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DOE OSTI2020

Green synthesis of reduced Ti 3 C 2 T x MXene nanosheets with enhanced conductivity, oxidation stability, and SERS activity

Transition metal carbides (MXenes) are an emerging family of highly conductive two-dimensional materials with additional functional properties introduced by surface terminations. Further modification of the surface terminations makes MXenes even more appealing for practical applications. Herein, we report a facile and environmentally benign synthesis of reduced Ti 3 C 2 T x MXene (r-Ti 3 C 2 T x ) via a simple treatment with L-ascorbic acid at room temperature. r-Ti 3 C 2 T x shows a six-fold increase in electrical conductivity, from 471 ± 49 for regular Ti 3 C 2 T x to 2819 ± 306 S m -1 for the reduced version. Additionally, we show an enhanced oxidation stability of r-Ti 3 C 2 T x as compared to regular Ti 3 C 2 T x . An examination of the surface-enhanced Raman scattering (SERS) activity reveals that the SERS enhancement factor of r-Ti 3 C 2 T x is an order of magnitude higher than that of regular Ti 3 C 2 T x . The improved SERS activity of r-Ti 3 C 2 T x is attributed to the charge transfer interaction between the MXene surface and probe molecules, re-enforced by an increased electronic density of states (DOS) at the Fermi level of r-Ti 3 C 2 T x . The findings of this study suggest that reduced MXene could be a superior choice over regular MXene, especially for the applications that employ high electronic conductivity, such as electrode materials for batteries and supercapacitors, photodetectors, and SERS-based sensors.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Binding of saturated and unsaturated C 6 -hydrocarbons to the electrophilic anion [B 12 Br 11 ] – : a systematic mechanistic study

The highly reactive gaseous ion [B 12 Br 11 ] – is a metal-free closed-shell anion which spontaneously forms covalent bonds with hydrocarbon molecules, including alkanes. Herein, we systematically investigate the reaction mechanism for binding of [B 12 Br 11 ] – to the five hexane isomers yielding [B 12 Br 11 (C 6 H 14 )] – , as well as to cyclohexane and several hexene isomers (yielding [B 12 Br 11 (C 6 H 12 )] – ) using collision-induced dissociation (CID), infrared photodissociation spectroscopy (IRPD) and computational methods. CID of the different [B 12 Br 11 (C 6 H 14 )] – ions results in distinct fragmentation patterns dependent on the structure of the hexane isomer. Further, the observed fragmentation reactions provide insights into the addition mechanism of [B 12 Br 11 ] – to hexane. Based on the observed CID patterns, we identified that either B–C bond formation through heterolytic C–C or C–H bond cleavages or B–H bond formation through heterolytic C–H cleavage occur dependent on the structure of the hexane isomer. Meanwhile, we observe identical CID spectra of adducts originating from isomers of C 6 H 12 . Spectroscopic investigations of adducts of 1-hexene and cyclohexane indicate the same product structure with an open C 6 chain. Computational investigations evidenced that low lying transition states are present, which enable a ring opening reaction of cyclohexane when binding to [B 12 Br 11 ] – .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

High-pressure high-temperature synthesis and characterization of B 10 C

The boron-rich boron carbide materials have been traditionally synthesized by adding boron powder to B 4 C material and subjecting it to hot pressing sintering for materials composition containing 8.8–20 at. % carbon in boron (composition range of B 10.4 C to B 4 C). Our study explores a synthesis route for B 10 C starting from high-purity boron and carbon and direct conversion under high pressure and high temperature (HPHT) conditions of 2000 °C and 6–8 GPa. Synthesis was verified via x-ray diffraction analysis, showing the conversion of the high-purity boron and carbon powder mixture into a hexagonal B 10 C structure (R-3m space group) with lattice parameters of a = b = 5.6115 Å and c = 12.197 Å. The concentration of boron was measured through x-ray photoelectron spectroscopy, confirming the B 10 C ratio. The measured nanoindentation mean hardness of B 10 C was 40 GPa. Raman spectroscopy of the HPHT synthesized sample shows characteristic vibrational breathing modes of boron icosahedron and an additional intense band at a vibrational frequency of 380 cm –1 . This Raman band, which appears notably weaker in earlier studies and B 4 C samples, is assigned to the linear chain of B–B–B and attributed to the maximal incorporation of boron within the hexagonal structure.

36 MATERIALS SCIENCE↗

DOE OSTI2020

One-colour (~220 nm) resonance-enhanced (S 1 –S 0 ) multi-photon dissociation of acetylene: probe of the C 2 A 1 Π u – X 1 Σ + g band by frequency-modulation spectroscopy

In a recent paper, we demonstrated that one-colour (~220 nm), resonance-enhanced (S 1 –S 0 ), photodissociation of acetylene generates strong $C_2$ Swan band ($d^3Π_g–a^3Π_u$) and $C_2$ Deslandres-d'Azambuja band ($C^1Π_g–A^1Π_u$) fluorescence, and long-lived (>3 µs) fluorescence from an electronically-excited $C_2$H * species. It was not known whether the $C_2A^1Π_u$ and $X^1Σ^+_g$ states are also directly populated in this process. In this paper, multiple vibration-rotation transitions between the $C_2A$-state v = 2 and the X-state v = 0 level are examined by time-resolved frequency-modulation (FM) spectroscopy. The photolysis laser wavelength is tuned into resonance at the one-photon level with S 1 –S 0 transitions that populate individual rotational levels of the S 1 $_{trans}$-conformer $3^2,3^3,$ and $3^4$ vibrational states. By comparing the phase of the FM signals from the $C_2A–X$ transitions with that from the Rb D 1 -line absorption transition, we determine that, for all of the probed A–X transitions, the X-state level is more populated than the A-state level. We propose that the acetylene S 1 level is excited by the second photon to an acetylene dissociation precursor state, which undergoes sequential C-H bond-breaking to produce the $C_2X$ state. The dissociation precursor is assigned as the $1^1B_g(trans-bent)/1^1B_1(cis-bent)$ valence state, which correlates to a doubly-excited configuration, $(1π_u)^2(1π_g)^2$, at linear geometry. Based on the rotational distributions of the $C_2X$-state fragments, we believe that at least one of the transition states involved in the photolysis via S 1 $3^4$ has a larger CC-H bend-angle for the departing H-atom than that involved in the S 1 $3^2$ and $3^3$ photolysis.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2024

A Quantum Mechanical MP2 Study of the Electronic Effect of Nonplanarity on the Carbon Pyramidalization of Fullerene C 60

Among C 60 ’s diverse functionalities, its potential application in CO 2 sequestration has gained increasing interest. However, the processes involved are sensitive to the molecule’s electronic structure, aspects of which remain debated and require greater precision. To address this, we performed structural optimization of fullerene C 60 using the QM MP2/6–31G* method. The nonplanarity of the optimized icosahedron is characterized by two types of dihedral angles: 138° and 143°. The 120 dihedrals of 138° occur between two hexagons intersecting at C–C bonds of 1.42 Å, while the 60 dihedrals of 143° are observed between hexagons and pentagons at C–C bonds of 1.47 Å. NBO analysis reveals less pyramidal sp 1.78 hybridization for carbons at the 1.42 Å bonds and more pyramidal sp 2.13 hybridization for the 1.47 Å bonds. Electrostatic potential charges range from -0.04 a.u. to 0.04 a.u. on the carbon atoms. Second-order perturbation analysis indicates that delocalization interactions in the C–C bonds of 1.42 Å (143.70 kcal/mol) and 1.47 Å (34.98 kcal/mol) are 22% and 38% higher, respectively, than those in benzene. MP2/Def2SVP calculations yield a correlation energy of 13.49 kcal/mol per electron for C 60 , slightly higher than the 11.68 kcal/mol for benzene. However, the results from HOMO-LUMO calculations should be interpreted with caution. This study may assist in the rational design of fullerene C 60 derivatives for CO 2 reduction systems.

36 MATERIALS SCIENCE↗

DOE OSTI2020

C{sub 2}N{sub 2} Vertical Profile in Titan’s Stratosphere

In this paper, we present the first measurements of the vertical distribution of cyanogen (C{sub 2}N{sub 2}) in Titan’s lower atmosphere at different latitudes and seasons, using Cassini’s Composite Infrared Spectrometer far-infrared data. We also study the vertical distribution of three other minor species detected in our data: methylacetylene (C{sub 3}H{sub 4}), diacetylene (C{sub 4}H{sub 2}), and H{sub 2}O, in order to compare them to C{sub 2}N{sub 2}, but also to get an overview of their seasonal and meridional variations in Titan’s lower stratosphere from 85 km to 225 km. We measured an average volume mixing ratio of C{sub 2}N{sub 2} of 6.2±0.8×10{sup −11} at 125 km at the equator, but poles exhibit a strong enrichment in C{sub 2}N{sub 2} (up to a factor 100 compared to the equator), greater than what was measured for C{sub 3}H{sub 4} or C{sub 4}H{sub 2}. Measuring C{sub 2}N{sub 2} profiles provides constraints on the processes controlling its distribution, such as bombardment by Galactic Cosmic Rays which seem to have a smaller influence on C{sub 2}N{sub 2} than predicted by photochemical models.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2022

First measurement of the $ {\Lambda}_c^{+} $ → pη ' decay

We present the first measurement of the branching fraction of the singly Cabibbo-suppressed (SCS) decay $ {\Lambda}_c^{+} $ → pη ' with η ' → ηπ + π – , using a data sample corresponding to an integrated luminosity of 981 fb – 1 , collected by the Belle detector at the KEKB e + e – asymmetric-energy collider. A significant $ {\Lambda}_c^{+} $ → pη ' signal is observed for the first time with a signal significance of 5.4 σ . The relative branching fraction with respect to the normalization mode $ {\Lambda}_c^{+} $ → pK – π + is measured to be $$ \frac{\mathcal{B}\left({\Lambda}_c^{+}\to p\eta^{\prime}\right)}{\mathcal{B}\left({\Lambda}_c^{+}\to {pK}^{-}{\pi}^{+}\right)}=\left(7.54\pm 1.32\pm 0.73\right)\times {10}^{-3}, $$ where the uncertainties are statistical and systematic, respectively. Using the world-average value of $ \mathcal{B}\left({\Lambda}_c^{+}\to {pK}^{-}{\pi}^{+}\right) $ = (6 . 28 ± 0 . 32) × 10 – 2 , we obtain $$ \mathcal{B}\left({\Lambda}_c^{+}\to p\eta^{\prime}\right)=\left(4.73\pm 0.82\pm 0.46\pm 0.24\right)\times {10}^{-4}, $$ where the uncertainties are statistical, systematic, and from $ \mathcal{B}\left({\Lambda}_c^{+}\to {pK}^{-}{\pi}^{+}\right) $ , respectively.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2022

Partial wave analysis of the charmed baryon hadronic decay $ {\Lambda}_c^{+} $ → Λ π +π 0

Based on e + e - collision samples corresponding to an integrated luminosity of 4.4 fb -1 collected with the BESIII detector at center-of-mass energies between 4.6 GeV and 4.7 GeV, a partial wave analysis of the charmed baryon hadronic decay ${\Lambda}_c^{+}$ → Λπ + π 0 is performed, and the decays ${\Lambda}_c^{+}$ → Λρ(770) + and ${\Lambda}_c^{+}$ → Σ(1385)π are studied for the first time. Making use of the world-average branching fraction $\mathcal{B}$ (${\Lambda}_c^{+}$ → Λπ + π 0 ), their branching fractions are determined to be $\mathcal{B}$(${\Lambda}_c^{+}$ → Λρ(770) + ) = (4.06 ± 0.30 ± 0.35 ± 0.23) x 10 -2 , $\mathcal{B}$(${\Lambda}_c^{+}$ → Σ(1385) + )π 0 ) = (5.86 ± 0.49 ± 0.52 ± 0.35) x 10 -3 , $\mathcal{B}$(${\Lambda}_c^{+}$ → Σ(1385) 0 )π + ) = (6.47 ± 0.59 ± 0.66 ± 0.38) x 10 -3 , where the first uncertainties are statistical, the second are systematic, and the third are from the uncertainties of the branching fractions $\mathcal{B}$ ${\Lambda}_c^{+}$ → Λπ + π 0 and $\mathcal{B}$(Σ(1385) → Λπ). In addition, the decay asymmetry parameters are measured to be αΛρ(770)+ = - 0.763 ± 0.053 ± 0.045, $\alpha$ Σ(1385)+π 0 = -0.917 ± 0.069 ± 0.056, and $\alpha$ Σ(1385) 0 π + = -0.789 ± 0.098 ± 0.056.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

Observation of χ c J → 3 ( K + K − )

By analyzing ( 27.12 ± 0.14 ) × 10 8 ψ ( 3686 ) events collected with the BESIII detector operating at the BEPCII collider, the decay processes χ c J → 3 ( K + K − ) ( J = 0 , 1, 2) are observed for the first time with statistical significances of 8.2 σ , 8.1 σ , and 12.4 σ , respectively. The product branching fractions of ψ ( 3686 ) → γ χ c J , χ c J → 3 ( K + K − ) are presented and the branching fractions of χ c J → 3 ( K + K − ) decays are determined to be B χ c 0 → 3 ( K + K − ) = ( 10.7 ± 1.8 ± 1.1 ) × 10 − 6 , B χ c 1 → 3 ( K + K − ) = ( 4.2 ± 0.9 ± 0.5 ) × 10 − 6 , and B χ c 2 → 3 ( K + K − ) = ( 7.2 ± 1.1 ± 0.8 ) × 10 − 6 , where the first uncertainties are statistical and the second are systematic. Published by the American Physical Society 2024

Astronomy & Astrophysics↗

DOE OSTI2024

Search for the light hadron decay χ c 1 ( 3872 ) → π + π − η

With a data sample corresponding to an integrated luminosity of 11.5 fb − 1 collected with the BESIII detector operating at the BEPCII storage ring, for the first time the light hadron decay χ c 1 ( 3872 ) → π + π − η is searched for. While no significant signal is observed, the upper limits at the 90% confidence level for σ [ e + e − → γ χ c 1 ( 3872 ) ] B [ χ c 1 ( 3872 ) → π + π − η ] at center of mass energies from 4.13 GeV to 4.34 GeV are determined. By normalizing to the χ c 1 ( 3872 ) → π + π − J / ψ decay channel, a 90% confidence level upper limit for the branching fraction ratio R = B [ χ c 1 ( 3872 ) → π + π − η ] / B [ χ c 1 ( 3872 ) → π + π − J / ψ ] < 0.12 is given. These measurements provide important inputs for understanding the internal structure of the χ c 1 ( 3872 ) resonance. Published by the American Physical Society 2024

Astronomy & Astrophysics↗

DOE OSTI2023

Iron‐Catalyzed Intermolecular C−H Amination Assisted by an Isolated Iron‐Imido Radical Intermediate

Here we report the use of a base metal complex [( tBu pyrpyrr 2 )Fe(OEt 2 )] (1-OEt 2 ) ( tBu pyrpyrr 2 2− =3,5-tBu 2 -bis(pyrrolyl)pyridine) as a catalyst for intermolecular amination of C sp3 −H bonds of 9,10-dihydroanthracene (2 a) using 2,4,6-trimethyl phenyl azide (3 a) as the nitrene source. The reaction is complete within one hour at 80 °C using as low as 2 mol % 1-OEt 2 with control in selectivity for single C−H amination versus double C−H amination. Catalytic C−H amination reactions can be extended to other substrates such as cyclohexadiene and xanthene derivatives and can tolerate a variety of aryl azides having methyl groups in both ortho positions. Under stoichiometric conditions the imido radical species [( tBu pyrpyrr 2 )Fe{=N(2,6-Me 2 -4-tBu-C 6 H 2 )] (1-imido) can be isolated in 56 % yield, and spectroscopic, magnetometric, and computational studies confirmed it to be an S = 1 Fe IV complex. Complex 1-imido reacts with 2 a to produce the ferrous aniline adduct [( tBu pyrpyrr 2 )Fe{NH(2,6-Me 2 -4-tBu-C 6 H 2 )(C 14 H 11 )}] (1-aniline) in 45 % yield. Lastly, it was found that complexes 1-imido and 1-aniline are both competent intermediates in catalytic intermolecular C−H amination.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2025

Quantifying spatial and vertical variations in soil C:N relationships in permafrost-affected landscapes

Permafrost regions are experiencing rapid changes that affect carbon (C) and nitrogen (N) cycles, with implications for vegetation dynamics and gas exchanges with the atmosphere. Soil C:N ratio is a key indicator of organic matter quality, yet spatial estimates of N stocks and C:N ratios lag behind those for C. We used quantile regression forests to compare direct and indirect digital soil mapping approaches for predicting soil C:N ratios at 0–30, 30–60, and 60–100 cm depths across a latitudinal transect in Alaska. The indirect approach – deriving C:N from separately predicted C and N stocks – outperformed direct mapping for the surface layer (0–30 cm), while direct mapping was marginally better at greater depths. However, prediction accuracy decreased with depth for both methods. Temperature and topography were the most important predictors. Both approaches overestimated low and underestimated high C:N ratios, with direct mapping showing greater bias. Our results underscore the challenges of modeling C:N ratios in heterogeneous, data-sparse permafrost soils, but also suggest that indirect mapping holds promise if supported by more extensive datasets.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2021

Long-term leaf C:N ratio change under elevated CO 2 and nitrogen deposition in China: Evidence from observations and process-based modeling

Climate change, elevating atmosphere CO 2 (eCO 2 ) and increased nitrogen deposition (iNDEP) are altering the biogeochemical interactions between plants, microbes and soils, which further modify plant leaf carbon-nitrogen (C:N) stoichiometry and their carbon assimilation capability. Many field experiments have observed large sensitivity of leaf C:N ratio to eCO 2 and iNDEP. However, the large-scale pattern of this sensitivity is still unclear, because eCO 2 and iNDEP drive leaf C:N ratio toward opposite directions, which are further compounded by the complex processes of nitrogen acquisition and plant-and-microbial nitrogen competition. Here, we attempt to map the leaf C:N ratio spatial variation in the past 5 decades in China with a combination of data-driven model and process-based modeling. These two approaches showed consistent results. Over different regions, we found that leaf C:N ratio had significant but uneven changes between 2 time periods (1960-1989 and 1990-2015): a 5% ± 8% increase for temperate grasslands in northern China, a 3% ± 6% increase for boreal grasslands in western China, and by contrast, a 7% ± 6% decrease for temperate forests in southern China, and a 3% ± 5% decrease for boreal forests in northeastern China. Additionally, the structural equation models indicated that the leaf C:N change was sensitive to ΔNDEP, ΔCO 2 and ΔMAT rather than ΔMAP and ecosystem types. In this work, process-based modeling suggested that iNDEP was the main source of soil mineral nitrogen change, dominating leaf C:N ratio change in most areas in China, while eCO 2 led to leaf C:N ratio increase in low iNDEP area. This study also indicates that the long-term leaf C:N ratio acclimation was dominated by climate constraint, especially temperature, but was constrained by soil N availability over decade scale.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2024

Mechanism of Ni-Catalyzed Photochemical Halogen Atom-Mediated C(sp 3 )–H Arylation

Within the context of Ni photoredox catalysis, halogen atom photoelimination from Ni has emerged as a fruitful strategy for enabling hydrogen atom transfer (HAT)-mediated C(sp 3 )–H functionalization. Despite the numerous synthetic transformations invoking this paradigm, a unified mechanistic hypothesis that is consistent with experimental findings on the catalytic systems and accounts for halogen radical formation and facile C(sp 2 )–C(sp 3 ) bond formation remains elusive. We employ kinetic analysis, organometallic synthesis, and computational investigations to decipher the mechanism of a prototypical Ni-catalyzed photochemical C(sp 3 )–H arylation reaction. Our findings revise the previous mechanistic proposals, first by examining the relevance of SET and EnT processes from Ni intermediates relevant to the HAT-based arylation reaction. Our investigation highlights the ability for blue light to promote efficient Ni–C(sp 2 ) bond homolysis from cationic Ni III and C(sp 2 )–C(sp 3 ) reductive elimination from bipyridine Ni II complexes. However interesting, the rates and selectivities of these processes do not account for the productive catalytic pathway. Instead, our studies support a mechanism that involves halogen atom evolution from in situ generated Ni II dihalide intermediates, radical capture by a Ni II (aryl)(halide) resting state, and key C–C bond formation from Ni III . Oxidative addition to Ni I , as opposed to Ni 0 , and rapid Ni III /Ni I comproportionation play key roles in this process. Furthermore, the findings presented herein offer fundamental insight into the reactivity of Ni in the broader context of catalysis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2020

Three Decades of Divergent Land Use and Plant Community Change Alters Soil C and N Content in Tallgrass Prairie

Frequent fire and grazing by megafauna are important determinants of tallgrass prairie plant community structure. However, fire suppression and removal of native grazers have altered these natural disturbance regimes and changed grassland plant communities with potential long-term consequences for soil carbon (C) and nitrogen (N) storage. We investigated multidecade changes in soil C and N pools in response to contrasting long-term burning and grazing treatments. Fire suppression with or without grazers and exclusion of grazers in annually burned prairie increased soil C content and shifted the δ 13 C signature of soil C over time, concomitant with changes in plant community composition. Soil δ 13 C values indicated that increased soil C content was associated with an increased contribution from plants using a C 3 photosynthetic pathway (i.e., woody shrubs) under fire suppression. Soil N content also increased when fire was suppressed, relative to frequently burned grassland, but the rate of increase was slower when grazers were present. Additionally, changes in δ 15 N values suggested that grazing increased the openness of the N cycle, presumably due to greater N losses. By coupling long-term fire and grazing treatments with plant community data and soil samples archived over three decades, we demonstrate that human-caused changes to natural disturbance regimes in a tallgrass prairie significantly alter soil C and N cycles through belowground changes associated with shifts in the plant community. Since natural disturbance regimes have been altered in grasslands across the world, our results are relevant for understanding the long-term biogeochemical consequences of these ongoing land use changes.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2024

Myosin-binding protein C regulates the sarcomere lattice and stabilizes the OFF states of myosin heads

Muscle contraction is produced via the interaction of myofilaments and is regulated so that muscle performance matches demand. Myosin-binding protein C (MyBP-C) is a long and flexible protein that is tightly bound to the thick filament at its C-terminal end (MyBP-C C8C10 ), but may be loosely bound at its middle- and N-terminal end (MyBP-C C1C7 ) to myosin heads and/or the thin filament. MyBP-C is thought to control muscle contraction via the regulation of myosin motors, as mutations lead to debilitating disease. We use a combination of mechanics and small-angle X-ray diffraction to study the immediate and selective removal of the MyBP-C C1C7 domains of fast MyBP-C in permeabilized skeletal muscle. We show that cleavage leads to alterations in crossbridge kinetics and passive structural signatures of myofilaments that are indicative of a shift of myosin heads towards the ON state, highlighting the importance of MyBP-C C1C7 to myofilament force production and regulation.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2021

Evolution of a σ–(c-di-GMP)–anti-σ switch

Filamentous actinobacteria of the genus Streptomyces have a complex lifecycle involving the differentiation of reproductive aerial hyphae into spores. We recently showed c-di-GMP controls this transition by arming a unique anti-σ, RsiG, to bind the sporulation-specific σ, WhiG. The Streptomyces venezuelae RsiG–(c-di-GMP) 2 –WhiG structure revealed that a monomeric RsiG binds c-di-GMP via two E(X) 3 S(X) 2 R(X) 3 Q(X) 3 D repeat motifs, one on each helix of an antiparallel coiled-coil. Here we show that RsiG homologs are found scattered throughout the Actinobacteria. Strikingly, RsiGs from unicellular bacteria descending from the most basal branch of the Actinobacteria are small proteins containing only one c-di-GMP binding motif, yet still bind their WhiG partners. Our structure of a Rubrobacter radiotolerans (RsiG) 2 –(c-di-GMP) 2 –WhiG complex revealed that these single-motif RsiGs are able to form an antiparallel coiled-coil through homodimerization, thereby allowing them to bind c-di-GMP similar to the monomeric twin-motif RsiGs. Further data show that in the unicellular actinobacterium R. radiotolerans , the (RsiG) 2 –(c-di-GMP) 2 –WhiG regulatory switch controls type IV pilus expression. Phylogenetic analysis indicates the single-motif RsiGs likely represent the ancestral state and an internal gene-duplication event gave rise to the twin-motif RsiGs inherited elsewhere in the Actinobacteria. Thus, these studies show how the anti-σ RsiG has evolved through an intragenic duplication event from a small protein carrying a single c-di-GMP binding motif, which functions as a homodimer, to a larger protein carrying two c-di-GMP binding motifs, which functions as a monomer. Consistent with this, our structures reveal potential selective advantages of the monomeric twin-motif anti-σ factors.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2021

INTEGRATED RISK ASSESSMENT OF DIGITAL I&C SAFETY SYSTEMS FOR NUCLEAR POWER PLANTS

Upgrading the existing analog instrumentation and control (I&C) systems to state-of-the-art digital I&C (DI&C) systems provides the foremost means to improve performance and reduce costs for existing light-water reactors (LWRs). However, qualification of digital technologies remains a challenge?especially the issue of software common cause failure (CCF), which has been difficult to address. Existing analyses of CCFs in I&C systems mainly focus on hardware failures. With the application and upgrading of new DI&C systems, software CCFs due to design flaws might become a potential threat to plant safety, considering that most redundancy designs use similar digital platforms or software in their operating and application systems. With complex multi-layer redundancy designs to meet the single failure criterion, these I&C safety systems are of particular concern in U.S. Nuclear Regulatory Commission (NRC) licensing procedures. In 2019, the Risk-Informed Systems Analysis (RISA) Pathway of the U.S. Department of Energy?s (DOE?s) Light Water Reactor Sustainability (LWRS) Program initiated a project to develop a risk assessment strategy for delivering a strong technical basis to support effective, licensable, secure DI&C technologies for digital upgrades/designs. An integrated risk assessment for the DI&C (RADIC) process was proposed for this strategy to identify potential key digital-induced failures, implement reliability analyses of related digital safety I&C systems, and evaluate the unanalyzed sequences introduced by these failures (particularly software CCFs) at the plant level. This paper summarizes these RISA efforts in the risk analysis of safety-related DI& systems at Idaho National Laboratory.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗