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- Marinova, Ts., et al.
(författare)
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Nickel based ohmic contacts on SiC
- 1997
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Ingår i: Materials Science and Engineering B. - 0921-5107. ; 46:1-3, s. 223-226
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Tidskriftsartikel (refereegranskat)abstract
- We have compared the chemical and structural properties of Ni/SiC and Ni2Si/SiC interfaces. In the case of Ni/SiC, the contact formation is initiated by the dissociation of SiC, due to the strong reactivity of nickel at 950 °C. Ni2Si is formed and carbon accumulates, both at the interface and throughout the metal layer. At the interface, many Kirkendall voids are observed by TEM. Despite this poor interface morphology, low contact resistances have been measured. But the presence of carbon in the contact layer and at the interface is a potential source of contact degradation at high temperature. In the case of Ni/Si multilayers evaporated on SiC instead of pure Ni, the contact formation is preceded by Ni and Si mutual diffusion in the deposited layer yielding Ni2Si. Therefore, a smaller amount of carbon is released from SiC. Low carbon segregation, abrupt interface and low contact resistance characterize this contact. The thermal stability of Ni2Si contacts is illustrated with ageing experiments carried out at 500 °C.
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- Zhang, Penghua, et al.
(författare)
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Engineering BspQI nicking enzymes and application of N.BspQI in DNA labeling and production of single-strand DNA
- 2010
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Ingår i: Protein Expression and Purification. - : Elsevier BV. - 1046-5928. ; 69:2, s. 226-234
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Tidskriftsartikel (refereegranskat)abstract
- BspQI is a thermostable Type IIS restriction endonuclease (REase) with the recognition sequence 5′GCTCTTC N1/N4 3′. Here we report the cloning and expression of the bspQIR gene for the BspQI restriction enzyme in Escherichia coli. Alanine scanning of the BspQI charged residues identified a number of DNA nicking variants. After sampling combinations of different amino acid substitutions, an Nt.BspQI triple mutant (E172A/E248A/E255K) was constructed with predominantly top-strand DNA nicking activity. Furthermore, a triple mutant of BspQI (Nb.BspQI, N235A/K331A/R428A) was engineered to create a bottom-strand nicking enzyme. In addition, we demonstrated the application of Nt.BspQI in optical mapping of single DNA molecules. Nt or Nb.BspQI-nicked dsDNA can be further digested by E. coli exonuclease III to create ssDNA for downstream applications. BspQI contains two potential catalytic sites: a top-strand catalytic site (Ct) with a D-H-N-K motif found in the HNH endonuclease family and a bottom-strand catalytic site (Cb) with three scattered Glu residues. BlastP analysis of proteins in GenBank indicated a putative restriction enzyme with significant amino acid sequence identity to BspQI from the sequenced bacterial genome Croceibacter atlanticus HTCC2559. This restriction gene was amplified by PCR and cloned into a T7 expression vector. Restriction mapping and run-off DNA sequencing of digested products from the partially purified enzyme indicated that it is an EarI isoschizomer with 6-bp recognition, which we named CatHI (CTCTTC N1/N4).
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