27,766 research outputs found
Catalysts on Demand: Selective Oxidations by Laboratory-Evolved Cytochrome P450 BM3
Efficient catalysts for selective oxidation of C-H bonds using atmospheric oxygen are highly desirable to decrease the economic and environmental costs associated with conventional oxidation processes. We have used methods of directed evolution to generate variants of bacterial cytochrome P450 BM3 that catalyze hydroxylation and epoxidation of a wide range of nonnative substrates. This fatty acid hydroxylase was converted to a propane monooxygenase (PMO) capable of hydroxylating propane at rates comparable to that of BM3 on its natural substrates. Variants along the PMO evolutionary lineage showed broadened substrate scope; these became the starting points for evolution of a wide array of enzymes that can hydroxylate and derivatize organic scaffolds. This work demonstrates how a single member of enzyme family is readily converted by evolution into a whole family of catalysts for organic synthesis
Enzymatic functionalization of carbon-hydrogen bonds
The development of new catalytic methods to functionalize carbon–hydrogen (C–H) bonds
continues to progress at a rapid pace due to the significant economic and environmental benefits
of these transformations over traditional synthetic methods. In nature, enzymes catalyze regio- and
stereoselective C–H bond functionalization using transformations ranging from hydroxylation to
hydroalkylation under ambient reaction conditions. The efficiency of these enzymes relative to
analogous chemical processes has led to their increased use as biocatalysts in preparative and
industrial applications. Furthermore, unlike small molecule catalysts, enzymes can be systematically
optimized via directed evolution for a particular application and can be expressed in vivo to
augment the biosynthetic capability of living organisms. While a variety of technical challenges
must still be overcome for practical application of many enzymes for C–H bond functionalization,
continued research on natural enzymes and on novel artificial metalloenzymes will lead to improved
synthetic processes for efficient synthesis of complex molecules. In this critical review, we discuss the
most prevalent mechanistic strategies used by enzymes to functionalize non-acidic C–H bonds, the
application and evolution of these enzymes for chemical synthesis, and a number of potential
biosynthetic capabilities uniquely enabled by these powerful catalysts (110 references)
Measurement of temperature profiles in hot gases and flames
Computer program was written for calculation of molecular radiative transfer from hot gases. Shape of temperature profile was approximated in terms of simple geometric forms so profile could be characterized in terms of few parameters. Parameters were adjusted in calculations using appropriate radiative-transfer expression until best fit was obtained with observed spectra
Selective decay by Casimir dissipation in fluids
The problem of parameterizing the interactions of larger scales and smaller
scales in fluid flows is addressed by considering a property of two-dimensional
incompressible turbulence. The property we consider is selective decay, in
which a Casimir of the ideal formulation (enstrophy in 2D flows, helicity in 3D
flows) decays in time, while the energy stays essentially constant. This paper
introduces a mechanism that produces selective decay by enforcing Casimir
dissipation in fluid dynamics. This mechanism turns out to be related in
certain cases to the numerical method of anticipated vorticity discussed in
\cite{SaBa1981,SaBa1985}. Several examples are given and a general theory of
selective decay is developed that uses the Lie-Poisson structure of the ideal
theory. A scale-selection operator allows the resulting modifications of the
fluid motion equations to be interpreted in several examples as parameterizing
the nonlinear, dynamical interactions between disparate scales. The type of
modified fluid equation systems derived here may be useful in modelling
turbulent geophysical flows where it is computationally prohibitive to rely on
the slower, indirect effects of a realistic viscosity, such as in large-scale,
coherent, oceanic flows interacting with much smaller eddies
Spin-Correlation Coefficients and Phase-Shift Analysis for p+He Elastic Scattering
Angular Distributions for the target spin-dependent observables A,
A, and A have been measured using polarized proton beams at
several energies between 2 and 6 MeV and a spin-exchange optical pumping
polarized He target. These measurements have been included in a global
phase-shift analysis following that of George and Knutson, who reported two
best-fit phase-shift solutions to the previous global p+He elastic
scattering database below 12 MeV. These new measurements, along with
measurements of cross-section and beam-analyzing power made over a similar
energy range by Fisher \textit{et al.}, allowed a single, unique solution to be
obtained. The new measurements and phase-shifts are compared with theoretical
calculations using realistic nucleon-nucleon potential models.Comment: Submitted to Phys. Rev.
Fourier transform infrared spectrometer for a single aerosol particle
A spectrometer is reported here for obtaining the infrared spectrum of a single aqueous aerosol particle by a Fourier transform technique. The particle is held in an electrodynamic balance and irradiated simultaneously by the infrared output from a Michelson interferometer and the visible light from a dye laser. The size of the particle is modulated by chopping the IR beam, and the resulting visible scattered light fluctuation is detected at 90° with a photomultiplier tube. The amplitude of the scattered light fluctuation is measured with a lock-in amplifier at each interferometer mirror position. The electronic circuitry for stepping the interferometer mirror is presented and discussed. Inverting the lock-in signal by a discrete fast Fourier transform routine (FFT) yields the particle absorption spectrum. The resulting spectrum for an (NH4)2SO4 droplet is presented
Measurement of temperature profiles in hot gases by emission-absorption spectroscopy Final report
Measurement of spectral radiances and absorptances in hot gase
Directed evolution of Vibrio fischeri LuxR for improved response to butanoyl-homoserine lactone
LuxR is the 3-oxohexanoyl-homoserine lactone (3OC6HSL) dependent transcriptional activator of the prototypical acyl-homoserine lactone (AHL) quorum sensing system of Vibrio fischeri. Wild-type LuxR exhibits no response to butanoyl-HSL (C4HSL) in quantitative bioassays at concentrations of up to 1 µM; a previously described LuxR variant (LuxR-G2E) exhibits a broadened response to diverse AHLs, including pentanoyl-HSL (C5HSL), but not to C4HSL. Here, two rounds of directed evolution of LuxR-G2E generated variants of LuxR that responded to C4HSL at concentrations as low as 10 nM. One variant, LuxR-G4E, had only one change, I45F, relative to the parent LuxR-G2E, which itself differs from wild-type at three residues. Dissection of the four mutations within LuxR-G4E demonstrated that at least three of these changes were simultaneously required to achieve any measurable C4HSL response. The four changes improved both sensitivity and specificity towards C4HSL relative to any of the other 14 possible combinations of those residues. These data confirm that LuxR is evolutionarily pliable and suggest that LuxR is not intrinsically asymmetric in its response to quorum sensing signals with different acyl-side chain lengths
Spaces of finite element differential forms
We discuss the construction of finite element spaces of differential forms
which satisfy the crucial assumptions of the finite element exterior calculus,
namely that they can be assembled into subcomplexes of the de Rham complex
which admit commuting projections. We present two families of spaces in the
case of simplicial meshes, and two other families in the case of cubical
meshes. We make use of the exterior calculus and the Koszul complex to define
and understand the spaces. These tools allow us to treat a wide variety of
situations, which are often treated separately, in a unified fashion.Comment: To appear in: Analysis and Numerics of Partial Differential
Equations, U. Gianazza, F. Brezzi, P. Colli Franzone, and G. Gilardi, eds.,
Springer 2013. v2: a few minor typos corrected. v3: a few more typo
correction
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