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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

The interplay between chromophore and protein determines the extended excited state dynamics in a single-domain phytochrome

Phytochromes are a diverse family of bilin-binding photoreceptors that regulate a wide range of physiological processes. Their photochemical properties make them attractive for applications in optogenetics and superresolution microscopy. Phytochromes undergo reversible photoconversion triggered by the Z ⇌ E photoisomerization about the double bond in the bilin chromophore. However, it is not fully understood at the molecular level how the protein framework facilitates the complex photoisomerization dynamics. In this work, we have studied a single-domain bilin-binding photoreceptor All2699g1 ( Nostoc sp. PCC 7120) that exhibits photoconversion between the red light-absorbing (P r ) and far red-absorbing (P fr ) states just like canonical phytochromes. We present the crystal structure and examine the photoisomerization mechanism of the P r form as well as the formation of the primary photoproduct Lumi-R using time-resolved spectroscopy and hybrid quantum mechanics/molecular mechanics simulations. We show that the unusually long excited state lifetime (broad lifetime distribution centered at ~300 picoseconds) is due to the interactions between the isomerizing pyrrole ring D and an adjacent conserved Tyr142. The decay kinetics shows a strongly distributed character which is imposed by the nonexponential protein dynamics. Our findings offer a mechanistic insight into how the quantum efficiency of the bilin photoisomerization is tuned by the protein environment, thereby providing a structural framework for engineering bilin-based optical agents for imaging and optogenetics applications.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

Observing 3-hydroxyanthranilate-3,4-dioxygenase in action through a crystalline lens

The synthesis of quinolinic acid from tryptophan is a critical step in the de novo biosynthesis of nicotinamide adenine dinucleotide (NAD + ) in mammals. Herein, the nonheme iron-based 3-hydroxyanthranilate-3,4-dioxygenase responsible for quinolinic acid production was studied by performing time-resolved in crystallo reactions monitored by UV-vis microspectroscopy, electron paramagnetic resonance (EPR) spectroscopy, and X-ray crystallography. Seven catalytic intermediates were kinetically and structurally resolved in the crystalline state, and each accompanies protein conformational changes at the active site. Among them, a monooxygenated, seven-membered lactone intermediate as a monodentate ligand of the iron center at 1.59-Å resolution was captured, which presumably corresponds to a substrate-based radical species observed by EPR using a slurry of small-sized single crystals. Other structural snapshots determined at around 2.0-Å resolution include monodentate and subsequently bidentate coordinated substrate, superoxo, alkylperoxo, and two metal-bound enol tautomers of the unstable dioxygenase product. These results reveal a detailed stepwise O-atom transfer dioxygenase mechanism along with potential isomerization activity that fine-tunes product profiling and affects the production of quinolinic acid at a junction of the metabolic pathway.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Imaging conical intersection dynamics during azobenzene photoisomerization by ultrafast X-ray diffraction

Significance Vibronic coherences are unique features that emerge in the decisive moments of photochemistry and photophysics. Here we demonstrate how X-ray diffraction can access fundamental information about these coherences. This is enabled by recent developments at free-electron lasers, working toward temporal resolution and single-molecule accessibility of scattering-based measurements. Time-resolved diffraction patterns of the isomerization of azobenzene are simulated. The process is monitored in time via the momentum-space signal, with a special focus on the mixed elastic/inelastic scattering from coherences. We give practical ideas on how this relatively weak contribution to the signal can potentially be retrieved. A direct connection to the real-space movie of the conical intersection dynamics is made, enabling the observation of quantum coherences that direct chemical processes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Bivalent molecular mimicry by ADP protects metal redox state and promotes coenzyme B 12 repair

Control over transition metal redox state is essential for metalloprotein function and can be achieved via coordination chemistry and/or sequestration from bulk solvent. Human methylmalonyl-Coenzyme A (CoA) mutase (MCM) catalyzes the isomerization of methylmalonyl-CoA to succinyl-CoA using 5′-deoxyadenosylcobalamin (AdoCbl) as a metallocofactor. During catalysis, the occasional escape of the 5′-deoxyadenosine (dAdo) moiety leaves the cob(II)alamin intermediate stranded and prone to hyperoxidation to hydroxocobalamin, which is recalcitrant to repair. In this study, we have identified the use of bivalent molecular mimicry by ADP, coopting the 5′-deoxyadenosine and diphosphate moieties in the cofactor and substrate, respectively, to protect against cob(II)alamin overoxidation on MCM. Crystallographic and electron paramagnetic resonance (EPR) data reveal that ADP exerts control over the metal oxidation state by inducing a conformational change that seals off solvent access, rather than by switching five-coordinate cob(II)alamin to the more air stable four-coordinate state. Subsequent binding of methylmalonyl-CoA (or CoA) promotes cob(II)alamin off-loading from MCM to adenosyltransferase for repair. This study identifies an unconventional strategy for controlling metal redox state by an abundant metabolite to plug active site access, which is key to preserving and recycling a rare, but essential, metal cofactor.

Science & Technology - Other Topics↗
DOE OSTI2025

Structural dynamics of laser-ionized cis-stilbene studied by ultrafast electron diffraction

Stilbene has been a model system for studying photoisomerization upon absorption of a UV photon. Although the dynamics of the first excited state have been investigated in great detail, the structural dynamics of cationic states remains largely experimentally unexplored. In this study, the dynamics following ionization via absorption of two UV photons was captured using ultrafast electron diffraction. cis-stilbene (CS) was optically pumped with 267 nm ultraviolet light with two different pump fluences and probed with 3.7 MeV electrons. We compare our experimental results to ab-initio multiple spawning simulations for single-photon excitation and molecular dynamics simulations for ionization in order to separate the contributions from the two channels. We found that with a fluence of 170 mJ cm −2 both the single and two-photon channels are present, while with higher fluence of 280 mJ cm −2 the two-photon channel dominates. We did not observe isomerization of the CS cation and found the main structural motion a to be a vibration of the phenyl rings about the central C–C bond.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2025

Structure of beta-decaying states in the deformed, neutron-rich nucleus 104 Nb

Excited structures in 104 Mo were populated by β decays of the ground and isomeric states in the neutron-rich nucleus 104 Nb. The beams were produced by the CARIBU facility at Argonne National Laboratory, re-accelerated by the ATLAS accelerator and implanted on a moving-tape system in the middle of the GAMMASPHERE array. Separate decay schemes for the two β-decaying states in 104 Nb were constructed for the first time. The structure of the isomers are discussed in the framework of the deformed Nilsson model and systematics of known quasiparticle structures in neighboring nuclei.

Nilsson model↗
DOE OSTI2021

The NUBASE2020 evaluation of nuclear physics properties *

The NUBASE2020 evaluation contains the recommended values of the main nuclear physics properties for all nuclei in their ground and excited, isomeric (T-1/2 >= 100 ns) states. It encompasses all experimental data published in primary (journal articles) and secondary (mainly laboratory reports and conference proceedings) references, together with the corresponding bibliographical information. In cases where no experimental data were available for a particular nuclide, trends in the behavior of specific properties in neighboring nuclei were examined and estimated values are proposed. Evaluation procedures and policies that were used during the development of this evaluated nuclear data library are presented, together with a detailed table of recommended values and their uncertainties.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2026

Lifetimes of two- and four-neutron-hole states in 204 Pb and 206 Pb

The centroid-shift method was used to determine half-lives of excited levels in 204 Pb and 206 Pb. For the states with 𝐼 𝜋 = 12 + , 17 − , and 19 − in 204 Pb, values of 𝑇 1/2 = 4.9⁢(3), 3.9(3), and 0.9(3) ns, respectively, were determined. The half-life of the 𝐼 𝜋 = 9 − level in 206 Pb was measured to be 1.1(3) ns. In 204 Pb, the 12 + state arises from two neutron holes in the 𝑖 13/2 subshell, while the 17 − and 19 − levels are built on 𝜈⁡(𝑖$^{−3}_{13/2}$, 𝑝$^{−1}_{1/2}$) and 𝜈⁡(𝑖$^{−3}_{13/2}$, 𝑓$^{−1}_{5/2}$) four-neutron-hole configurations, respectively. The 9 − state in 206 Pb results from a 𝜈⁡(𝑖$^{−1}_{13/2}$, 𝑓${−1}_{5/2}$) excitation. As a result, the transition probabilities for the decay of these isomeric levels are reasonably consistent with the expected single-particle values, and these states are described well by shell-model calculations using the kshell code with the KHHE interaction.

190 ≤ A ≤ 219↗
DOE OSTI2026

First Observation of Multiphonon 𝛾-Vibrations in an Odd-Odd Nuclear System

Here, the identification of the first multiphonon 𝛾-vibrational bands in an odd-odd neutron-rich nucleus of the nuclear chart is presented. These high-spin structures of hard to access $^{104}_{41}\textrm{Nb}_{63}$, produced in fission, were studied by combining a spectrometer with isotopic resolution coupled to a 𝛾-ray tracking array and independently from high-fold 𝛾 coincidence measurements. Triaxial projected shell model calculations for the high-spin states are in good agreement with the measured observables for the yrast, one-phonon, and two-phonon 𝛾 bands. The possibility of an oblate shape of an isomeric state and coexistence of triaxial and oblate configurations is investigated from the decay of the 141 keV isomer. The present Letter illustrates the robustness of vibration excitations in the presence of an odd valence proton and neutron as well as the possibly coexisting shapes beyond the 𝑁 = 60 transitional region.

Wang, E. H. [Shandong Univ., Jinan (China); Vander↗
DOE OSTI2025

Sensitivity of nEXO to 136 Xe charged-current interactions: Background-free searches for solar neutrinos and fermionic dark matter

We study the sensitivity of nEXO to solar neutrino charged-current interactions, 𝜈 𝑒 + 136 Xe → 136 Cs*+𝑒 − , as well as analogous interactions predicted by models of fermionic dark matter. Due to the recently observed low-lying isomeric states of 136 Cs, these interactions will create a time-delayed coincident signal observable in the scintillation channel. Here we develop a detailed Monte Carlo simulation of scintillation emission, propagation, and detection in the nEXO detector to model these signals under different assumptions about the timing resolution of the photosensor readout. We show this correlated signal can be used to achieve background discrimination on the order of 10 −9 , enabling nEXO to make background-free measurements of solar neutrinos above the reaction threshold of 0.668 MeV. We project that nEXO could measure the flux of neutrinos from the carbon-nitrogen-oxygen cycle with a statistical uncertainty of 25%, thus contributing a novel and competitive measurement toward addressing the solar metallicity problem. Additionally, nEXO could measure the mean energy of the 7 Be neutrinos with a precision of 𝜎 ≤ 1.5 keV and could determine the survival probability of 7 Be and pep solar 𝜈𝑒 with precision comparable to the state of the art. These quantities are sensitive to the Sun’s core temperature and to nonstandard neutrino interactions, respectively. Here, the strong background suppression would allow nEXO to search for charged-current interactions of fermionic dark matter in the mass range 𝑚 𝜒 = 0.668–7 MeV with a sensitivity up to three orders of magnitude better than current limits.

47 OTHER INSTRUMENTATION↗
DOE OSTI2025

Core-excited microsecond isomer in 207 Pb

An isomeric state with T 1/2 = 1.46(10) μs and E x = 8835 keV has been established in 207 Pb. Here, this state likely arises from the occupation of neutron holes in the high-j i 13/2 and j 15/2 subshells following the excitation of the 208 Pb core. The high-spin excited level structure of 207 Pb has been extended beyond the previously known states up to E x ≈ 6 MeV. Additionally, a more precise value of the half-life of the I π = 10 + isomer in 208 Pb has been obtained. Shell-model calculations provide a good account of the structure up to the 31/2 + level in 207 Pb, while the energies of higher-spin states built from exclusively high-j nucleons are considerably underestimated.

190 ≤ A ≤ 219↗
DOE OSTI2025

Reduction in Nuclear Size and Quadrupole Deformation of High-Spin Isomers of 127,129 In

We employed laser spectroscopy of atomic transitions to measure the nuclear charge radii and electromagnetic properties of the high-spin isomeric states in neutron-rich indium isotopes (𝑍 =49) near the closed proton and neutron shells at 𝑍 = 50 and 𝑁 = 82. Our data reveal a reduction in the nuclear charge radius and intrinsic quadrupole moment when protons and neutrons are fully aligned in 129 In ⁢(𝑁 = 80), to form the high spin isomer. Such a reduction is not observed in 127 In ⁢(𝑁 = 78), where more complex configurations can be formed by the existence of four neutron holes. These observations are not consistently described by nuclear theory.

high spin states↗
DOE OSTI2021

Measurements of Short-Lived Isomers from Photofission as a Method of Active Interrogation for Special Nuclear Materials

Isomeric fission products, such as 134m Te (T 1/2 = 164 ns) and 136m Xe (T 1/2 = 2.95 μs), are proposed as a signature of special nuclear materials in active interrogation applications. To test this conjecture, monoenergetic 9, 11, and 13 MeV photons from the HIγS facility are used to induce fission of 235 U, 238 U, and 239 Pu targets. The de-excitation γ rays of the isomers are measured using time-gated spectroscopy with high-purity germanium detectors. The 134m Te and 136m Xe isomers are detected and identified by the energy and decay half-life of their characteristic γ-ray transitions. The ratio of yields for these two signature γ rays, corresponding to 134m Te/( 136m Xe + 136 I), is found to be strongly correlated with the identity of the fissioning nuclei. These results show that fission-product isomers may be used in active interrogation to detect and identify special nuclear materials, even providing information on the isotopic enrichment. In conclusion, the feasibility of an active interrogation scenario using a bremsstrahlung beam is discussed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Analysis of the 18 F g,m (d,p) 19 F reactions in the rotational model

Spectroscopic strengths for the 18 F g , m ( d , p ) F 19 reactions to members of the ground-state band in F 19 are interpreted in the rotational model. The analysis considers two different coupling schemes, namely, the strong and decoupled limits of the particle rotor model to describe the ground and isomeric states of F 18 , as well as the transitions to the ground-state band in F 19 . Our results, obtained using Nilsson amplitudes for the neutron levels 1 2 [ 220 ] , 3 2 [ 211 ] , and 5 2 [ 202 ] calculated at a deformation ε 2 = 0.32 are in agreement with the measurements and USDB shell-model calculations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

New, low-energy excitations in 107 Mo and 109 Mo

New ground-state level with spin-parity 1/2 + is established in 109 Mo, 69.8 keV below the previously reported 5/2(+) ground state in this nucleus, based on precise spectroscopy measurements of gamma radiation following spontaneous fission of 248 Cm performed using the Eurogam2 array of anti-Compton spectrometers. Analogous measurement of gamma radiation following spontaneous fission of 252 Cf, performed using the Gammasphere array, confirms the new 1/2 + ground state of 107 Mo, proposed recently and establishes the isomeric character of the 5/2 + first excited state in 107 Mo. Two beta-decaying isomers are suggested in 111 Mo nucleus based on regular energy systematics, supporting previous predictions. Low-energy excitations in Mo isotopes are interpreted and compared to calculations reported in the literature. The results suggest shape transition from prolate to oblate deformation at N >= 67.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Shell-model studies of the astrophysical r p -process reactions S 34 ( p , γ ) Cl 35 and Cl 34 g , m ( p , γ ) Ar 35

Background: Dust grains condensed in the outflows of presolar classical novae should have been present in the protosolar nebula. Candidates for such presolar nova grains have been found in primitive meteorites and can in principle be identified by their isotopic ratios, but the ratios predicted by state-of-the-art one-dimensional hydrodynamic models are uncertain due to nuclear-physics uncertainties. Purpose: To theoretically calculate the thermonuclear rates and uncertainties of the S 34 ( p , γ ) Cl 35 and Cl 34 g , m ( p , γ ) Ar 35 reactions and investigate their impacts on the predicted 34 S/ 32 S isotopic ratio for presolar nova grains. Method: A shell-model approach in a (0 +1 ) ℏ ω model space was used to calculate the properties of resonances in the 34 S(p ,γ ) 35 Cl and 34 g,m Cl (p ,γ ) 35 Ar reactions and their thermonuclear rates. Uncertainties were estimated using a Monte Carlo method. The implications of these rates and their uncertainties on sulfur isotopic nova yields were investigated using a postprocessing nucleosynthesis code. The rates for transitions from the ground state of 34 Cl as well as from the isomeric first excited state of 34 Cl were explicitly calculated. Results: At energies in the resonance region near the proton-emission threshold, many negative-parity states appear. Energies, spectroscopic factors, and proton-decay widths are reported. The resulting thermonuclear rates are compared with previous determinations. Conclusions: The shell-model calculations alone are sufficient to constrain the variation of the 34 S/ 32 S ratios to within about 30%. Uncertainties associated with other reactions must also be considered, but in general we find that the 34 S/ 32 S ratios are not a robust diagnostic to clearly identify presolar grains made from nova ejecta.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Structure of Nd 155 and Gd 163 from Cf 252 spontaneous fission

Background: A puzzle has arisen recently caused by the apparent shift in maximum deformation from the expected 66 Dy isotopic chain to the Nd 60 isotopic chain in the 82 155 Nd and 163 Gd, useful for constraining parameters in models that seek to answer the six proton shift in maximum deformation. Method: Data from the spontaneous fission of 252 Cf were taken by the Gammasphere detector array at Lawrence Berkeley National Laboratory to observe the excited states of 155 Nd and 163 Gd. Results: The structure of 163 Gd has been expanded with the addition of two new levels and three new γ rays, which are found to be consistent with previously published calculations and the structure of 165 Dy. In 155 Nd, nine new levels and 12 new γ rays are observed. The spins and parities of the previously known levels in 155 Nd have been reassigned from a ν3/2 - [521] ground state configuration to a ν5/2 + [642] isomeric configuration by comparison of these newly observed levels with levels in 153 Nd and 155 Sm. Conclusion: Further experimentation is required to determine the energy of the newly reassigned ν5/2+[642] level in 155 Nd with respect to the suspected ν3/2-[521] ground state. Additionally, more experiments should be conducted to further determine the structure of neutron rich nuclei, rarely produced in the spontaneous fission of 252 Cf, such as 163 Gd.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗