123 research outputs found
The Ser176 of T4 endonuclease IV is crucial for the restricted and polarized dC-specific cleavage of single-stranded DNA implicated in restriction of dC-containing DNA in host Escherichia coli
Endonuclease (Endo) IV encoded by denB of bacteriophage T4 is an enzyme that cleaves single-stranded (ss) DNA in a dC-specific manner. Also the growth of dC-substituted T4 phage and host Escherichia coli cells is inhibited by denB expression presumably because of the inhibitory effect on replication of dC-containing DNA. Recently, we have demonstrated that an efficient cleavage by Endo IV occurs exclusively at the 5ā²-proximal dC (dC1) within a hexameric or an extended sequence consisting of dC residues at the 5ā²-proximal and the 3ā²-proximal positions (dCs tract), in which a third dC residue within the tract affects the polarized cleavage and cleavage rate. Here we isolate and characterize two denB mutants, denB(W88R) and denB(S176N). Both mutant alleles have lost the detrimental effect on the host cell. Endo IV(W88R) shows no enzymatic activity (<0.4% of that of wild-type Endo IV). On the other hand, Endo IV(S176N) retains cleavage activity (17.5% of that of wild-type Endo IV), but has lost the polarized and restricted cleavage of a dCs tract, indicating that the Ser176 residue of Endo IV is implicated in the polarized cleavage of a dCs tract which brings about a detrimental effect on the replication of dC-containing DNA
Heteroepitaxial growth and optoelectronic properties of layered iron oxyarsenide, LaFeAsO
Epitaxial thin films of LaFeAsO were fabricated on MgO (001) and
mixed-perovskite (La, Sr)(Al, Ta)O3 (001) single-crystal substrates by pulsed
laser deposition using a Nd:YAG second harmonic source and a 10 at.% F-doped
LaFeAsO disk target. Temperature dependences of the electrical resistivities
showed no superconducting transition in the temperature range of 2-300 K, and
were similar to those of undoped polycrystalline bulk samples. The
transmittance spectrum exhibited a clear peak at ~0.2 eV, which is explained by
ab-initio calculations.Comment: Submission: 31st July 2008, Accepted for publication in Appl. Phys.
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Degenerate p-type conductivity in wide-gap LaCuOS1āxSex (x = 0ā1) epitaxial films
Epitaxial films of LaCuOS1āxSex (x = 0ā1) solid solution were grown on MgO (001) substrates and their electrical and optical properties were examined. Sharp emission due to room-temperature exciton with binding energy of ~50 meV is observed for all x values. Hall mobility becomes large with an increase in the Se content and it reaches 8.0 cm2Vā1sā1 in LaCuOSe, a comparable value to that of p-type GaN:Mg. Doping of Mg2+ ions at La3+ sites enhances a hole concentration up to 2.2Ć1020 cmā3, while maintaining the Hall mobility as large as 4.0 cm2Vā1sā1. Consequently, a degenerate p-type electrical conduction with a conductivity of 140 S cmā1 was achieved
Single-atomic-layered quantum wells built in wide-gap semiconductors LnCuOCh (Ln=lanthanide, Ch=chalcogen)
LnCuOCh (Ln=lanthanide, Ch=chalcogen) layered oxychalcogenides are wide-gap p-type semiconductors composed of alternately stacked (Ln2O2)2+ oxide layers and (Cu2Ch2)2- chalcogenide layers. Energy band calculations revealed that Cu-Ch hybridized bands only spread in the (Cu2Ch2)2- layers, which suggests that hole carriers in these bands are confined by the potential barriers formed by the (Ln2O2)2+ layers. Stepwise absorption spectra of a series of LnCuOCh experimentally verified that an exciton in the (Cu2Ch2)2- layers shows a two-dimensional behavior. These theoretical and experimental results indicate that LnCuOCh has ānatural multiple quantum wellsā built into its layered structure
Excitonic blue luminescence from p-LaCuOSe/n-InGaZn5O8 light-emitting diode at room temperature
A hetero p/n junction diode was fabricated by laminating an amorphous n-type InGaZn5O8 layer to a p-type LaCuOSe film epitaxially grown on a MgO (001) substrate. It exhibited a relatively sharp blue electroluminescence (EL) that peaked at ~430 nm at room temperature when a forward bias voltage above 8 V was applied. The wavelength and bandwidth of the EL band agreed well with those of the excitonic photoluminescence band in LaCuOSe, which indicates that the EL band originates from the exciton in LaCuOSe. This experiment strongly suggests that layered compounds, LnCuOCh (Ln=lanthanide, Ch=chalcogen), are promising as the light-emitting layer in optoelectronic devices that operate in the blueāultraviolet region
Intrinsic excitonic photoluminescence and band-gap engineering of wide-gap p-type oxychalcogenide epitaxial films of LnCuOCh (Ln = La, Pr, and Nd; Ch = S or Se) semiconductor alloys
The optical spectroscopic properties of layered oxychalcogenide semiconductors LnCuOCh (Ln = La, Pr, and Nd; Ch = S or Se) on epitaxial films were thoroughly investigated near the fundamental energy band edges. Free exciton emissions were observed for all the films between 300 and ~30 K. In addition, a sharp emission line, which was attributed to bound excitons, appeared below ~80 K. The free exciton energy showed a nonmonotonic relationship with lattice constant and was dependent on lanthanide and chalcogen ion substitutions. These results imply that the exciton was confined to the (Cu2Ch2)2ā layer. Anionic and cationic substitutions tune the emission energy at 300 K from 3.21 to 2.89 eV and provide a way to engineer the electronic structure in light-emitting devices
Third-order optical nonlinearity originating from room-temperature exciton in layered compounds LaCuOS and LaCuOSe
We have studied the third-order optical nonlinearity ((3)) of epitaxial thin films of layered compounds LaCuOS and LaCuOSe at room temperature by a spectrally resolved degenerative four-wave mixing technique with femtosecond time resolution. The (3) values in both films are sharply resonant to optical absorption bands in the ultraviolet (UV) light region due to room-temperature exciton. The peak values are evaluated to be as large as 2ā4Ć10ā9 esu with a fast time response of 250ā300 fs. These findings indicate that LaCuOS and LaCuOSe are promising materials for emerging optical nonlinear devices that operate in the UV light region compatible for GaN-based lasers
Intrinsic excitonic photoluminescence and band-gap engineering of wide-gap p-type oxychalcogenide epitaxial films of LnCuOCh (Ln = La, Pr, and Nd; Ch = S or Se) semiconductor alloys
The optical spectroscopic properties of layered oxychalcogenide semiconductors LnCuOCh (Ln = La, Pr, and Nd; Ch = S or Se) on epitaxial films were thoroughly investigated near the fundamental energy band edges. Free exciton emissions were observed for all the films between 300 and ~30 K. In addition, a sharp emission line, which was attributed to bound excitons, appeared below ~80 K. The free exciton energy showed a nonmonotonic relationship with lattice constant and was dependent on lanthanide and chalcogen ion substitutions. These results imply that the exciton was confined to the (Cu2Ch2)2ā layer. Anionic and cationic substitutions tune the emission energy at 300 K from 3.21 to 2.89 eV and provide a way to engineer the electronic structure in light-emitting devices
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