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At least 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Surface Dynamics of Selective Hydration of Rutile TiO 2 : A Kinetic Monte Carlo Approach

The utility of atomic layer deposition (ALD) continues to expand beyond conformal thin-film fabrication to include area- or site-selective ALD. We previously identified a strategy for site-selective ALD (SS-ALD) via the evaluation of thermodynamically preferred hydration of rutile TiO 2 surfaces, as deduced from electronic structure calculations. Here, we report a novel kinetic Monte Carlo (KMC) model that allows for the investigation of surface dynamics and kinetics that improves our understanding of and intuition for the selective hydration strategy. Here we demonstrate the validity of the strategy with respect to step-edge defects for the lowest energy (110) facet as well as report results for the other common facets which agree with experimental STM observations. The results here indicate that the selective hydration strategy is feasible both thermodynamically (evaluated in our previous publication) and kinetically (from the KMC model). Because diffusion has a slower rate than others, we find that any proximity effects between terrace and defect sites are unlikely to affect the selective hydration strategy for rutile TiO 2 . The KMC model further provides relevant timescales for achieving selectivity experimentally and establishes the kinetic viability of the selective hydration approach to SS-ALD.

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

Atomically Dispersed Tin-Modified $\gamma$-alumina for Selective Propane Dehydrogenation under H 2 S Co-feed

Developing an earth-abundant catalyst that is sulfur-tolerant, active, and highly selective is of great interest for valorizing natural gas streams containing sour gas. Here, a tin-modified alumina catalyst is reported that is stable and selective for propane dehydrogenation in the presence of percent quantities of H 2 S in the feed. In particular, Sn/Al 2 O 3 –S catalysts with 1.5–5% Sn content exhibit 98% selectivity with up to 16% conversion at 560 °C during the fourth cycle. Experimental and computational characterization shows that the active sites are the defect tricoordinated Al atoms. H 2 S pretreatment further modifies a portion of these sites via exchanging a neighboring oxygen atom with sulfur, thereby rendering them more active and selective. At low loadings, Sn is atomically dispersed and selectively binds to hydroxyl groups or oxygen atoms on Al 2 O 3 . This prevents the formation of original (unmodified) defect sites on Al 2 O 3 and improves overall selectivity. The activity and selectivity of the catalyst are heavily dependent on the chemical potential of sulfur and hydrogen because they influence both the relative concentration of the two types of sites and the overall reaction mechanism. Finally, the catalyst can be regenerated fully under a pure H 2 S stream, thereby precluding treatment under oxygen, which can lead to sintering.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Operando XAS and DFT Uncover Structure-Performance Relationships in Re/TiO 2 for Selective CO 2 Hydrogenation to Methanol

The conversion of CO 2 into value-added chemicals, such as methanol, offers a promising pathway toward a renewable energy future. However, a precise kinetic control and a highly selective catalyst are necessary to overcome the thermodynamic preference for CO 2 hydrogenation to methane. Rhenium-based catalysts, particularly Re/TiO 2 , demonstrate high activity and selectivity for methanol under high-pressure conditions. For example, at 100 bar and 200 °C, a methanol selectivity of 97−99% was obtained. Catalysts with 1 wt % Re and 5 wt % Re/ TiO 2 were used to study the effect of cluster sizes. At 250 °C, the 1 wt % catalyst achieves 97% selectivity at 23% conversion, whereas 5 wt % Re/TiO 2 achieves 74% selectivity at 40% conversion, corresponding to a drop in space-time yield from 65 to 16 g CH 3 OH ·g Re −1 ·h −1 , respectively. X-ray absorption spectroscopy provided insights into the structure of the active sites, while density functional theory calculations revealed the effects of cluster size on the energy barriers for H 2 activation, CH 3 OH dissociation, and CH 3 OH desorption, all of which directly influence conversion and selectivity. These results underscore the importance of balancing cluster size for optimal catalyst performance and provide insights into the design of efficient and selective catalysts for renewable methanol production.

XAS↗
DOE OSTI2018

Selective CO Production by Photoelectrochemical Methane Oxidation on TiO 2

The inertness of the C–H bond in CH 4 poses significant challenges to selective CH 4 oxidation, which often proceeds all the way to CO 2 once activated. Selective oxidation of CH 4 to high-value industrial chemicals such as CO or CH 3 OH remains a challenge. Presently, the main methods to activate CH 4 oxidation include thermochemical, electrochemical, and photocatalytic reactions. Of them, photocatalytic reactions hold great promise for practical applications but have been poorly studied. Existing demonstrations of photocatalytic CH 4 oxidation exhibit limited control over the product selectivity, with CO 2 as the most common product. The yield of CO or other hydrocarbons is too low to be of any practical value. In this work, we show that highly selective production of CO by CH4 oxidation can be achieved by a photoelectrochemical (PEC) approach. Under our experimental conditions, the highest yield for CO production was 81.9%. The substrate we used was TiO 2 grown by atomic layer deposition (ALD), which features high concentrations of Ti 3+ species. The selectivity toward CO was found to be highly sensitive to the substrate types, with significantly lower yield on P25 or commercial anatase TiO 2 substrates. Moreover, our results revealed that the selectivity toward CO also depends on the applied potentials. Based on the experimental results, we proposed a reaction mechanism that involves synergistic effects by adjacent Ti sites on TiO 2 . Spectroscopic characterization and computational studies provide critical evidence to support the mechanism. Furthermore, the synergistic effect was found to parallel heterogeneous CO 2 reduction mechanisms. Our results not only present a new route to selective CH 4 oxidation, but also highlight the importance of mechanistic understandings in advancing heterogeneous catalysis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Selectivity-Determining Steps in O 2 Reduction Catalyzed by Iron(tetramesitylporphyrin)

The oxygen reduction reaction (ORR) is the cathode reaction in fuel cells and its selectivity for water over hydrogen peroxide production is important for these technologies. Iron porphyrin catalysts have long been studied for the ORR, but the origins of their selectivity are not well understood because the selectivity-determining step(s) usually occur after the rate-determining step. We report here the effects of acid concentration, as well as other solution conditions such as acid pK a , on the H 2 O 2 /H 2 O selectivity in electrocatalytic ORR by iron(tetramesitylporphyrin) (Fe(TMP)) in DMF. The results show that selectivity reflects a kinetic competition in which the dependence on [HX] is one order greater for the production of H 2 O than H 2 O 2 . Based on such experimental results and computational studies, we propose that the selectivity is governed by competition between protonation of the hydroperoxo intermediate, Fe III (TMP)(OOH), to produce water versus dissociation of the HOO – ligand to yield H 2 O 2 . The data rule out a bifurcation based on the regioselectivity of protonation of the hydroperoxide, as suggested in the enzymatic systems. Furthermore, the analysis developed in this paper should be generally valuable to the study of selectivity in other multi-proton/multi-electron electrocatalytic reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Controlling solvation in conducting redox polymers for selective electrochemical separation of nitrate from wastewater

Selective capture of nitrate from wastewater is crucial for ensuring safe drinking water and promoting resource circularity. This study investigated alkylated polyaniline redox polymers as highly-selective electrosorbents to address this challenge. By controlling polymer solvation properties through synthetic functionalization, poly(N-methylaniline) (PNMA) achieves a nitrate uptake of up to 1.38 mmol g −1 -polymer and a separation factor of 7 over chloride. Poly(N-butylaniline) (PNBA) further enhances selectivity, achieving a separation factor beyond 14 due to increased hydrophobicity. The mechanisms underlying this selectivity are investigated using ab initio molecular dynamics (AIMD) and in-situ electrochemical quartz crystal microbalance (EQCM) studies, which reveal that hydrophobicity reduces chloride binding. A technoeconomic analysis indicates that methylation on PANI reduces nitrate removal costs by 50% compared to non-functionalized PANI, due to enhanced selectivity and uptake, and decreased energy consumption. PNMA electrodes demonstrate practical nitrate selectivity over 20 versus chloride in real wastewater, while avoiding sulfate binding. This study highlights the potential of controlling solvation at electroactive polymers to enhance nitrate selectivity, offering a promising design path for redox-mediated electrochemical separations.

chemical engineering↗
DOE OSTI2024

Inverse chirality-induced spin selectivity effect in chiral assemblies of π -conjugated polymers

Coupling of spin and charge currents to structural chirality in non-magnetic materials, known as chirality-induced spin selectivity, is promising for application in spintronic devices at room temperature. Although the chirality-induced spin selectivity effect has been identified in various chiral materials, its Onsager reciprocal process, the inverse chirality-induced spin selectivity effect, remains unexplored. Here we report the observation of the inverse chirality-induced spin selectivity effect in chiral assemblies of π-conjugated polymers. Using spin-pumping techniques, the inverse chirality-induced spin selectivity effect enables quantification of the magnitude of the longitudinal spin-to-charge conversion driven by chirality-induced spin selectivity in different chiral polymers. By widely tuning conductivities and supramolecular chiral structures via a printing method, we found a very long spin relaxation time of up to several nanoseconds parallel to the chiral axis. Furthermore, our demonstration of the inverse chirality-induced spin selectivity effect suggests possibilities for elucidating the puzzling interplay between spin and chirality, and opens a route for spintronic applications using printable chiral assemblies.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Discovery and molecular basis of subtype-selective cyclophilin inhibitors

Abstract Although cyclophilins are attractive targets for probing biology and therapeutic intervention, no subtype-selective cyclophilin inhibitors have been described. We discovered novel cyclophilin inhibitors from the in vitro selection of a DNA-templated library of 256,000 drug-like macrocycles for cyclophilin D (CypD) affinity. Iterated macrocycle engineering guided by ten X-ray co-crystal structures yielded potent and selective inhibitors (half maximal inhibitory concentration (IC 50 ) = 10 nM) that bind the active site of CypD and also make novel interactions with non-conserved residues in the S2 pocket, an adjacent exo-site. The resulting macrocycles inhibit CypD activity with 21- to >10,000-fold selectivity over other cyclophilins and inhibit mitochondrial permeability transition pore opening in isolated mitochondria. We further exploited S2 pocket interactions to develop the first cyclophilin E (CypE)-selective inhibitor, which forms a reversible covalent bond with a CypE S2 pocket lysine, and exhibits 30- to >4,000-fold selectivity over other cyclophilins. These findings reveal a strategy to generate isoform-selective small-molecule cyclophilin modulators, advancing their suitability as targets for biological investigation and therapeutic development.

Biochemistry & Molecular Biology↗
DOE OSTI2026

A family portrait of lanmodulin selectivity for enhanced rare-earth separations

Proteins offer a molecular design space to create bespoke ligands for the separation of critical metals like rare earth elements (REs). However, data-intensive approaches to tune metalloprotein selectivity are constrained by the low-throughput nature of existing characterization methods. Here we invented an assay called ‘SpyTag-Catcher Immobilization of Lanmodulin for Assaying Metal-Binding Selectivity’ (SpyCI-LAMBS) to measure metalloprotein selectivity en masse. This 96-format workflow was used to study the selectivity of 621 lanmodulin (LanM) orthologs for 15 REs, revealing eight distinct selectivity profiles based on sequence-to-function analyses. We discovered >200 LanMs with stronger selectivity against low-value LaIII relative to the prototypical LanM. This includes a LanM that can perform a challenging one-stage separation of PrIII from LaIII with up to >99.9 mol% purity and 83% yield. SpyCI-LAMBS is a powerful tool that can rapidly collect high-fidelity selectivity data to inform metal ion separations and machine-learning-assisted metalloprotein design.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2017

Hydroisomerization of n-hexadecane: remarkable selectivity of mesoporous silica post-synthetically modified with aluminum

As the impact of acids on catalytically driven chemical transformations is tremendous, fundamental understanding of catalytically relevant factors is essential for the design of more efficient solid acid catalysts. In this work, we employed a post-synthetic doping method to synthesize a highly selective hydroisomerization catalyst and to demonstrate the effect of acid strength and density, catalyst microstructure, and platinum nanoparticle size on the reaction rate and selectivity. Aluminum doped mesoporous silica catalyzed gas-phase n-hexadecane isomerization with remarkably high selectivity to monobranched isomers (~95%), producing a substantially higher amount of isomers than traditional zeolite catalysts. Mildly acidic sites generated by post-synthetic aluminum grafting were found to be the main reason for its high selectivity. The flexibility of the post-synthetic doping method enabled us to systematically explore the effect of the acid site density on the reaction rate and selectivity, which has been extremely difficult to achieve with zeolite catalysts. We found that a higher density of Brønsted acid sites leads to higher cracking of n-hexadecane presumably due to an increased surface residence time. Furthermore, regardless of pore size and microstructure, hydroisomerization turnover frequency linearly increased as a function of Brønsted acid site density. In addition to strength and density of acid sites, platinum nanoparticle size affected catalytic activity and selectivity. The smallest platinum nanoparticles produced the most effective bifunctional catalyst presumably because of higher percolation into aluminum doped mesoporous silica, generating more 'intimate' metallic and acidic sites. Finally, the aluminum doped silica catalyst was shown to retain its remarkable selectivity towards isomers even at increased reaction conversions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Structural trends in the dehydrogenation selectivity of palladium alloys

Alloying is well-known to improve the dehydrogenation selectivity of pure metals, but there remains considerable debate about the structural and electronic features of alloy surfaces that give rise to this behavior. To provide molecular-level insights into these effects, a series of Pd intermetallic alloy catalysts with Zn, Ga, In, Fe and Mn promoter elements was synthesized, and the structures were determined using in situ X-ray absorption spectroscopy (XAS) and synchrotron X-ray diffraction (XRD). The alloys all showed propane dehydrogenation turnover rates 5–8 times higher than monometallic Pd and selectivity to propylene of over 90%. Moreover, among the synthesized alloys, Pd 3 M alloy structures were less olefin selective than PdM alloys which were, in turn, almost 100% selective to propylene. This selectivity improvement was interpreted by changes in the DFT-calculated binding energies and activation energies for C–C and C–H bond activation, which are ultimately influenced by perturbation of the most stable adsorption site and changes to the d-band density of states. Furthermore, transition state analysis showed that the C–C bond breaking reactions require 4-fold ensemble sites, which are suggested to be required for non-selective, alkane hydrogenolysis reactions. These sites, which are not present on alloys with PdM structures, could be formed in the Pd 3 M alloy through substitution of one M atom with Pd, and this effect is suggested to be partially responsible for their slightly lower selectivity.

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

Solar cell contacts: quantifying the impact of interfacial layers on selectivity, recombination, charge transfer, and V oc

Interfacial layers (IFLs) are ubiquitous in solar cells, but their precise impact on carrier transfer rates and the relation of these rates to performance metrics and the concepts of selectivity and recombination is lacking. We report the use of a well-defined interdigitated back-contact (IBC) silicon solar cell to determine the precise role of such IFLs. We characterize the action of the common IFL spiro-OMeTAD by making it a third contact to the IBC cell. This architecture creates three solar cells within a single structure that, with numerical simulation, provide the exchange current densities (i.e., charge transfer rates) for electrons ( J 0n ) and holes ( J 0p ) and the quasi Fermi-level splitting in the absorber, which measures the balance of generation and recombination. Further, we describe the relation of V oc to contact recombination, the asymmetry in electron/hole collection rates at a single contact (contact selectivity), and the asymmetry in collection rates of the same carrier at separate contacts (carrier selectivity). Relative to bare gold, neat spiro-OMeTAD reduces J 0n and J 0p (their geometric mean ( J 0n J 0p ) 0.5 decreases by 10 4 ), decreasing contact recombination. Addition of the common dopant Li-TFSI and air increase J 0p / J 0n by 10 6 with little effect on ( J 0n J 0p ) 0.5 , increasing contact hole selectivity. The significant increase in V oc observed by introducing spiro-OMeTAD/Li-TFSI IFLs into the cells studied, however, is due to an increase in the carrier selectivity rather than the contact selectivity or recombination of the spiro-OMeTAD-modified contact. Operando measurements further show voltage-dependent changes in the J 0 s, demonstrating that spiro-OMeTAD contributes to hysteresis. Broader context : Solar cells operate by photogenerating excess charge carriers in an absorber material and, in competition with recombination, asymmetrically extracting them at so-called carrier-selective contacts, one that ideally collects only electrons and the other, only holes. Particularly in emerging photovoltaics such as perovskites, thin layers of organic semiconductors or related materials are introduced between the absorber and contact to improve power conversion efficiency. In terms of interfacial charge transfer, a prevailing view is that such interfacial layers improve performance by helping block the collection of the undesired carrier, considered a form of recombination. We use a novel platform to study the simultaneous impact of spiro-OMeTAD, a common interfacial layer, on the collection of electrons, the collection of holes, and on the recombination of electrons and holes. We quantitatively demonstrate that spiro-OMeTAD layers indeed passivate the contact toward recombination, but that larger improvements in the open-circuit voltage, a key cell metric, can come not only from this but also from increasing the asymmetry of the collection of electrons in the system. Further, operando measurements show transient changes in the properties of spiro-OMeTAD which suggest that it contributes to hysteresis phenomena commonly observed in perovskite and other solar cells.

36 MATERIALS SCIENCE↗
DOE OSTI2022

The mechanism for acetate formation in electrochemical CO (2) reduction on Cu: selectivity with potential, pH, and nanostructuring

Nanostructured Cu catalysts have increased the selectivities and geometric activities for high value C–C coupled (C 2 ) products (ethylene, ethanol, and acetate) in the electrochemical CO (2) reduction reaction (CO (2) RR). The selectivity among the high-value C 2 products is also altered, where for instance the yield of acetate increases with alkalinity and is dependent on the catalyst morphology. The reaction mechanisms behind the selectivity towards acetate vs. other C 2 products remain controversial. In this work, we elucidate the reaction mechanism for acetate formation by using ab initio simulations, a coupled kinetic-transport model, and loading dependent experiments. We find that trends in acetate selectivity can be rationalized from variations in electrolyte pH and the local mass transport properties of the catalyst and not from changes in Cu's intrinsic activity. The selectivity mechanism originates from the transport of ketene, a stable (closed shell) intermediate, away from the catalyst surface into solution where it reacts to form acetate. While this type of mechanism has not yet been discussed in the CO (2) RR, variants of it may explain similar selectivity fluctuations observed for other stable intermediates like CO and acetaldehyde. Our proposed mechanism suggests that acetate selectivity increases with increasing pH, decreasing catalyst roughness and significantly varies with the applied potential.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Diacylglycerol enantiomer selectivity of diacylglycerol acyltransferases highlights metabolic specialization in triacylglycerol synthesis across the tree of life

Triacylglycerols are the major energy storage lipids in plants, animals, and microorganisms, and are predominantly produced by acyl-CoA:diacylglycerol (DAG) acyltransferases (DGATs). Two enantiomers of the DAG substrate, sn -1,2 and sn -2,3, can be produced by different biological mechanisms; however, little is known about which species produce each enantiomer, the selectivity of DGAT isoforms for either enantiomer, or whether DGAT enantiomer selectivity varies across organisms. Here, DAG enantiomer selectivity of DGAT1 and DGAT2 was measured from eight seed plants, two mammals, one oleaginous yeast, and one photosynthetic microalga using enantiomer-specific in vitro DGAT assays. Across most plants, DGAT1 favored sn -1,2-DAG, whereas DGAT2 preferentially utilized sn -2,3-DAG. However, there were several exceptions. Mammalian DGAT1, DGAT2, and microbial DGAT1s efficiently used both DAG enantiomers, while microbial DGAT2s had unique selectivity. The selectivity of several DGATs for combined acyl-CoA and DAG enantiomer molecular species were also evaluated for biotechnical applications. Therefore, DGAT DAG enantiomer selectivity is common yet strongly dependent on lineage and isoform and likely shaped in part by species-specific metabolic context of triacylglycerol synthesis, turnover, and remodeling. This work expands our understanding of DGAT function and establishes a foundation for leveraging enantiomer-selective acyltransferases in metabolic engineering of tailored lipid products.

Arabidopsis thaliana↗
DOE OSTI2018

Nitride Layer Screening as Carrier-Selective Contacts for Silicon Heterojunction Solar Cells

A three-architecture method for screening new materials' viability as carrier-selective contacts for silicon solar cells is presented. Test-structure solar cells were fabricated with a standard silicon heterojunction contact for the front side, and one of three treatments on the back: bare silicon, intrinsic amorphous silicon (i-aSi:H), or n-type amorphous silicon on an i-aSi:H passivation layer. Then, the candidate contact material of interest was deposited on the back of each test structure and the cells were finished with evaporated Al. By analysing the current-voltage characteristics of each type of architecture, the carrier selectivity and surface passivation quality of novel contact materials or material stacks can be rapidly screened. To demonstrate the utility of this method, we present a preliminary investigation of nitride compounds as electron-selective contacts in silicon solar cells, in particular zinc tin nitride (ZnSnN2), which is naturally n-type and has favourable band alignments with c-Si. ZnSnN2 was deposited by reactive sputtering as an electron contact on each test structure. No passivation was observed, but decent electron-selectivity was observed when in direct contact with the silicon wafer. When combined with intrinsic amorphous silicon, poor performances were obtained with poor selectivity and the occurrence of an S-shape, likely due to insufficient selectivity of the ZTN. Similar results were obtained for 2 nm and 20 nm thick layers, indicating selectivity did not result from the Al over-layer, but was in fact due to the ZTN itself. Overall, this work shows the three-architecture screening method is a useful tool for assessing novel contact materials for silicon heterojunction solar cells.

14 SOLAR ENERGY↗
DOE OSTI2019

Passivation, conductivity, and selectivity in solar cell contacts: Concepts and simulations based on a unified partial-resistances framework

Passivation, conductivity, and selectivity are often acknowledged as the three requirements for optimal contacts to photovoltaic solar cells. Although there are generally accepted definitions and metrics for passivation and conductivity, a common understanding of the concept of selectivity is emerging only now. In this contribution, we present a generalized model of solar cell contacts based on the distinct lumped resistances encountered by electrons and holes traversing a contact, which we refer to as partial specific contact resistances. The relations between electron and hole partial current densities, quasi-Fermi level separation, and external voltage are derived from these partial specific contact resistances, leading to simple metrics for the aforementioned contact properties: the sum of the electron and hole resistances is a metric for passivation, their ratio is a metric for selectivity, and the majority-carrier resistance is a metric for conductivity. Using PC1D, we validate our model by simulating 10 500 cases of homojunction contacts to crystalline silicon solar cells, although our framework is material agnostic and can be equally applied to any other type of absorber. In these simulations, the hole contact and absorber are assumed to be ideal, whereas we vary the partial specific contact resistances in the electron contact by orders of magnitude by adjusting the electron and hole mobilities, their densities (through variations of the donor doping density), and the contact thickness. The simulations confirm the finding of the model that, when the contact fraction cannot be adjusted—as is the case with full-area contacts—combined passivation and conductivity are necessary and sufficient for optimal solar cell performance, and they imply selectivity. However, the reciprocal is not true: contacts can be selective but lack conductivity—causing a deleterious drop in fill factor—or can be selective but provide poor passivation—leading to a reduction in implied open-circuit voltage and, hence, actual open-circuit voltage. Furthermore, selectivity is a meaningful metric in the sole case of partial-area contacts, where the contact fraction can be adjusted arbitrarily.

14 SOLAR ENERGY↗
DOE OSTI2023

Spin selective charge recombination in chiral donor-bridge-acceptor triads

In this paper, we outline a physically motivated framework for describing spin-selective recombination processes in chiral systems, from which we derive spin-selective reaction operators for recombination reactions of donor–bridge–acceptor molecules, where the electron transfer is mediated by chirality and spin–orbit coupling. In general, the recombination process is selective only for spin-coherence between singlet and triplet states, and it is not, in general, selective for spin polarization. We find that spin polarization selectivity only arises in hopping-mediated electron transfer. We describe how this effective spin-polarization selectivity is a consequence of spin-polarization generated transiently in the intermediate state. Furthermore, the recombination process also augments the coherent spin dynamics of the charge separated state, which is found to have a significant effect on the recombination dynamics and to destroy any long-lived spin polarization. Although we only consider a simple donor–bridge–acceptor system, the framework we present here can be straightforwardly extended to describe spin-selective recombination processes in more complex systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Tuning the ion selectivity of tetrameric cation channels by changing the number of ion binding sites

Selective ion conduction across ion channel pores is central to cellular physiology. To understand the underlying principles of ion selectivity in tetrameric cation channels, we engineered a set of cation channel pores based on the nonselective NaK channel and determined their structures to high resolution. These structures showcase an ensemble of selectivity filters with a various number of contiguous ion binding sites ranging from 2 to 4, with each individual site maintaining a geometry and ligand environment virtually identical to that of equivalent sites in K{sup +} channel selectivity filters. Combined with single channel electrophysiology, we show that only the channel with four ion binding sites is K{sup +} selective, whereas those with two or three are nonselective and permeate Na{sup +} and K{sup +} equally well. These observations strongly suggest that the number of contiguous ion binding sites in a single file is the key determinant of the channel's selectivity properties and the presence of four sites in K{sup +} channels is essential for highly selective and efficient permeation of K{sup +} ions.

Derebe, Mehabaw G.↗