Amga Baldansuren

Rensselaer Polytechnic Institute | RPI · Department of Chemistry and Chemical Biology

Lytic polysaccharide monooxygenases (LPMOs) are copper-containing enzymes that oxidatively break down recalcitrant polysaccharides such as cellulose and chitin. Since their discovery, LPMOs have become integral factors in the industrial utilization of biomass, especially in the sustainable generation of cellulosic bioethanol. We report here a struc...

Mixed silver−sodium containing zeolite LTA was prepared by aqueous ion exchange. Reduction of oxidized Ag/NaA (12 wt %) using hydrogen leads to the formation of a paramagnetic cluster possessing a well-defined structure of six equivalent Ag nuclei. The hyperfine structure of the silver clusters appeared only in Ag+-exchanged zeolite. Continuous wav...

Silver containing catalysts were prepared by aqueous ion exchange of Ag(+) against Na(+) in an LTA zeolite. A well-defined paramagnetic cluster consisting of six equivalent silver nuclei was obtained after oxidation and hydrogen reduction. Continuous wave EPR demonstrates that the reduced Ag(6)(+) clusters are isolated and all silver atoms are clos...

The flavoenzyme monoamine oxidase (MAO) regulates mammalian behavioral patterns by modulating neurotransmitters such as adrenaline and serotonin. The mechanistic basis which underpins this enzyme is far from agreed upon. Reported herein is that the combination of a synthetic flavin and alloxan generates a catalyst system which facilitates biomimeti...

The structural and electrical properties of metastable defects in various types of hydrogenated amorphous silicon have been studied using a powerful combination of continuous wave electron-paramagnetic resonance spectroscopy, electron spin echo (ESE) decay measurements, and Doppler broadening positron annihilation spectroscopy. The observed depende...

In comparison to the bulk 2.889 Å distance, the inter-atomic Ag-Ag distance was contracted on the order of ≈ 1/10 (i.e. 9.5%) of an Ångström due to size effects on supported silver clusters. Paramagnetism appears when a conduction electronic band becomes indeed discrete once an energy level separation is become comparable to Zeeman energy levels (µ...

We report a synthesis and structure of the most highly distorted four-coordinate d3 ion known to date that also serves as the second known example of a bis(biphenolato) transition metal...

The solar water-splitting protein complex, photosystem II (PSII), catalyzes one of the most energetically demanding reactions in nature by using light energy to drive a catalyst capable of oxidizing water. The water oxidation reaction is catalyzed at the Mn4Ca-oxo cluster in the oxygen-evolving complex (OEC), which cycles through five light-driven...

Chlorophylls (Chl)s exist in a variety of flavors and are ubiquitous in both the energy and electron transfer processes of photosynthesis. The functions they perform often occur on the ultrafast (fs – ns) time scale and until recently, these have been difficult to measure in real time. Further, the complexity of the binding pockets and the resultin...

This research addresses one of the most compelling issues in the field of photosynthesis, namely, the role of the accessory chlorophyll molecules in primary charge separation. Using a combination of empirical and computational methods, we demonstrate that the primary acceptor of photosystem (PS) I is a dimer of accessory and secondary chlorophyll m...

The photosynthetic reaction center, photosystem II (PSII), catalyzes one of the most energetically demanding reactions in nature by using light energy to drive water oxidation. The four-electron water oxidation reaction occurs at the tetranuclear manganese-calcium-oxo (Mn4Ca-oxo) cluster that is present in the oxygen-evolving complex (OEC) of PSII....

The photosynthetic water-oxidation reaction is catalyzed by the oxygen-evolving complex in photosystem II (PSII) that is comprised of the Mn4CaO5 cluster, with participation of the redox-active tyrosine residue (YZ) and a hydrogen-bonded network of amino acids and water molecules. It has been proposed that the strong hydrogen bond between YZ and D1...

p>Light-driven water oxidation in algae, cyanobacteria, and higher plants generates dioxygen that supports life on Earth. The water-oxidation reaction is catalyzed by the oxygen-evolving complex (OEC) in photosystem II (PSII) that is comprised of the tetranuclear manganese calcium-oxo (Mn₄CaO₅) cluster, with participation of t...

Annealing [Fe L 2 ][BF 4 ] 2 ∙2H 2 O ( L = 2,6- bis -[5-methyl-1 H -pyrazol-3-yl]pyridine) affords an anhydrous material, which undergoes a spin-transition at T ½ = 205 K with a 65 K thermal hysteresis loop. This occurs via a sequence of phase changes, which were monitored by powder diffraction in an earlier study. [Cu L 2 ][BF 4 ] 2 ∙2H 2 O and [Z...

Treatment of MOF-5 with NbCl4(THF)2 in acetonitrile leads to incorporation of Nb(IV) centers in a fashion that diverges from the established cation metathesis reactivity of this iconic material. A combination of X-ray absorption spectroscopy analysis and reactivity studies altogether supported by density functional theory computational studies docu...

The catecholate groups in [{Pt(L)}3(μ3-tctq)] (H6tctq = 2,3,6,7,10,11-hexahydroxy-4b,8b,12b,12d-tetramethyltribenzotriquinacene; L = a diphosphine chelate) undergo sequential oxidation to their semiquinonate forms by voltammetry, with ΔE½ = 160–170 mV. The monoradical [{Pt(dppb)}3(μ3-tctq•)]⁺ is valence-localised, with no evidence for intervalence...

A family of heterometallic Anderson-type ‘wheels’ of general formula [MIII2MII5(hmp)12]4+ (MIII = Cr or Al and MII = Ni or Zn, hmpH = 2-pyridinemethanol) has been extended to include MIII = Cr or Al and MII = Co, Fe, Mn or Cu, affording five new species of formulae [Cr2Co5(hmp)12](ClO4)4 (1), [Cr2Fe5(hmp)12](ClO4)4 (2), [Cr2Mn5(hmp)12](ClO4)4 (3),...

p>The reaction of the simple metalloligand [Fe^IIIL₃] [HL = 1-(4-pyridyl)butane-1,3-dione] with a variety of different M^II salts results in the formation of a family of heterometallic cages of formulae [Fe^III ₈Pd^II ₆L₂₄]Cl₁₂ (1), [Fe^III <sub...

The reaction of the simple metalloligand [FeIIIL3] [HL = 1-(4-pyridyl)butane-1,3-dione] with a variety of different MII salts results in the formation of a family of heterometallic cages of formulae [FeIII8PdII6L24]Cl12 (1), [FeIII8CuII6L24(H2O)4Br4]Br8 (2), [FeIII8CuII6L24(H2O)10](NO3)12 (3), [FeIII8NiII6L24(SCN)11Cl] (4), and [FeIII8CoII6L24(SCN)...

Michael addition (MA) is one of the most well studied chemical transformation in synthetic chemistry. Here, we report the synthesis and crystal structures of a library of 3d/4f coordination clusters (CCs) formulated as [Zn(II)2Y(III)2L4(solv)X(Z)Y] and study their catalytic properties toward the MA of nitrostyrenes with barbituric acid derivatives....

A family of five [MIII2MII3]n+ trigonal bipyramidal cages (MIII = Fe, Cr and Al; MII = Co, Zn and Pd; n = 0 for 1-3 and n = 6 for 4- 5) of formulae [Fe2Co3L6Cl6] (1), [Fe2Zn3L6Br6] (2), [Cr2Zn3L6Br6] (3), [Cr2Pd3L6(dppp)3](OTf)6 (4) and [Al2Pd3L6(dppp)3](OTf)6 (5) (where HL is 1-(4-pyridyl)butane-1,3-dione and dppp is 1,3-bis(diphenylphosphino)prop...

The solid and solution state structure of the vanadium(II) N-heterocyclic silylene (NHSi) complex, [(SiIPr)V(Cp)2] (1) is reported (SiIPr: 1,3-bis(2,6-diisopropylphenyl)-1,3-diaza-2-silacyclopent-4-en-2-ylidene). The electronic structure of 1 is probed using combination of magnetic measurements, EPR spectroscopy and computational studies. The V–Si...

At 20 K, as long as Tm~800−1000 ns were measured for the [2Fe-2S] cluster in its intrinsic protein environment. Such relaxations are a relatively long coherence time for the low-spin S=1/2 system. For this biological cluster, the phase memory time was found significantly affected by the nuclear hyperfine interactions of 14N with I=1. When labeling...

A well-defined, monodisperse Ag6+ cluster was prepared by mild chemical treatments including aqueous ion-exchange, dehydration, oxygen calcination at 673K and hydrogen reduction 293K, rather than autoreduction and irradiations with γ-ray and X-ray. H2 reduction was proved as a crucial step to form the nanosize cluster with six equivalent silver ato...

Chlorophylls are an essential class of cofactors found in all photo-synthetic organisms. Upon absorbing a photon, the excited state energy of the chlorophyll can either be transferred to another acceptor molecule, or be used to drive electron transfer. When acting as the primary donor in the bacterial photosynthetic reaction center, light-induced c...

An investigation of the redox-active tris[4-(pyridin-4-yl)phenyl]amine (NPy 3) ligand in the solution state and upon its incorporation into the solid-state metal− organic framework (MOF) [Zn(NPy 3)(NO 2) 2 ·xMeOH· xDMF] n (MeOH = methanol and DMF = N,N-dimethylfor-mamide) was conducted using in situ UV/vis/near-IR, electron paramagentic resonance (...

Three complexes of cyclotricatechylene (H6ctc), [{PtL}3(μ3-ctc)], have been synthesised: (L = 1,2-bis(diphenylphosphino)benzene {dppb}, 1; L = 1,2-bis(diphenylphosphino)ethane {dppe}, 2; L = 4,4′-bis(tert-butyl)-2,2′-bipyridyl {tBu2bipy}, 3). The complexes show three low-potential, chemically reversible voltammetric oxidations separated by ca. 180...

In-situ EXAFS experiments were performed at the Ag $K$-edge after adsorbing NO on the hydrogen reduced silver cluster. The coordination parameters, i.e. inter-atomic distance, coordination number and Debye-Waller factor were derived from a three-shell fitting model including a split Ag-Ag shell and a single Ag-O(N) shell. The multiple-shell fitting...

Three complexes of cyclotricatechylene (H6ctc), [{PtL}3(μ3-ctc)], have been synthesised: (L = 1,2-bis(diphenylphosphino)benzene {dppb}, 1; L = 1,2-bis(diphenylphosphino)ethane {dppe}, 2; L = 4,4′-bis(tert-butyl)-2,2′-bipyridyl {tBu2bipy}, 3). The complexes show three low-potential, chemically reversible voltammetric oxidations separated by ca. 180...

Three complexes of cyclotricatechylene (H6ctc), [{PtL}3(μ3-ctc)], have been synthesised: (L = 1,2-bis(diphenylphosphino)benzene {dppb}, 1; L = 1,2-bis(diphenylphosphino)ethane {dppe}, 2; L = 4,4'-bis(tert-butyl)-2,2'-bipyridyl { t Bu2bipy}, 3). The complexes show three low-potential, chemically reversible voltammetric oxidations separated by ca. 1...

The flavoenzyme monoamine oxidase (MAO) regulates mammalian behavioral patterns by modulating neurotransmitters such as adrenaline and serotonin. The mechanistic basis which underpins this enzyme is far from agreed upon. Reported herein is that the combination of a synthetic flavin and alloxan generates a catalyst system which facilitates biomimeti...

The biological [2Fe-2S](Cys) 4-n (His) n (n=0,1,2) cluster is one of the most common conduits in various redox systems from all three domains of life. Metalloenzymes with these cofactors play versatile physiological functions, and in some cases have been implicated as targets for pharmaceutical, bioengineering, and medical applications. Pulsed EPR...

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The structural and electrical properties of metastable defects in various types of hydrogenated amorphous silicon have been studied using a powerful combination of continuous wave electron-paramagnetic resonance spectroscopy, electron spin echo (ESE) decay measurements, and Doppler broadening positron annihilation spectroscopy. The observed depende...

12\% (wt.) silver containing catalysts were prepared by aqueous ion-exchange of Ag$^+$ against Na$^+$ in NaA zeolite. Hydrogen reduction leads to the formation of silver clusters. The results of X-ray absorption spectroscopy experiments at the Ag $K$-edge using direct {\it in-situ} reduction yield a Ag-Ag coordination number of 4.00, which is compa...

The phase coherence relaxation times as long as $T_2\sim830-1030\pm20$ ns were measured for the [2Fe-2S] cluster in the intrinsic protein environment. This relaxation corresponds to a relatively long lasting coherence of the low-spin $S=1/2$ state. For this biological cluster, the phase coherence relaxation time was significantly affected by the nu...

Due to the increasing use of their unique properties,metal nano-clusters are very promising candidates for developments in new age nano-electronic and nano-magnetic devices. In many respects, metal clusters no longer follow classical physical laws as most bulk materials do, but are correctly to be considered by means of quantum mechanics. The param...

Specific contributions of particular residues in the reaction mechanisms and/or folding dynamics of a metalloenzyme of interest can be best addressed by using magnetic resonance (e.g. NMR and EPR) and vibrational (e.g. FTIR and resonance Raman) spectroscopic techniques, often aided by X-ray crystallographic analysis. In this work, we report a set o...

The variation of a putative hydrogen bond network around a biological redox-active metal center with the polypeptide backbone frame is a common theme in tuning the structure and function of many metalloemzymes. The number of putative hydrogen bonds is usually estimated qualitatively from a (medium- to high-resolution) protein crystal structure of i...

A histidine ligand residue plays a crucial role in the coupled electron-proton transfer reaction in the Rieske [2Fe-2S](His)2(Cys)2 and mitoNEET-type [2Fe-2S](His)1(Cys)3 proteins. For example, in the cytochrome bc1/b6f family, extraction of a proton, as well as an electron, from bound quinol to the Rieske protein is a crucial step, and the pKa val...

The in vitro evolution and engineering of quorum-quenching lactonases with enhanced reactivities was achieved using a thermostable GKL enzyme as template, yielding the E101G/R230C GKL mutant with increased catalytic activity and broadened substrate range [Chow, J. Y., Xue, B., Lee, K. H., Tung, A., Wu, L., Robinson, R. C., and Yew, W. S. (2010) J B...

The electronic structure and geometry of redox-active metal cofactors in proteins are tuned by the pattern of hydrogen bonding with the backbone peptide matrix. In this study we developed a method for selective amino acid labeling of a hyperthermophilic archaeal metalloprotein with engineered Escherichia coli auxotroph strains, and applied this to...

In the photosynthetic reaction center from Rhodobacter sphaeroides, the primary (QA) and secondary (QB) electron acceptors are both ubiquinone-50 (Q-10), but with very different properties and functions. QB is always seen to occupy the proximal location when the RC was frozen under illumination — indicating that it is this conformation which traps...

In the QB site of the Rba. sphaeroides photosynthetic reaction centre the donation of a hydrogen bond from the hydroxyl group of Ser-L223 to the ubisemiquinone formed after the first flash is debatable. In this study we use a combination of spectroscopy and quantum mechanics/molecular mechanics (QM/MM) calculations to comprehensively explore this t...

Selective 15N isotope labeling of the cytochrome bo3 ubiquinol oxidase from Escherichia coli with auxotrophs was used to characterize the hyperfine couplings with the side-chain nitrogens from residues R71, H98, and Q101 and peptide nitrogens from residues R71 and H98 around the semiquinone (SQ) at the high-affinity QH site. The two-dimensional ESE...

MitoNEET (a mammalian mitochondrial outer membrane [2Fe-2S] protein) is a potential pharmacological/clinical target of the insulin-sensitizer pioglitazone for the treatment of the II diabetes. In this presentation, we report the 1.80-A structure and pulsed EPR spectroscopic analysis of a mitoNEET homolog (namely TthNEET0026) from an extreme thermop...

The cytochrome bo3 ubiquinol oxidase from Escherichia coli resides in the bacterial cytoplasmic membrane and catalyzes the two-electron oxidation of ubiquinol-8 and four-electron reduction of O2 to water. The one-electron reduced semiquinone forms transiently during the reaction, and the enzyme has been demonstrated to stabilize the semiquinone. Th...

Ag/NaA was prepared by aqueous ion-exchange of Ag(+) against Na(+) and investigated using X-band EPR spectroscopy. After hydrogen reduction, the EPR signal at a loading of 6% (wt.) exhibiting axial g- and hyperfine tensors (g(perpendicular to) = 1.998 and g(parallel to) = 2.214, coaxial with vertical bar A(perpendicular to)vertical bar = 23 G and v...

We present a Q-band spectrometer which was built recently at the Institute of Physical Chemistry of the University of Stuttgart. It allows us to perform the field-sweep electron spin echo (ESE), pulsed electron–nuclear double resonance (ENDOR), relaxation and electron spin echo envelope modulation experiments both at room and low (down to 1.5 K) te...

Size-selected clusters are important to fundamental investigations. Silver containing catalysts were prepared by aqueous ion exchange of Ag+ against Na+ cations in an LTA zeolite. A recent project deals with continuous wave and pulse EPR investigations into adsorption interactions of ethylene, oxygen, and nitrogen monoxide with supported silver clu...