More on the dynamics of a stirred tank with consecutive reactions

The literature on the stirred tank reactor with consecutive reactions is reviewed and a census of multiplicity diagrams, bifurcation diagrams and phase portraits is given. The location of the organizing centre, introduced by Balakotaiah and Luss, and some features of the butterfly singularity are discu particularly with reference to variations in the ratio of activation energies. The forms that the regions of attraction of the various steady states ma take in the three-dimensional state space are also discussed.     

Regional myocardial motion and thickening assessed at rest by ECG-gated 99mTc-MIBI emission tomography and by magnetic resonance imaging

We have validated ECG-gated emission tomography using technetium-99m methoxyisobutylisonitrile for the assessment of regional ventricular function by comparing it with cine magnetic resonance imaging (MRI). Gated tomography was performed at rest in 24 patients referred for myocardial perfusion imaging [17 males and seven females with a mean age of 58 years, nine of whom had had a previous myocardial infarction (MI)]. Scores were assigned to each of nine myocardial segments for wall motion and for thickening. Cine MRI was analysed in an identical fashion. Four out of 216 (2%) segments were uninterpretable by gated tomography because of inadequate tracer uptake. In eight patients without coronary artery disease (CAD), wall motion and thickening were normal by both methods. Gated tomography showed abnormal wall motion or thickening in all patients with previous MI and in five of seven patients with CAD but no prior MI. Association between wall motion and thickening was good (rs=0. 86). Overall, there was good agreement between gated tomography and MRI for both wall motion (178/212 segments, kappa=0.66) and wall thickening (184/212 segments, kappa=0.69). In segments with severely reduced perfusion, however, there was poorer agreement (kappa=0.31). Interobserver and intraobserver agreement was high (kappa from 0.61 to 0.78). Thus, in patients investigated for CAD, there is good overall agreement between gated tomography and MRI but the agreement is lower in segments with severe perfusion defects.     

Effectiveness of catalytic archipelagos—II: Random arrays of random islands

Voronoi polygons are used to simulate a random distribution of irregularly shaped catalytic islands. Finite element mathematics is used to determine the catalytic effectiveness of the islands when adsorbed reactants diffuse and disappear by a first order reaction at their boundaries. It is found that the catalytic effectiveness of the islands at any fractional coverage can be estimated reasonably accurately either from asymptotic formulae, valid for certain values of diffusion, adsorption, and reaction constants, or from the equivalent annulus approximation found earlier to be accurate for certain regular arrays of regular islands. The equivalent annulus approximation allows analytic prediction of the catalytic effectiveness for any set of parameter conditions given only the fraction area ratio Γ/Ω. Both of these quantities of which can be determined directly from a photomicrog of a representative sample.     

ON THE CONCEPT OF THE STEADY STATE IN CHEMICAL REACTOR ANALYSIS

The inlet and outlet streams of a chemical reactor may reach a steady state before the internal distributions of concentration, etc. have come to the steady state. Such a steady state, in which the reactor is viewed only as a system with certain inputs and outputs, may be called the black-box steady state (BBSS). The existence of a BBSS does not always imply that of a true SS, and the BBSS may be attained in a finite time even though the SS is only attained asymptotically. Indeed under some circumstances the BBSS can be achieved immediately upon start-up. Provided that the internal transients are not catastrophic, the practical, or operator's, concept of a steady state is really that of a BBSS.     

Effectiveness of catalytic archipelagos—I Regular arrays of regular islands

The interplay of the adsorption and desorption of reactants onto a catalytically active surface and their diffusion in that surface is an important process in catalysis. A surface with regularly distributed circular or rectangular catalyst islands in a plane has been investigated here under the assumption that the adsorbed reactant diffuses to the boundaries of the islands and disappears there by a first-order reaction. With the aid of the finite element method, effectiveness factors of the model systems have been calculated in terms of two dimensionless parameters ψ and μ, one measuring the ratio of the adsorption and desorption rates to that of surface diffusion and the other the rate of reaction to the rate of surface diffusion. It is found that the effectiveness factor for arrays whose symmetry elements are not too elongate can be approximated closely by that of an equivalent annulus whose inner and outer radii are calculated in terms of the geometrical parameters of the various systems. The equivalent annulus approximation is fairly accurate for these arrays in the parameter range 0.01 ?ψ, μ?10. Furthermore, outside this parameter range, asymptotic formulas are given which are accurate for all systems considered.     

Nop5p is a small nucleolar ribonucleoprotein component required for pre-18 S rRNA processing in yeast

We have identified a novel nucleolar protein, Nop5p, that is essential for growth in Saccharomyces cerevisiae. Monoclonal antibodies B47 and 37C12 recognize Nop5p, which has a predicted size of 57 kDa and possesses a KKX repeat motif at its carboxyl terminus. Truncations that removed the KKX motif were functional and localized to the nucleolus, but conferred slow growth at 37 degreesC. Nop5p shows significant sequence homology with yeast Sik1p/Nop56p, and putative homologues in archaebacteria, plants, and human. Depletion of Nop5p in a GAL-NOP5 strain lengthened the doubling time about 5-fold, and selectively reduced steady-state levels of 40 S ribosomal subunits and 18 S rRNA relative to levels of free 60 S subunits and 25 S rRNA. Northern blotting and primer extension analyses showed that Nop5p depletion impairs processing of 35 S pre-rRNA at the A0 and A2 cleavage sites. Nop5p is associated with the small nucleolar RNAs U3, snR13, U14, and U18. Depletion of Nop5p caused the nucleolar protein Nop1p (yeast fibrillarin) to be localized to the nucleus and cytosol. Also, 37C12 co-immunoprecipitated Nop1p. These results suggest that Nop5p functions with Nop1p in the execution of early pre-rRNA processing steps that lead to formation of 18 S rRNA.     

Mutations and expression of the ras family genes in leukemias

The levels of expression and the incidence of codon 12 point mutations of the ras family genes were studied in 18 cases of leukemia, seven with acute myeloblastic leukemia (AML), three with acute lymphoblastic leukemia (ALL), four cases with chronic myelogenic leukemia (CML) and four cases with chronic lymphocytic leukemia (CLL). Elevated expression of the ras genes was found for 39%, 61% and 67% of the specimens for the H-ras, K-ras and N-ras, respectively. A trend was found between the overexpression of the N-ras gene and the acute leukemias: all 10 acute leukemias exhibited overexpression of the N-ras gene, while only two of the CML cases, both in blastic crisis, showed elevated levels of the N-ras gene. Codon 12 point mutations at the N-ras gene were found in two of seven cases (28%) with AML and one of four cases (25%) with CML. The only K-ras codon 12 point mutation was found in a patient with CLL. No mutations were found in the codon 12 of H-ras. Our data suggest that apart from the point mutations, overexpression of the ras family genes is important in the development of the disease.