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The enzyme MiaB catalyzes the attachment of a methylthio (-SCH3) group at the C2 position of N6-(isopentenyl)adenosine (i6A) in the final step of the biosynthesis of the hypermodified tRNA nucleotide 2-methythio-N6-(isopentenyl)adenosine (ms2i6A). MiaB belongs to the expanding subgroup of enzymes of the radical S-adenosylmethionine (SAM) superfamily that harbor one or more auxiliary [4Fe-4S] clusters in addition to the [4Fe-4S] cluster that all family members require for the reductive cleavage of SAM to afford the common 5'-deoxyadenosyl 5'-radical (5'-dA•) intermediate. While the role of the radical SAM cluster in generating the 5'-dA• is well understood, the detailed role of the auxiliary cluster, which is essential for MiaB catalysis, remains unclear. It has been proposed that the auxiliary cluster may serve as a coordination site for exogenously derived sulfur destined for attachment to the substrate or that the cluster itself provides the sulfur atom and is sacrificed during turnover. In this work, we report spectroscopic and biochemical evidence that the auxiliary [4Fe-4S]2+ cluster in Bacteroides thetaiotaomicron (Bt) MiaB is converted to a [3Fe-4S]0-like cluster during the methylation step of catalysis. Mossbauer characterization of the MiaB [3Fe-4S]0-like cluster revealed unusual spectroscopic properties compared to those of other well-characterized cuboidal [3Fe-4S]0 clusters. Specifically, the Fe sites of the mixed-valent moiety do not have identical Mossbauer parameters. Our results support a mechanism where the auxiliary [4Fe-4S] cluster is the direct sulfur source during catalysis.

The complex structure of plant cell walls resists chemical or biological degradation, challenging the breakdown of lignocellulosic biomass into renewable chemical precursors that could form the basis of future production of green chemicals and transportation fuels. Here, experimental and computational results reveal that the effect of the tetrahydrofuran (THF)-water cosolvents on the structure of lignin and on its interactions with cellulose in the cell wall drives multiple synergistic mechanisms leading to the efficient breakdown and fractionation of biomass into valuable chemical precursors. Molecular simulations show that THF-water is an excellent "theta" solvent, such that lignin dissociates from itself and from cellulose and expands to form a random coil. The expansion of the lignin molecules exposes interunit linkages, rendering them more susceptible to depolymerization by acid-catalyzed cleavage of aryl-ether bonds. Nanoscale infrared sensors confirm cosolvent-mediated molecular rearrangement of lignin in the cell wall of micrometer-thick hardwood slices and track the disappearance of lignin. At bulk scale, adding dilute acid to the cosolvent mixture liberates the majority of the hemicellulose and lignin from biomass, allowing unfettered access of cellulolytic enzymes to the remaining cellulose-rich material, allowing them to sustain high rates of hydrolysis to glucose without enzyme deactivation. Through this multiscale analysis, synergistic mechanisms for biomass deconstruction are identified, portending a paradigm shift toward first-principles design and evaluation of other cosolvent methods to realize low cost fuels and bioproducts.

The enzyme MiaB catalyzes the attachment of a methylthio (−SCH(3)) group at the C2 position of N(6)-(isopentenyl)adenosine (i(6)A) in the final step of the biosynthesis of the hypermodified tRNA nucleotide 2-methythio-N(6)-(isopentenyl)adenosine (ms(2)i(6)A). MiaB belongs to the expanding subgroup of enzymes of the radical S-adenosylmethionine (SAM) superfamily that harbor one or more auxiliary [4Fe-4S] clusters in addition to the [4Fe-4S] cluster that all family members require for the reductive cleavage of SAM to afford the common 5′-deoxyadenosyl 5′-radical (5′-dA(•)) intermediate. While the role of the radical SAM cluster in generating the 5′-dA(•) is well understood, the detailed role of the auxiliary cluster, which is essential for MiaB catalysis, remains unclear. It has been proposed that the auxiliary cluster may serve as a coordination site for exogenously derived sulfur destined for attachment to the substrate or that the cluster itself provides the sulfur atom and is sacrificed during turnover. In this work, we report spectroscopic and biochemical evidence that the auxiliary [4Fe–4S](2+) cluster in Bacteroides thetaiotaomicron (Bt) MiaB is converted to a [3Fe-4S](0)-like cluster during the methylation step of catalysis. Mössbauer characterization of the MiaB [3Fe-4S](0)-like cluster revealed unusual spectroscopic properties compared to those of other well-characterized cuboidal [3Fe-4S](0) clusters. Specifically, the Fe sites of the mixed-valent moiety do not have identical Mössbauer parameters. Our results support a mechanism where the auxiliary [4Fe-4S] cluster is the direct sulfur source during catalysis.

The synthesis and properties of the hybrid organic/inorganic germanium perovskite compounds, AGeI3, are reported (A = Cs, organic cation). The systematic study of this reaction system led to the isolation of 6 new hybrid semiconductors. Using CsGeI3 (1) as the prototype compound, we have prepared methylammonium, CH3NH3GeI3 (2), formamidinium, HC(NH2)2GeI3 (3), acetamidinium, CH3C(NH2)2GeI3 (4), guanidinium, C(NH2)3GeI3 (5), trimethylammonium, (CH3)3NHGeI3 (6), and isopropylammonium, (CH3)2C(H)NH3GeI3 (7) analogues. The crystal structures of the compounds are classified based on their dimensionality with 1–4 forming 3D perovskite frameworks and 5–7 1D infinite chains. Compounds 1–7, with the exception of compounds 5 (centrosymmetric) and 7 (nonpolar acentric), crystallize in polar space groups. The 3D compounds have direct band gaps of 1.6 eV (1), 1.9 eV (2), 2.2 eV (3), and 2.5 eV (4), while the 1D compounds have indirect band gaps of 2.7 eV (5), 2.5 eV (6), and 2.8 eV (7). Herein, we report on the second harmonic generation (SHG) properties of the compounds, which display remarkably strong, type I phase-matchable SHG response with high laser-induced damage thresholds (up to ∼3 GW/cm(2)). The second-order nonlinear susceptibility, χS(2), was determined to be 125.3 ± 10.5 pm/V (1), (161.0 ± 14.5) pm/V (2), 143.0 ± 13.5 pm/V (3), and 57.2 ± 5.5 pm/V (4). First-principles density functional theory electronic structure calculations indicate that the large SHG response is attributed to the high density of states in the valence band due to sp-hybridization of the Ge and I orbitals, a consequence of the lone pair activation.

Hydrogen bonds profoundly influence the architecture and activity of biological macromolecules. Deep appreciation of hydrogen bond contributions to biomolecular function thus requires a detailed understanding of hydrogen bond structure and energetics and the relationship between these properties. Hydrogen bond formation energies (ΔGf) are enormously more favorable in aprotic solvents than in water, and two classes of contributing factors have been proposed to explain this energetic difference, focusing respectively on the isolated and hydrogen-bonded species: (I) water stabilizes the dissociated donor and acceptor groups much better than aprotic solvents, thereby reducing the driving force for hydrogen bond formation; and (II) water lengthens hydrogen bonds compared to aprotic environments, thereby decreasing the potential energy within the hydrogen bond. Each model has been proposed to provide a dominant contribution to ΔGf, but incisive tests that distinguish the importance of these contributions are lacking. Here we directly test the structural basis of model II. Neutron crystallography, NMR spectroscopy, and quantum mechanical calculations demonstrate that O-H···O hydrogen bonds in crystals, chloroform, acetone, and water have nearly identical lengths and very similar potential energy surfaces despite ΔGf differences8 kcal/mol across these solvents. These results rule out a substantial contribution from solvent-dependent differences in hydrogen bond structure and potential energy after association (model II) and thus support the conclusion that differences in hydrogen bond ΔGf are predominantly determined by solvent interactions with the dissociated groups (model I). These findings advance our understanding of universal hydrogen-bonding interactions and have important implications for biology and engineering.

Two-dimensional (2D) electronic systems are of wide interest due to their richness in chemical and physical phenomena and potential for technological applications. Here we report that [Pb2BiS3][AuTe2], known as the naturally occurring mineral buckhornite, hosts 2D carriers in single-atom-thick layers. The structure is composed of stacking layers of weakly coupled [Pb2BiS3] and [AuTe2] sheets. The insulating [Pb2BiS3] sheet inhibits interlayer charge hopping and confines the carriers in the basal plane of the single-atom-thick [AuTe2] layer. Magneto-transport measurements on synthesized samples and theoretical calculations show that [Pb2BiS3][AuTe2] is a multiband semimetal with a compensated density of electrons and holes, which exhibits a high hole carrier mobility of ∼1360 cm(2)/(V s). This material possesses an extremely large anisotropy, Γ = ρ(c)/ρ(ab) ≈ 10(4), comparable to those of the benchmark 2D materials graphite and Bi2Sr2CaCu2O(6+δ). The electronic structure features linear band dispersion at the Fermi level and ultrahigh Fermi velocities of 10(6) m/s, which are virtually identical to those of graphene. The weak interlayer coupling gives rise to the highly cleavable property of the single crystal specimens. Our results provide a novel candidate for a monolayer platform to investigate emerging electronic properties.

Electronic impurity doping of bulk semiconductors is an essential component of semiconductor science and technology. Yet there are only a handful of studies demonstrating control of electronic impurities in semiconductor nanocrystals. Here, we studied electronic impurity doping of colloidal PbSe quantum dots (QDs) using a postsynthetic cation exchange reaction in which Pb is exchanged for Ag. We found that varying the concentration of dopants exposed to the as-synthesized PbSe QDs controls the extent of exchange. The electronic impurity doped QDs exhibit the fundamental spectroscopic signatures associated with injecting a free charge carrier into a QD under equilibrium conditions, including a bleach of the first exciton transition and the appearance of a quantum-confined, low-energy intraband absorption feature. Photoelectron spectroscopy confirms that Ag acts as a p-type dopant for PbSe QDs and infrared spectroscopy is consistent with k·p calculations of the size-dependent intraband transition energy. We find that to bleach the first exciton transition by an average of 1 carrier per QD requires that approximately 10% of the Pb be replaced by Ag. We hypothesize that the majority of incorporated Ag remains at the QD surface and does not interact with the core electronic states of the QD. Instead, the excess Ag at the surface promotes the incorporation of1% Ag into the QD core where it causes p-type doping behavior.

Aromatic hydrogenation is a challenging transformation typically requiring alkali or transition metal reagents and/or harsh conditions to facilitate the process. In sharp contrast, the aromatic heterocycle 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene is shown to be reduced under 4 atm of H2 to give [3.1.0]bicylo reduction products, with the structure of the major isomer being confirmed by X-ray crystallography. NMR studies show this reaction proceeds via a reversible 1,4-H2 addition to generate an intermediate species, which undergoes an irreversible suprafacial hydride shift concurrent with P-P bond formation to give the isolated products. Further, para-hydrogen experiments confirmed the addition of H2 to triphosphabenzene is a bimolecular process. Density functional theory (DFT) calculations show that facile distortion of the planar triphosphabenzene toward a boat-conformation provides a suprafacial combination of vacant acceptor and donor orbitals that permits this direct and uncatalyzed reduction of the aromatic molecule.

We report the synthesis and characterization of Pb-chalcogenide fused quantum-dot (QD) dimer structures. The resulting QD dimers range in length from 6 to 16 nm and are produced by oriented attachment of single QD monomers with diameters of 3.1-7.8 nm. QD monomers with diameters exceeding about 5 nm appear to have the greatest affinity for QD dimer formation and, therefore, gave the greatest yields of fused structures. We find a new absorption feature in the first exciton QD dimer spectra and assign this to a splitting of the 8-fold degenerate 1S-level. The dimer splitting increases from 50 to 140 meV with decrease of the QD-monomer size, and we present a mechanism that accounts for this splitting. We also demonstrate the possibility of fusing two QDs with different sizes into a heterostructure.

RimO and MiaB are radical S-adenosylmethionine (SAM) enzymes that catalyze the attachment of methylthio (−SCH3) groups to macromolecular substrates. RimO attaches a methylthio group at C3 of aspartate 89 of protein S12, a component of the 30S subunit of the bacterial ribosome. MiaB attaches a methylthio group at C2 of N6-(isopentenyl)adenosine, found at nucleotide 37 in several prokaryotic tRNAs. These two enzymes are prototypical members of a subclass of radical SAM enzymes called methylthiotransferases (MTTases). It had been assumed that the sequence of steps in MTTase reactions involves initial sulfur insertion into the organic substrate followed by capping of the inserted sulfur atom with a SAM-derived methyl group. In this work, however, we show that both RimO and MiaB from Thermotoga maritima catalyze methyl transfer from SAM to an acid/base labile acceptor on the protein in the absence of their respective macromolecular substrates. Consistent with the assignment of the acceptor as an iron–sulfur cl...

The relationships among the macroscopic compositional parameters of a Cu-exchanged SSZ-13 zeolite catalyst, the types and numbers of Cu active sites, and activity for the selective catalytic reduction (SCR) of NOx with NH3 are established through experimental interrogation and computational analysis of materials across the catalyst composition space. Density functional theory, stochastic models, and experimental characterizations demonstrate that within the synthesis protocols applied here and across Si:Al ratios, the volumetric density of six-membered-rings (6MR) containing two Al (2Al sites) is consistent with a random Al siting in the SSZ-13 lattice subject to Löwenstein's rule. Further, exchanged Cu(II) ions first populate these 2Al sites before populating remaining unpaired, or 1Al, sites as Cu(II)OH. These sites are distinguished and enumerated ex situ through vibrational and X-ray absorption spectroscopies (XAS) and chemical titrations. In situ and operando XAS follow Cu oxidation state and coordination environment as a function of environmental conditions including low-temperature (473 K) SCR catalysis and are rationalized through first-principles thermodynamics and ab initio molecular dynamics. Experiment and theory together reveal that the Cu sites respond sensitively to exposure conditions, and in particular that Cu species are solvated and mobilized by NH3 under SCR conditions. While Cu sites are spectroscopically and chemically distinct away from these conditions, they exhibit similar turnover rates, apparent activation energies and apparent reaction orders at the SCR conditions, even on zeolite frameworks other than SSZ13.

AAsSe(2) (A = Li, Na) have been identified as a new class of polar direct-band gap semiconductors. These I-V-VI(2) ternary alkali-metal chalcoarsenates have infinite single chains of (1/infinity)[AsQ(2)(-)] derived from corner-sharing pyramidal AsQ(3) units with stereochemically active lone pairs of electrons on arsenic. The conformations and packing of the chains depend on the structure-directing alkali metals. This results in at least four different structural types for the Li(1-x)Na(x)AsSe(2) stoichiometry (alpha-LiAsSe(2), beta-LiAsSe(2), gamma-NaAsSe(2), and delta-NaAsSe(2)). Single-crystal X-ray diffraction studies showed an average cubic NaCl-type structure for alpha-LiAsSe(2), which was further demonstrated to be locally distorted by pair distribution function (PDF) analysis. The beta and gamma forms have polar structures built of different (1/infinity)[AsSe(2)(-)] chain conformations, whereas the delta form has nonpolar packing. A wide range of direct band gaps are observed, depending on composition: namely, 1.11 eV for alpha-LiAsSe(2), 1.60 eV for LiAsS(2), 1.75 eV for gamma-NaAsSe(2), 2.23 eV for NaAsS(2). The AAsQ(2) materials are soluble in common solvents such as methanol, which makes them promising candidates for solution processing. Band structure calculations performed with the highly precise screened-exchange sX-LDA FLAPW method confirm the direct-gap nature and agree well with experiment. The polar gamma-NaAsSe(2) shows very large nonlinear optical (NLO) second harmonic generation (SHG) response in the wavelength range of 600-950 nm. The theoretical studies confirm the experimental results and show that gamma-NaAsSe(2) has the highest static SHG coefficient known to date, 337.9 pm/V, among materials with band gaps larger than 1.0 eV.

The Cs(5)BiP(4)Se(12) salt grows naturally as nanowires that crystallize in the polar space group Pmc2(1), with a = 7.5357(2) A, b = 13.7783(6) A, c = 28.0807(8) A, and Z = 4 at 293(2) K. The compound features octahedral [Bi(P(2)Se(6))(2)](5-) coordination complexes that stack via weak intermolecular Se...Se interactions to form long, flexible fibers and nanowires. The Cs(5)BiP(4)Se(12) fibers are transparent in the near- and mid-IR ranges and were found to exhibit a nonlinear optical second harmonic generation response at 1 microm that is approximately twice that of the benchmark material AgGaSe(2). The material has a nearly direct band gap of 1.85 eV and melts congruently at 590 degrees C. Ab initio electronic structure calculations performed with the full-potential linearized augmented plane wave (FLAPW) method show that the band gap increases from its local density approximation (LDA) spin-orbit coupling value of 1.15 eV to the higher value of 2.0 eV when the screened-exchange LDA method is invoked and explain how the long nanowire nature of Cs(5)BiP(4)Se(12) emerges.

Reaction of NaBH4 with (tBuPOCOP)IrHCl affords the previously reported complex (tBuPOCOP)IrH2(BH3) (1) (tBuPOCOP = kappa(3)-C6H3-1,3-[OP(tBu)2]2). The structure of 1 determined from neutron diffraction data contains a B-H sigma-bond to iridium with an elongated B-H bond distance of 1.45(5) A. Compound 1 crystallizes in the space group P1 (Z = 2) with a = 8.262 (5) A, b = 12.264 (5) A, c = 13.394 (4) A, and V = 1256.2 (1) A(3) (30 K). Complex 1 can also be prepared by reaction of BH3 x THF with (tBuPOCOP)IrH2. Reaction of (tBuPOCOP)IrH2 with pinacol borane gave initially complex 2, which is assigned a structure analogous to that of 1 based on spectroscopic measurements. Complex 2 evolves H2 at room temperature leading to the borane complex 3, which is formed cleanly when 2 is subjected to dynamic vacuum. The structure of 3 has been determined by X-ray diffraction and consists of the (tBuPOCOP)Ir core with a sigma-bound pinacol borane ligand in an approximately square planar complex. Compound 3 crystallizes in the space group C2/c (Z = 4) with a = 41.2238 (2) A, b = 11.1233 (2) A, c = 14.6122 (3) A, and V = 6700.21 (19) A(3) (130 K). Reaction of (tBuPOCOP)IrH2 with 9-borobicyclononane (9-BBN) affords complex 4. Complex 4 displays (1)H NMR resonances analogous to 1 and exists in equilibrium with (tBuPOCOP)IrH2 in THF solutions.