Filtration in artificial porous media and natural sands under microgravity conditions

The goal of the present paper is to investigate experimentally and theoretically the capillary driven filtration in porous media with homogeneous and inhomogeneous porosity and permeability under microgravity conditions. The motivation having determined the choice of the environment was to study nonequilibrium phenomena in two-phase filtration in porous media. The paper contains the results illustrating the sensitivity of capillary forces to variations of porous media characteristics. The experimental results obtained for fluid imbibition into unsaturated artificial and natural porous media are compared. Theoretical and experimental results on determination of mixing fluxes in two-phase filtration are discussed.     

Optimization of high-voltage self-blast interrupters by gas flow and electric field computations

Self-blast interrupters have the advantage of operating with low mechanical energy as they build up the pressure of the arc extinguishing gas flow from the thermal energy of the arc. This creates a higher pressure, but also a higher gas temperature than in interrupters with mechanical compression. Detailed flow field and electric field computations are necessary to achieve under such conditions the energy removal and dielectric recovery in view of an increase in breaking capacity and voltage rating. In particular, the optimization of the electric field/gas density distribution E//spl rho/ is a tool to improve the short-circuit and capacitive current breaking performance.     

Ricin A-chain: kinetics, mechanism, and RNA stem-loop inhibitors

Ricin A-chain (RTA) catalyzes the depurination of a single adenine at position 4324 of 28S rRNA in a N-ribohydrolase reaction. The mechanism and specificity for RTA are examined using RNA stem-loop structures of 10-18 nucleotides which contain the required substrate motif, a GAGA tetraloop. At the optimal pH near 4.0, the preferred substrate is a 14-base stem-loop RNA which is hydrolyzed at 219 min-1 with a kcat/Km of 4.5 x 10(5) M-1 s-1 under conditions of steady-state catalysis. Smaller or larger stem-loop RNAs have lower kcat values, but all have Km values of approximately 5 microM. Both the 10- and 18-base substrates have kcat/Km near 10(4) M-1 s-1. Covalent cross-linking of the stem has a small effect on the kinetic parameters. Stem-loop DNA (10 bases) of the same sequence is also a substrate with a kcat/Km of 0.1 that for RNA. Chemical mechanisms for enzymatic RNA depurination reactions include leaving group activation, stabilization of a ribooxocarbenium transition state, a covalent enzyme-ribosyl intermediate, and ionization of the 2'-hydroxyl. A stem-loop RNA with p-nitrophenyl O-riboside at the depurination site is not a substrate, but binds tightly to the enzyme (Ki = 0.34 microM), consistent with a catalytic mechanism of leaving group activation. The substrate activity of stem-loop DNA eliminates ionization of the 2'-hydroxyl as a mechanism. Incorporation of the C-riboside formycin A at the depurination site provides an increased pKa of the adenine analogue at N7. Binding of this analogue (Ki = 9.4 microM) is weaker than substrate which indicates that the altered pKa at this position is not an important feature of transition state recognition. Stem-loop RNA with phenyliminoribitol at the depurination site increases the affinity substantially (Ki = 0.18 microM). The results are consistent with catalysis occurring by leaving group protonation at ring position(s) other than N7 leading to a ribooxocarbenium ion transition state. Small stem-loop RNAs have been identified with substrate activity within an order of magnitude of that reported for intact ribosomes.     

染色织物的非甲醛耐久压烫整理:棉交联聚羧酸的评价

1　引言已经得到证实 ,聚羧酸类 (PCAs) ,尤其是 1 ,2 ,3,4 丁烷四甲酸 (BTCA)或柠檬酸 (CA) (图 1 )与像次磷酸钠 (SHP)这样的含磷催化剂一起使用 ,是释放甲醛的交联剂如N 羟甲基化合物二羟甲基二羟基亚乙基脲 (DMDHEU)的最有效替代品。在常规 1 80℃、90s的焙烘过程中 ,PCA的羧基与纤维素链的羟苯基通过酐媒介形成酯键 ,结果发生交联反应 ,赋予棉织物耐久压烫性能。这些酯键可经受多次家庭碱洗。图 1　交联剂PCA、催化剂SHP、在CA整理织物的焙烘过程中产生的不饱和PCA类物质的结构示意图一些学者对PCA整理过的棉织物用多种…     

Intersection problems of parametric surfaces in CAGD

In Computer Aided Geometric Design the task of computing intersections between surfaces must be performed by numerical methods. In the present paper a new kind of marching method is considered which employs interval arithmetic for proofing the correct topology of the solution set.     

Alumina deposition by activated reactive evaporation

Alumina deposits were synthesized at various deposition temperatures from 700 to 1200°C using the activated reactive evaporation process by evaporating aluminum in the presence of a partial pressure of oxygen. The deposits were characterized by X-ray diffraction, scanning electron microscopy and microhardness measurements. The structure is predominantly amorphous at lower deposition temperatures and the equilibrium α-Al₂O₃ at higher deposition temperatures. The results compare closely with those of Al₂O₃ deposits produced by direct evaporation and sputtering.     

Treatment of coumarin-induced skin necrosis with a monoclonal antibody purified protein C concentrate

BACKGROUND AND DESIGN--Protein C is a vitamin K-dependent plasma protein that is converted to the serine protease activated protein C by the thrombin-thrombomodulin complex. Activated protein C functions as a natural anticoagulant by inactivating the cofactors of the coagulation cascade, factors Va and VIIIa. Coumarin (warfarin)-induced skin necrosis is thought to be due to a rapid elimination of protein C relative to other vitamin K-dependent factors during the initial phase of oral anticoagulation. We have used a highly purified protein C concentrate to treat a patient with acquired protein C deficiency who developed skin necrosis during the initial phase of oral anticoagulant therapy. OBSERVATIONS AND CONCLUSIONS--During protein C concentrate therapy, no further skin lesions appeared, and the healing process of necrotic areas was facilitated. Replacement therapy with protein C concentrate appears to be safe and effective as an adjunctive treatment for coumarin-induced skin necrosis.     

Interaction between neuronal nitric oxide synthase and inhibitory G protein activity in heart rate regulation in conscious mice

Nitric oxide (NO) synthesized within mammalian sinoatrial cells has been shown to participate in cholinergic control of heart rate (HR). However, it is not known whether NO synthesized within neurons plays a role in HR regulation. HR dynamics were measured in 24 wild-type (WT) mice and 24 mice in which the gene for neuronal NO synthase (nNOS) was absent (nNOS-/- mice). Mean HR and HR variability were compared in subsets of these animals at baseline, after parasympathetic blockade with atropine (0.5 mg/kg i.p.), after beta-adrenergic blockade with propranolol (1 mg/kg i.p.), and after combined autonomic blockade. Other animals underwent pressor challenge with phenylephrine (3 mg/kg i.p.) after beta-adrenergic blockade to test for a baroreflex-mediated cardioinhibitory response. The latter experiments were then repeated after inactivation of inhibitory G proteins with pertussis toxin (PTX) (30 microgram/kg i.p.). At baseline, nNOS-/- mice had higher mean HR (711+/-8 vs. 650+/-8 bpm, P = 0.0004) and lower HR variance (424+/-70 vs. 1,112+/-174 bpm2, P = 0.001) compared with WT mice. In nNOS-/- mice, atropine administration led to a much smaller change in mean HR (-2+/-9 vs. 49+/-5 bpm, P = 0.0008) and in HR variance (64+/-24 vs. -903+/-295 bpm2, P = 0.02) than in WT mice. In contrast, propranolol administration and combined autonomic blockade led to similar changes in mean HR between the two groups. After beta-adrenergic blockade, phenylephrine injection elicited a fall in mean HR and rise in HR variance in WT mice that was partially attenuated after treatment with PTX. The response to pressor challenge in nNOS-/- mice before PTX administration was similar to that in WT mice. However, PTX-treated nNOS-/- mice had a dramatically attenuated response to phenylephrine. These findings suggest that the absence of nNOS activity leads to reduced baseline parasympathetic tone, but does not prevent baroreflex-mediated cardioinhibition unless inhibitory G proteins are also inactivated. Thus, neuronally derived NO and cardiac inhibitory G protein activity serve as parallel pathways to mediate autonomic slowing of heart rate in the mouse.