Sebastian C. Peter

Dimensional Reduction: A Design Tool for New Radiation Detection Materials

John Androulakis , Sebastian C. Peter , Hao Li , Christos D. Malliakas , John A. Peters , Zhifu Liu , Bruce W W. essels , Jung-Hwan Song , Hosub Jin , Arthur J. reeman , F and Mercouri G. Kanatzidis *

Selective hydroxylation of alkanes by an extracellular fungal peroxygenase

Fungal peroxygenases are novel extracellular heme‐thiolate biocatalysts that are capable of catalyzing the selective monooxygenation of diverse organic compounds, using only H2O2 as a cosubstrate. Little is known about the physiological role or the catalytic mechanism of these enzymes. We have found that the peroxygenase secreted by Agrocybe aegerita catalyzes the H2O2‐dependent hydroxylation of linear alkanes at the 2‐position and 3‐position with high efficiency, as well as the regioselective monooxygenation of branched and cyclic alkanes. Experiments with n‐heptane and n‐octane showed that the hydroxylation proceeded with complete stereoselectivity for the (R)‐enantiomer of the corresponding 3‐alcohol. Investigations with a number of model substrates provided information about the route of alkane hydroxylation: (a) the hydroxylation of cyclohexane mediated by H218O2 resulted in complete incorporation of 18O into the hydroxyl group of the product cyclohexanol; (b) the hydroxylation of n‐hexane‐1,1,1,2,2,3,3‐D7 showed a large intramolecular deuterium isotope effect [(kH/kD)obs] of 16.0 ± 1.0 for 2‐hexanol and 8.9 ± 0.9 for 3‐hexanol; and (c) the hydroxylation of the radical clock norcarane led to an estimated radical lifetime of 9.4 ps and an oxygen rebound rate of 1.06 × 1011 s−1. These results point to a hydrogen abstraction and oxygen rebound mechanism for alkane hydroxylation. The peroxygenase appeared to lack activity on long‐chain alkanes (> C16) and highly branched alkanes (e.g. tetramethylpentane), but otherwise exhibited a broad substrate range. It may accordingly have a role in the bioconversion of natural and anthropogenic alkane‐containing structures (including alkyl chains of complex biomaterials) in soils, plant litter, and wood.

Driving Force for Oxygen Atom Transfer by Heme-Thiolate Enzymes

The heme-thiolate peroxygenase AaeAPO from Agrocybe aegerita is an important biocatalyst and P450 analog. We have found that AaeAPO compound I can be formed via oxidation of the ferric protein with HOBr and HOCl. The rate constant for the formation of AaeAPO-I induced by HOBr at pH 5.0, 4 °C was 7.1 × 10 M−1s−1. AaeAPO-I reacts with bromide and chloride ions to regenerate the resting ferric protein. Similar measurements were made for chloroperoxidase (CPO). The rate constant for the reaction of AaeAPO-I with bromide ion at pH 5.0, 4 °C was 2.6 × 105 M−1s−1. By measuring the rates of the forward and reverse reactions over a wide range of pH, Nernst plots of the driving force for oxygen atom transfer from AaeAPO-I and CPO-I can be constructed. It is found that CPO-I and AaeAPO-I have a two-electron redox potential similar to that of HOBr and about 200 mV less than that of HOCl. Interestingly, CPO-I and AaeAPO-I are both much more oxidizing than HRP compound I. The results are informative with regard to the reactivity of these proteins toward C–H bonds.

Metallic Yb2AuGe3: An Ordered Superstructure in the AlB2-Type Family with Mixed-Valent Yb and a High-Temperature Phase Transition

The intermetallic compound Yb(2)AuGe(3) was synthesized from indium flux. Yb(2)AuGe(3) crystallizes in the orthorhombic Ca(2)AgSi(3)-type structure which is an ordered superstructure of the AlB(2) structure type. The structure was refined in the Fmmm space group with lattice parameters a = 8.5124(17) Å, b = 14.730(3) Å, c = 8.4995(17) Å. Temperature-dependent powder X-ray diffraction studies show that Yb(2)AuGe(3) undergoes a phase transition from orthorhombic to hexagonal upon heating above 773 K. The compound shows weak paramagnetism that derives from a combination of Curie and Pauli paramagnetism with a magnetic moment value of 0.33(2) μ(B)/Yb atom. Magnetic ordering was not observed down to 2 K. Yb(2)AuGe(3) is metallic, and at low temperature the resistivity varies as T(2), indicating possible Fermi liquid behavior. Heat capacity measurements suggest that Yb(2)AuGe(3) is possibly a moderate heavy fermion system.

Anomalous thermal expansion in the square-net compounds RE4TGe8 (RE = Yb, Gd; T = Cr-Ni, Ag).

The family of materials RE(4)TGe(8) (RE = Yb, Gd; T = transition metal) exhibits directional zero thermal expansion (ZTE) via a process that is associated with the linking of planar square nets in the third dimension. The Ge square nets in these compounds exhibit commensurate long-range modulations similar to those observed in charge-density-wave compounds. The ZTE is manifested in the plane of the square nets from 10 to 300 K with negligible volume expansion below ∼160 K. The specific atomic arrangement in RE(4)TGe(8) enables a Poisson-like mechanism that allows the structure to contract along one direction as it expands only slightly in the perpendicular direction.

Crystal structure of YbCu6In6 and mixed valence behavior of Yb in YbCu(6-x)In(6+x) (x = 0, 1, and 2) solid solution.

High quality single crystals of YbCu(6)In(6) have been grown using the flux method and characterized by means of single crystal X-ray diffraction data. YbCu(6)In(6) crystallizes in the CeMn(4)Al(8) structure type, tetragonal space group I4/mmm, and the lattice constants are a = b = 9.2200(13) Å and c = 5.3976(11) Å. The crystal structure of YbCu(6)In(6) is composed of pseudo-Frank-Kasper cages filled with one ytterbium atom in each ring. The neighboring cages share corners along [100] and [010] to build the three-dimensional network. YbCu(6-x)In(6+x) (x = 0, 1, and 2) solid solution compounds were obtained from high frequency induction heating and characterized using powder X-ray diffraction. The magnetic susceptibilities of YbCu(6-x)In(6+x) (x = 0, 1, and 2) were investigated in the temperature range 2-300 K and showed Curie-Weiss law behavior above 50 K, and the experimentally measured magnetic moment indicates mixed valent ytterbium. A deviation in inverse susceptibility data at 200 K suggests a valence transition from Yb(2+) to Yb(3+) as the temperature decreases. An increase in doping of Cu at the Al2 position enhances the disorder in the system and enhancement in the trivalent nature of Yb. Electrical conductivity measurements show that all compounds are of a metallic nature.

Epoxidation of linear, branched and cyclic alkenes catalyzed by unspecific peroxygenase.

Unspecific peroxygenases (EC 1.11.2.1) represent a group of secreted heme-thiolate proteins that are capable of catalyzing the mono-oxygenation of diverse organic compounds, using only H2O2 as a co-substrate. Here we show that the peroxygenase secreted by the fungus Agrocybe aegerita catalyzed the oxidation of 20 different alkenes. Five branched alkenes, among them 2,3-dimethyl-2-butene and cis-2-butene, as well as propene and butadiene were epoxidized with complete regioselectivity. Longer linear alkenes with a terminal double bond (e.g. 1-octene) and cyclic alkenes (e.g. cyclohexene) were converted into the corresponding epoxides and allylic hydroxylation products; oxidation of the cyclic monoterpene limonene yielded three oxygenation products (two epoxides and an alcohol). In the case of 1-alkenes, the conversion occurred with moderate stereoselectivity, in which the preponderance for the (S)-enantiomer reached up to 72% ee for the epoxide product. The apparent Michaelis-Menten constant (Km) for the epoxidation of the model substrate 2-methyl-2-butene was 5mM, the turnover number (kcat) 1.3×10(3)s(-1) and the calculated catalytic efficiency, kcat/Km, was 2.5×10(5)M(-1)s(-1). As epoxides represent chemical building blocks of high relevance, new enzymatic epoxidation pathways are of interest to complement existing chemical and biotechnological approaches. Stable and versatile peroxygenases as that of A. aegerita may form a promising biocatalytic platform for the development of such enzyme-based syntheses.

Yb5Ga2Sb6: A Mixed Valent and Narrow-Band Gap Material in the RE5M2X6 Family

A new compound Yb5Ga2Sb6 was synthesized by the metal flux technique as well as high frequency induction heating. Yb5Ga2Sb6 crystallizes in the orthorhombic space group Pbam (no. 55), in the Ba5Al2Bi6 structure type, with a unit cell of a = 7.2769(2) Å, b = 22.9102(5) Å, c = 4.3984(14) Å, and Z = 2. Yb5Ga2Sb6 has an anisotropic structure with infinite anionic double chains (Ga2Sb6)(10-) cross-linked by Yb(2+) and Yb(3+) ions. Each single chain is made of corner-sharing GaSb4 tetrahedra. Two such chains are bridged by Sb2 groups to form double chains of 1/∞ [Ga2Sb6(10-)]. The compound satisfies the classical Zintl-Klemm concept and is a narrow band gap semiconductor with an energy gap of around 0.36 eV calculated from the electrical resistivity data corroborating with the experimental absorption studies in the IR region (0.3 eV). Magnetic measurements suggest Yb atoms in Yb5Ga2Sb6 exist in the mixed valent state. Temperature dependent magnetic susceptibility data follows the Curie-Weiss behavior above 100 K and no magnetic ordering was observed down to 2 K. Experiments are accompanied by all electron full-potential linear augmented plane wave (FP-LAPW) calculations based on density functional theory to calculate the electronic structure and density of states. The calculated band structure shows a weak overlap of valence band and conduction band resulting in a pseudo gap in the density of states revealing semimetallic character.

Structural phase transitions in a new compound Eu2AgGe3.

A new intermetallic compound Eu2AgGe3 has been synthesized using high-frequency induction heating method. Single-crystal X-ray diffraction data showed that Eu2AgGe3 crystallizes in the orthorhombic Ba2LiSi3 structure type, with Fddd space group and lattice parameters a = 8.7069(17) Å, b = 15.011(3) Å, c = 17.761(4) Å. Eu2AgGe3 is composed of infinite arrays of hexagonal [Ag3Ge3] units stacked along the [001] direction, and the Eu sites are sandwiched between these parallel hexagonal networks. Temperature-dependent powder XRD data and DTA hint toward a structural phase transition from orthorhombic to hexagonal above 477 K and an unusual reversible transition to the original phase, i.e., orthorhombic phase at around 718 K. Magnetic measurements on Eu2AgGe3 sample show paramagnetic behavior above 100 K and weak ferromagnetic interactions below 80 K. Mössbauer spectroscopy and X-ray absorption near-edge spectroscopic (XANES) studies reveal that Eu atoms in Eu2AgGe3 exist in the divalent oxidation state.

Single crystal growth of europium and ytterbium based intermetallic compounds using metal flux technique

AbstractThis article covers the use of indium as a potential metal solvent for the crystal growth of europium and ytterbium-based intermetallic compounds. A brief view about the advantage of metal flux technique and the use of indium as reactive and non-reactive flux are outlined. Large single crystals of EuGe2, EuCoGe3 and Yb2AuGe3 compounds were obtained in high yield from the reactions of the elements in liquid indium. The results presented here demonstrate that considerable advances in the discovery of single crystal growth of complex phases are achievable utilizing molten metals as solvents. Graphical AbstractMetal flux is an important synthetic tool in solid-state chemistry for both crystal-growth studies and exploratory studies of europium and ytterbium-based intermetallic compounds. Large single crystals of EuGe2, EuCoGe3 and Yb2AuGe3 compounds were obtained in high yield from the reactions of the elements in liquid indium.