Method for rapid determination of ion gauge sensitivity factors

In ultrahigh vacuum thin film growth processes using gas phase growth precursors, the pressure of the gas at or near the substrate is a critical parameter since it is directly related to the collision frequency of the precursor with the substrate and ultimately to the growth rate. These pressures are usually measured using a nude Bayrd-Alpert-type ion gauges, which are generally calibrated for nitrogen. Consequently, it is necessary to know the ion gauge sensitivity factor that relates the measured pressure to the actual pressure of the growth precursor. The purpose of this article is to describe a simple method to obtain such sensitivity factors. This method uses a simple gas manifold comprised of equipment commonly found in laboratory settings where ultrahigh vacuum work is performed. Results are reported for dimethyl silane, monomethyl silane, methane, and hydrogen. The gauge sensitivity factors for the latter two gases are known and, therefore, provide a basis for validating the method.     

用小型散热器对DirectFET封装功率MOSFET进行双面散热

本文探讨了可以用于生产厂封装的器件的各种热管理方案,并且引用了国际整流器公司的测试数据,来评估使用小型散热器改善这些器件散热性能的优点,并研究了在进行通电循环的情况下,在器件上直接安装散热器对器件可靠性的影响。     

Electrically evoked compound action potentials of guinea pig and cat: responses to monopolar, monophasic stimulation

On the basis of sequence comparison with the M subunit of the reaction center of purple bacteria, no residues in photosystem II can be clearly identified that may be predicted to correspond to the His residue that binds one of the accessory bacteriochlorophylls in the purple bacterial reaction center. However, the Arg180 residue of the D2 protein is close to where this residue is predicted to be and could conceivably serve as a chlorophyll ligand. To analyze the function of Arg180, it was changed to nine different amino acids in the cyanobacterium Synechocystis sp. PCC 6803. Except for the Arg180-->Gln (R180Q) mutant, the resulting strains were no longer photoautotrophic. The properties of photosystem II upon mutation of Arg180 were probed in strains from which photosystem I had been deleted genetically. Mutations at the Arg180 residue affected oxygen evolution capacity and the amount of photosystem II that was present in thylakoids. Surprisingly, in the Arg180 mutants, EPR signals that may originate from the oxidized redoxactive Tyr160 of the D2 protein (Y(D)ox) were small and generally did not resemble the usual signal IIs, signifying an effect of the Arg180 mutations on the environment surrounding Tyr160. In addition, in most mutants, the charge recombination kinetics between the primary electron-accepting quinone in photosystem II (Q(A)-) and oxidized species on the donor side were faster upon introducing mutations at Arg180 suggesting an increased steady-state concentration of P680+ in the mutants. However, Arg180 mutations also affected Q(A)- oxidation by the secondary electron-accepting quinone (Q(B)). HPLC analysis showed that, in the Arg180 mutants that were assayed, the pheophytin/chlorophyll ratio of photosystem II had not changed, indicating that the mutations did not lead to a pheophytinization of one of the chlorophyll molecules. Even though the results presented do not provide positive evidence that Arg180 of the D2 protein corresponds in function to the ligand to the central Mg in an accessory bacteriochlorophyll in reaction centers of purple bacteria, it is clear that changes in Arg180 greatly affect Tyr160 and P680. Various scenarios are discussed that are compatible with the data presented, and include an apparently close interaction between Arg180, His189, and Tyr160, and the possibility of the involvement of multiple chlorophylls to together form P680.     

Ligand-induced conformational change in transferrins: crystal structure of the open form of the N-terminal half-molecule of human transferrin

Serum transferrin binds ferric ions in the bloodstream and transports them to cells, where they are released in a process involving receptor-mediated endocytosis. Iron release is believed to be pH dependent and is coupled with a large conformational change. To help define the steps in iron release, we have determined the three-dimensional structure of the iron-free (apo) form of the recombinant N-lobe half-molecule of human serum transferrin (ApoTfN) by X-ray crystallography. Two crystal forms were obtained, form 1 with four molecules in the asymmetric unit and form 2 with two molecules in the asymmetric unit. The structures of both forms were determined by molecular replacement and were refined at 2.2 and 3.2 A resolution, respectively. Final R-factors were 0.203 (free R = 0. 292) for form 1 and 0.217 (free R = 0.312) for form 2. All six copies of the ApoTfN structure are essentially identical. Comparison with the holo form (FeTfN) shows that a large rigid-body domain movement of 63 degrees has occurred in ApoTfN, to give an open binding cleft. The extent of domain opening is the same as in the N-lobe of human lactoferrin, showing that it depends on internal constraints that are conserved in both proteins, and that it is unaffected by the presence or absence of the C-lobe. Although the conformational change is primarily a rigid-body motion, several local adjustments occur. In particular, two iron ligands, Asp 63 and His 249, change conformation to form salt bridges, with Lys 296 and Glu 83, respectively, in the binding cleft of the apo protein. Both salt bridges would have to break for iron coordination to occur. Most importantly, the structure, determined at a pH (5.3) that is close to the pH of physiological iron release, indicates that protonation of His 249 is a key step in iron release.     

Muscle-splitting thoracotomy

Muscle-splitting thoracotomy avoids transection of the latissimus dorsi and the serratus anterior muscles, thereby decreasing post thoracotomy pain and preserving the function and viability of these two muscles. The exposure provided for most intrathoracic procedures is excellent.     

1H,13C] NMR determination of the order of lobe loading of human transferrin with iron: comparison with other metal ions

The human calmodulin-1 gene (hCALM1) contains a (CAG)7 repeat in its 5'-untranslated region (5'-UTR). We found this repeat to be stable and nonpolymorphic in the human population. To determine whether the repeat region affects hCALM1 expression and whether repeat expansions to numbers known to be associated with disease in other genes may alter expression, we tested luciferase reporter genes driven by the hCALM1 promoter and 5'-UTR containing 0, 7 (wild-type), 20, and 45 CAG repeats in human NT2/D1 teratoma cells. Interestingly, the repeat deletion, (CAG)0, decreased expression by 45%, while repeat expansions to (CAG)20 and (CAG)45, or the insertion of a scrambled (C,A,G)7 sequence did not alter gene expression. These data indicate (1) that the endogenous repeat element is required for full expression of hCALM1, and (2) that some triplet repeat expansions in the 5'-UTR of protein-coding genes may be well tolerated and even optimize gene expression.     

Surface chemistry of Ni induced graphite formation on the 6H-SiC (0 0 0 1) surface and its implications for graphene synthesis

Ni films ranging in thickness from 0.4 nm to 50 nm were deposited by evaporation onto terraced SiC (0 0 0 1) substrates at room temperature and annealed at 700 °C. The resulting changes in surface composition and morphology were characterized using Auger electron spectroscopy and atomic force microscopy. In all cases, graphitic films dominate the surface chemistry. There appears to be three different thickness dependent morphology regimes. For the thinnest Ni films (0.4 nm), there is a uniform carbon-overlayer. For slightly thicker Ni films (0.6-9.6 nm), clustering and platelet formation are observed, and for still thicker films (50 nm), the platelets give way to hillocks. Within the platelet regime, there is a critical thickness at which surface roughening occurs. These results reveal a potential parametric window in which graphene may be produced and harvested.