, the quasifission component is dominant in the evolution of dinuclear system, and the fusion process is extremely hindered.

Stochastic Analog Neutron Transport with TRIPOLI-4 and FREYA: Bayesian Uncertainty Quantification for Neutron Multiplicity Counting

From nuclear safeguards to homeland security applications, the need for the better modeling of nuclear interactions has grown over the past decades. Current Monte Carlo radiation transport codes compute average quantities with great accuracy and performance; however, performance and averaging come at the price of limited interaction-by-interaction modeling. These codes often lack the capability of modeling interactions exactly: for a given collision, energy is not conserved, energies of emitted particles are uncorrelated, and multiplicities of prompt fission neutrons and photons are uncorrelated. Many modern applications require more exclusive quantities than averages, such as the fluctuations in certain observables (e.g., the neutron multiplicity) and correlations between neutrons and photons. In an effort to meet this need, the radiation transport Monte Carlo code TRIPOLI-4® was modified to provide a specific mode that models nuclear interactions in a full analog way, replicating as much as possible the underlying physical process. Furthermore, the computational model FREYA (Fission Reaction Event Yield Algorithm) was coupled with TRIPOLI-4 to model complete fission events. FREYA automatically includes fluctuations as well as correlations resulting from conservation of energy and momentum.Neutron multiplicity counting (NMC) exploits the correlated nature of fission chains and thus requires analog neutron transport. With the latest analog neutron transport developments in TRIPOLI-4, we show that NMC can now be properly simulated by reconstructing the mass and multiplication of two objects by analyzing the measured signal from 3He tubes in a well counter.

A hemi-fission intermediate links two mechanistically distinct stages of membrane fission.

Fusion and fission drive all vesicular transport. Although topologically opposite, these reactions pass through the same hemi-fusion/fission intermediate1,2, characterized by a ‘stalk’ in which only the inner monolayers of the two compartments have merged to form a localized non-bilayer connection1-3. Formation of the hemi-fission intermediate requires energy input from proteins catalyzing membrane remodeling; however the relationship between protein conformational rearrangements and hemi-fusion/fission remains obscure. Here we analyzed how the GTPase cycle of dynamin, the prototypical membrane fission catalyst4-6, is directly coupled to membrane remodeling. We used intra-molecular chemical cross-linking to stabilize dynamin in its GDP•AlF4--bound transition-state. In the absence of GTP this conformer produced stable hemi-fission, but failed to progress to complete fission, even in the presence of GTP. Further analysis revealed that the pleckstrin homology domain (PHD) locked in its membrane-inserted state facilitated hemi-fission. A second mode of dynamin activity, fueled by GTP hydrolysis, couples dynamin disassembly with cooperative diminishing of the PHD wedging, thus destabilizing the hemi-fission intermediate to complete fission. Molecular simulations corroborate the bimodal character of dynamin action and indicate radial and axial forces as dominant, although not independent drivers of hemi-fission and fission transformations, respectively. Mirrored in the fusion reaction7-8, the force bimodality might constitute a general paradigm for leakage-free membrane remodeling.

Fission Reaction Event Yield Algorithm, FREYA — For event-by-event simulation of fission

From nuclear materials accountability to detection of special nuclear material, SNM, the need for better modeling of fission has grown over the past decades. Current radiation transport codes compute average quantities with great accuracy and performance, but performance and averaging come at the price of limited interaction-by-interaction modeling. For fission applications, these codes often lack the capability of modeling interactions exactly: energy is not conserved, energies of emitted particles are uncorrelated, prompt fission neutron and photon multiplicities are uncorrelated. Many modern applications require more exclusive quantities than averages, such as the fluctuations in certain observables (e.g. the neutron multiplicity) and correlations between neutrons and photons. The new computational model, FREYA (Fission Reaction Event Yield Algorithm), aims to meet this need by modeling complete fission events. Thus it automatically includes fluctuations as well as correlations resulting from conservation of energy and momentum. FREYA has been integrated into the LLNL Fission Library, and will soon be part of MCNPX2.7.0, MCNP6, TRIPOLI-4.9, and Geant4.10. Program summaryProgram title: FREYA 1.0Catalogue identifier: AEVS_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEVS_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Special LLNL licenceNo. of lines in distributed program, including test data, etc.: 187792No. of bytes in distributed program, including test data, etc.: 2585089Distribution format: tar.gzProgramming language: Fortran 90, C++.Computer: Any computer with a Fortran 90 and a C++ compiler, tested with(a) Intel Xeon CPU X5660, 2.8 GHz, 48 GB RAM,(b) Intel(R) Xeon(R) CPU E5-2620, 2 GHz, 64 GB RAM,(c) Intel Xeon CPU W3520, 2.67 GHz, 6 GB RAM.Operating system:(a) Red Hat Enterprise Linux Server release 6.5 (GNU Fortran and g++ (GCC) 4.4.7 20120313 (Red Hat 4.4.7-4)),(b) CentOS release 6.4 (GNU Fortran and g++ (GCC) 4.4.7 20120313 (Red Hat 4.4.7-3)),(c) MacBook Pro OSX 10.6.8 (GNU Fortran and g++-mp-4.7 (MacPorts gcc47 4.7.3 5) 4.7.3).RAM: 6 GBClassification: 17.8.Nature of problem: Modeling of fission events.Solution method: Simulation of complete fission events, production of secondary fission fragments, fission neutrons and photons.Restrictions: Restricted to spontaneous fission of 238 U, 240 Pu, 244 Cm, 252 Cf; neutron-induced fission of 233 U, 235 U, 239 Pu, for incident neutron energies less than 20 MeV.Running time: 8 s for 1M events

Inclusion of Angular Momentum in FREYA

The event-by-event fission model FREYA generates large samples of complete fission events from which any observable can extracted, including fluctuations of the observables and the correlations between them. We describe here how FREYA was recently refined to include angular momentum throughout. Subsequently we present some recent results for both neutron and photon observables.

Event-by-event Modeling of Prompt Neutrons and Photons from Neutron-induced and Spontaneous Fission with FREYA

The fast event-by-event fission code FREYA generates large samples of complete fission events. Using FREYA, it is possible to obtain the fission products as well as the prompt neutrons and photons emitted during the fission process, all with complete kinematic information. We can therefore extract any desired correlation observables. Concentrating on 239Pu(n,f), 235U(n,f) and 252Cf(sf), we compare our FREYA results with available data on prompt neutron and photon emission.

Event-by-event Modeling of Prompt Neutrons and Photons from Neutron-induced and Spontaneous Fission with FREYA

Abstract The fast event-by-event fission code FREYA generates large samples of complete fission events. Using FREYA, it is possible to obtain the fission products as well as the prompt neutrons and photons emitted during the fission process, all with complete kinematic information. We can therefore extract any desired correlation observables. Concentrating on 235 U( n ,f), 239 Pu( n ,f) and 252 Cf(sf), we compare our FREYA results with available data on prompt neutron and photon emission.

Computation of the bond dissociation enthalpies and free energies of hydroxylic antioxidants using the ab initio Hartree–Fock method

IntroductionA new method for calculating theoretical bond dissociation enthalpy (BDE) and bond dissociation free energy (BDFE) of hydroxylic antioxidants is forwarded. BDE and BDFE may be understood as activation energies accompanying the formation of transition states, which may undergo downhill homolytic dissociation. The new method does not involve the complete fission of O–H bonds.MethodTheoretical gas phase BDE values were calculated with the ab initio unrestricted Hartree–Fock (UHF) method, as changes in enthalpy between ground singlet states (GS) and triplet dissociative states (DS). Similarly, gas phase BDFEs were estimated from the corresponding changes in Gibbs free energy. The results were then compared with reliable experimental reports.ResultsThe proposed theoretical approach of BDE and BDFE determination was tested using 10 simple phenols, 5 flavonoids, and l-ascorbic acid derivatives. The agreement between our calculated gas phase results and the adopted experimental values were generally within 0.5 kcal mol−1, with a very few exceptions.DiscussionGenerally, steric interactions as well as intramolecular hydrogen bonding involving the dissociating OH group should be minimized in the GS. The DS are both electronically and vibrationally exited transition states. They have one unpaired electron on the carbon atom, which bears the homolytically dissociating OH group and are second order saddle points with a fixed <C–O–H bond angel of 180°.ConclusionIt was concluded that ab initio UHF was well suited for the estimation of gas phase BDE and BDFE. The method presented has a good potential for application across a range of hydroxylic antioxidants. Currently, work is underway to extend its application in other class of antioxidants.

Computation of the bond dissociation enthalpies and free energies of hydroxylic antioxidants using the ab initio Hartree-Fock method

Introduction A new method for calculating theoretical bond dissociation enthalpy (BDE) and bond dissociation free energy (BDFE) of hydroxylic antioxidants is forwarded. BDE and BDFE may be understood as activation energies accompanying the formation of transition states, which may undergo downhill homolytic dissociation. The new method does not involve the complete fission of O–H bonds. Method Theoretical gas phase BDE values were calculated with the ab initio unrestricted Hartree–Fock (UHF) method, as changes in enthalpy between ground singlet states (GS) and triplet dissociative states (DS). Similarly, gas phase BDFEs were estimated from the corresponding changes in Gibbs free energy. The results were then compared with reliable experimental reports. Results The proposed theoretical approach of BDE and BDFE determination was tested using 10 simple phenols, 5 flavonoids, and l-ascorbic acid derivatives. The agreement between our calculated gas phase results and the adopted experimental values were genera...

Spatio-Temporal Organization of a Minimal Dynamin Machinery Producing Membrane Fission

Dynamin I, a prototype GTPase controlling membrane remodeling in cells, converts the energy of nucleotide hydrolysis into elastic stress driving membrane fission. The energy transduction requires the concerted action of many dynamin subunits, however, it remains unclear how many subunits, and how they are coordinated in time and space. Using lipid nanotubes pulled from different membrane reservoirs we found that 30-50nm long metastable scaffolds formed by dynamin in the presence of GTP are the most fission-effective. Such scaffolds correspond to just a few rungs of dynamin spiral locally squeezing the tube. The shorter structures fail to create the required elastic stress while the longer ones become self-inhibitory. The optimal scaffold produces elastic stress sufficient to induce fission; the stress is further converted into a hemifission-type intermediate structure by the concerted action of the plecstrin homology (PH) domains of dynamin. The PH domains rearrange during the transition state of the GTP hydrolysis allowing for the reversible closure of the nanotube, resembling the fusion pore flickering. This flickering stage is followed by the complete fission which facilitates disassembly of the dynamin scaffold. In this pathway, changes of dynamin scaffold geometry and rigidity that are associated with the GTP hydrolysis become stochastically coupled to the localized remodeling of the lipid bilayer.