11,109 research outputs found
Cold Adaptation of a Mesophilic Subtilisin-like Protease by Laboratory Evolution
Enzymes isolated from organisms native to cold environments generally exhibit higher catalytic efficiency at low temperatures and greater thermosensitivity than their mesophilic counterparts. In an effort to understand the evolutionary process and the molecular basis of cold adaptation, we have used directed evolution to convert a mesophilic subtilisin-like protease from Bacillus sphaericus, SSII, into its psychrophilic counterpart. A single round of random mutagenesis followed by recombination of improved variants yielded a mutant, P3C9, with a catalytic rate constant (kcat) at 10 °C 6.6 times and a catalytic efficiency (kcat/KM) 9.6 times that of wild type. Its half-life at 70 °C is 3.3 times less than wild type. Although there is a trend toward decreasing stability during the progression from mesophile to psychrophile, there is not a strict correlation between decreasing stability and increasing low temperature activity. A first generation mutant with a >2-fold increase in kcat is actually more stable than wild type. This suggests that the ultimate decrease in stability may be due to random drift rather than a physical incompatibility between low temperature activity and high temperature stability. SSII shares 77.4% identity with the naturally psychrophilic protease subtilisin S41. Although SSII and S41 differ at 85 positions, four amino acid substitutions were sufficient to generate an SSII whose low temperature activity is greater than that of S41. That none of the four are found in S41 indicates that there are multiple routes to cold adaptation
Stable Marriage with Multi-Modal Preferences
We introduce a generalized version of the famous Stable Marriage problem, now
based on multi-modal preference lists. The central twist herein is to allow
each agent to rank its potentially matching counterparts based on more than one
"evaluation mode" (e.g., more than one criterion); thus, each agent is equipped
with multiple preference lists, each ranking the counterparts in a possibly
different way. We introduce and study three natural concepts of stability,
investigate their mutual relations and focus on computational complexity
aspects with respect to computing stable matchings in these new scenarios.
Mostly encountering computational hardness (NP-hardness), we can also spot few
islands of tractability and make a surprising connection to the \textsc{Graph
Isomorphism} problem
Isotropic photonic band gap and anisotropic structures in transmission spectra of two-dimensional 5-fold and 8-fold symmetric quasiperiodic photonic crystals
We measured and calculated transmission spectra of two-dimensional
quasiperiodic photonic crystals (PCs) based on a 5-fold (Penrose) or 8-fold
(octagonal) symmetric quasiperiodic pattern. The photonic crystal consisted of
dielectric cylindrical rods in air placed normal to the basal plane on vertices
of tiles composing the quasiperiodic pattern. An isotropic photonic band gap
(PBG) appeared in the TM mode, where electric fields were parallel to the rods,
even when the real part of a dielectric constant of the rod was as small as
2.4. An isotropic PBG-like dip was seen in tiny Penrose and octagonal PCs with
only 6 and 9 rods, respectively. These results indicate that local multiple
light scattering within the tiny PC plays an important role in the PBG
formation. Besides the isotropic PBG, we found dips depending on the incident
angle of the light. This is the first report of anisotropic structures clearly
observed in transmission spectra of quasiperiodic PCs. Based on rod-number and
rod-arrangement dependence, it is thought that the shapes and positions of the
anisotropic dips are determined by global multiple light scattering covering
the whole system. In contrast to the isotropic PBG due to local light
scattering, we could not find any PBGs due to global light scattering even
though we studied transmission spectra of a huge Penrose PC with 466 rods.Comment: One tex file for manuscript and 12 PNG files for figures consisting
of Fig.1a-d, 2,3, ...
Inverse versus Normal NiAs Structure as High-Pressure Phase of FeO and MnO
The high-pressure phases of FeO and MnO were studied by the first principles
calculations. The present theoretical study predicts that the high-pressure
phase of MnO is a metallic normal B8 structure (nB8), while that of FeO should
take the inverse B8 structure (iB8). The novel feature of the unique
high-pressure phase of stoichiometric FeO is that the system should be a band
insulator in the ordered antiferromagnetic (AF) state and that the existence of
a band gap leads to special stability of the phase. The observed metallicity of
the high-pressure and high-temperature phase of FeO may be caused by the loss
of AF order and also by the itinerant carriers created by non-stoichiometry.
Analysis of x-ray diffraction experiments provides a further support to the
present theoretical prediction for both FeO and MnO. Strong stability of the
high-pressure phase of FeO will imply possible important roles in Earth's core.Comment: 7 pages, 3 figures and 1 table; submitted to "Nature
Equivalence between Schwinger and Dirac schemes of quantization
This paper introduces the modified version of Schwinger's quantization
method, in which the information on constraints and the choice of gauge
conditions are included implicitly in the choice of variations used in
quantization scheme. A proof of equivalence between Schwinger- and
Dirac-methods for constraint systems is given.Comment: 12pages, No figures, Latex, The proof is improved and one reference
is adde
Novel phase diagram for antiferromagnetism and superconductivity in pressure-induced heavy-fermion superconductor CeRhIn probed by In-NQR
We present a novel phase diagram for the antiferromagnetism and
superconductivity in CeRhIn probed by In-NQR studies under pressure
(). The quasi-2D character of antiferromagnetic spin fluctuations in the
paramagnetic state at = 0 evolves into a 3D character because of the
suppression of antiferromagnetic order for 1.36 GPa (QCP:
antiferromagnetic quantum critical point). Nuclear-spin-lattice-relaxation rate
measurements revealed that the superconducting order occurs in the
range 1.36 -- 1.84 GPa, with maximum 0.9 K around
1.36 GPa.Comment: 5 pages, 5 figures, submitted to PR
Crystal-field-induced magnetostrictions in the spin reorientation process of NdFeB-type compounds
Volume expansion associated with the spin reorientation
process of NdFeB-type compounds has been investigated in terms of
simple crystalline-electric-field (CEF) model. In this system,
is shown to be a direct measure of second order CEF energy. Calculated
anomalies in associated with the first-order magnetization
process of NdFeB are presented, which well reproduced the
observations.Comment: 2 pages, 2 figures, to appear in J. Magn. Magn. Mate
Structure and magnetism in nanocrystalline Ca(La)B films
Nanocrystalline films of La-doped CaB have been fabricated by using a
rf-magnetron sputtering. Lattice expansion of up to 6% with respect to the bulk
value was observed along the direction perpendicular to the film plane, which
arises from the trapping of Ar gas into the film. Large ferromagnetic moment of
3 ~ 4 Bohr magneton per La has been observed in some La-doped films only when
the lattice expansion rate is larger than 2.5%.Comment: 2 pages, 2 figures, to appear in J. Magn. Magn. Mate
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